Friday, May 10, 2013
ENDURANCE WAVE ANALYSIS
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1. INTRODUCTION
For an offshore structure, wind, wave and gravitational forces are all important sources of loading. The dominant load, however, is normally due to wind-generated random waves. If the structure is located in shallow waters, and in cases that the dominant wave period is obviously higher than the natural period of the structure, wave loads can be applied statically to the structural system. But, a non-linear time history analysis will become necessary to obtain more accurate results for the structure’s response, in particular for deep water structures under extreme loading conditions. It is important to account for the randomness of the loading also.
A recently emerged approach called Endurance Time Analysis (ETA) is a novel method for analyzing structures subjected to ground motion excitations. The basic idea of this method was introduced by Estekanchi et al. ETA comes under the category of time domain analysis that in which in the specific times (target times), the input response spectrum will be the same as the design spectra corresponding to the desired levels of risk. In this approach, structure is exposed to an artificial intensifying acceleration time history. Damage indices or any other engineering demand parameters such as base shear, drift, and stress in structural members can be studied through the time variable. ETA method can be applied to any structural system irrespective of the complexity of the structural modeling, and different design criteria can be studied with less cost and computational time. Studies show that this method is more efficient and accurate in the evaluation of structures during the earthquakes previously used methods.
Though there are similarities between structures subjected to earthquake and sea wave excitations in basic principles of design and performance, they differ in some aspects. Firstly, time duration of storm wave loading is very large (several hours) compared to earthquake loading occurrence. Secondly, temporal evolution of a stormy sea state may take several hours which covers a sufficient part of the growth and decay phases of the storm. Seismic excitations are almost abrupt and reach to their maximum potential in a short period of time (a few seconds).
Here a new approach, called Endurance Wave Analysis (EWA), has been introduced which can be used for non-linear dynamic analysis and assessment of offshore structures subjected to irregular wave forces. This is an extension of the Endurance Time Analysis (ETA) mentioned above for seismic assessment of the structures. In EWA method, the offshore
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structure is subjected to a predefined Intensifying Wave Train Function (IWTF), which represents different sea states at a specific site. This function is designed so that it characterizes increasing roughness of the sea state over the time which even goes well beyond the design sea state. A non-linear dynamic analysis is conducted by introducing IWTF as the source of external excitations of the offshore structure. From the results of the analysis the structural integrity of an offshore platform can be evaluated.
The EWA method has the ability to consider spectral features of the sea state, to incorporate waves of different heights and frequencies in a single dynamic analysis and to take into account the irregularity and randomness of the sea waves. It requires relatively short simulation times than the other time domain analysis procedures. The non-linear behavior of offshore structures can be evaluated. And it is able to consider the different damage indices for evaluating the structural integrity. The method is useful for both design and as well as the assessment and collapse analysis of existing offshore platforms.
2. THE APPROACH OF ENDURANCE WAVE ANALYSIS (EWA)
2.1 The Analogy to the Cardiopulmonary Exercise Test
The approach of EWA is owed from the ETA method which was inspired from the standard protocol (exercise test) that is used by cardiologists in order to evaluate the physical condition of the cardiovascular system in the human body. Even though the cardiovascular organization of humankind is one of the most complex systems known in nature, the performance of this system is evaluated with a simple test called as the exercise test. Cardiopulmonary exercise testing is a well-established procedure which is non-invasive, relatively inexpensive, a strong and independent predictor of cardiovascular disease and mortality. The analogy between a cardiopulmonary exercise test and the EWA method is shown in figure 2.1.
2.2 The ETA Method
Endurance Time Analysis (ETA) method is a time history based analysis procedure that applies special intensifying acceleration functions for estimating the seismic performance of structures at different excitation levels in each single analysis. The Endurance Time method is a
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procedure that can be used in both the linear and nonlinear seismic analysis of structures. Its simplicity is
Figure 2.1
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one of the priorities of ET method over response history analysis under actual ground motions. In the ET method, the response of a structure can be monitored against time which is correlated to the intensity of excitation. ET method is evaluated by comparing its results with results of time history analysis under actual records. To simulate excitations, intensifying accelerograms called “ET acceleration functions” (ETAF), are imposed to the structure. These ETAF are generated in such a way that their response spectra increase by the time, hence response of structure under this kind of accelerogram gradually increases with time. A typical ETAF is depicted in Figure 2.2.
Figure 2.2 A typical ET Acceleration function
2.3 The EWA Method
Endurance Wave Analysis (EWA) method is somewhat similar to the Endurance Time Analysis (ETA). In the EWA method, after a preliminary study on sea state conditions and wave spectrum characteristics of the site, the offshore structure is exposed to an intensifying time history of waves. For this, consecutive time series of stepwisely intensifying constrained irregular waves are joined together to form a single time history of the sea surface. This is called an Intensifying Wave Train Function (IWTF). It is similar to ET Acceleration Function (ETAF) in ETA method. Overall, it represents an artificial gradually deteriorating sea state at the site. This function is a relatively short duration time series of the irregular water surface elevation. It is designed so that it characterizes increasing roughness of the sea state, over the time, which even goes well beyond the design sea state.
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Similar to a cardiopulmonary exercise test, the non-linear dynamic response of the offshore structure can be evaluated when it is simulated under the IWTF. An arbitrary damage index, for example a drift limit or collapse occurrence, can be defined as a monitoring parameter in the temporal response events. Now, in principle, the sea state which causes the structure to reach the predefined damage target can be easily evaluated by a single dynamic analysis. The dynamic behavior of the structure, from its elastic part to complete failure level, and temporal variations of the desired damage indices can be monitored during an EWA. Using the EWA procedure and considering appropriate target criterion, different levels of performance for the offshore structure can be investigated. The Endurance Wave Analysis (EWA) method provides a powerful and practical approach for the dynamic, time domain and nonlinear analysis of the offshore structures subjected to random sea wave excitations.
2.4 The Hypothetical Shaking Experiment
The concept of EWA can be explained with a hypothetical shaking experiment. Three different designs for an offshore jacket type platform with various (say unknown) load bearing capacities, dynamic characteristics and geometrical properties of the structural members are considered. All three structures are considered to be placed in similar offshore locations with identical environments. These structures will be evaluated using an EWA method. For this all three structures become subject to an identical IWTF (Figure 2.3). In the beginning, the wave train and its corresponding wave energy density spectrum are assumed to represent a rough sea state (HS1 and TP1). The EWA analysis results indicate that all three structures exhibit stable responses under this sea environment (Fig. 2.3a). As the time goes on, the sea conditions is intensified (Figure. 2.3b) and the wave train and its corresponding wave energy density spectrum are now assumed to represent a high sea state (HS2 and TP2 , HS2 > HS1 ). This sea state has gone beyond that structure (2) can withstand. As a result a failure occurs in structure (2) (Fig. 2.3b). The failure has been identified by damage indices to exceed a target limit or by unbounded structural responses. Under this sea state, structures (1) and (3) are still performing well and exhibit stable and limited responses. With further intensification of the sea conditions the wave train and its corresponding wave energy density spectrum now, assumedly, represent a phenomenal sea state (HS3 and TP3 , HS3 > HS2). The wave load has now caused structure (3) to fail while structure (1) is still doing well and exhibits stable and limited responses but structure
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(2) is seriously damaged (Fig. 2.3c). So with a EWA procedure, the non-linear structural dynamic performance of an offshore structure, or its reserve capacity, can be easily evaluated by a single dynamic analysis.
Figure. 2.3. Three hypothetical offshore platforms subjected to an Intensifying Wave Train Function (IWTF)
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Endurance Wave Height (EWH) of an offshore structure refers to the maximum sea state that the structure can withstand under an IWTF. EWH can be defined by monitoring an appropriate damage index when it meets a damage threshold. The EWH corresponds to a time in the analysis when the amount of damage observed in the structure exceeds a target value. As a result, for a particular structure, based on demand levels and the desired performance criteria different EWHs can be identified. The method can also effectively describe the behavior of structure from linear elastic to non-linear plastic zone toward its final collapse by a relatively short duration time domain analysis.
3. THE WAVE TRAIN FUNCTION EMPLOYED
3.1 Intensifying Wave Train Functions (IWTFS)
The most important element in the Endurance Wave Analysis (EWA) method is producing appropriate Intensifying Wave Train Functions (IWTFs). Based on the structure type, the water depth, the wave climate condition and the analysis, different wave theories can be used for constructing the IWTF. IWTF may be accordingly selected to be regular or irregular. In practice, ocean and sea waves are both random and irregular and simultaneously contain waves with different shapes, heights, speeds, periods and directions. For engineering purposes, the wave condition may be described either by a deterministic or a stochastic approach using a wave spectrum. The sea state is usually assumed to be a stationary random process. A stationary sea state can be characterized by a set of environmental parameters such as the significant wave height HS and the peak period TP. Three hours has been introduced as a standard time between registrations of sea states when measuring waves, but the period of stationarity can range from 30 min to 10 h.
For design applications, regular wave theories such as fifth order Stokes (in deep water) and conidial (in shallow water) are commonly employed to calculate the wave loads on offshore platform. In this approach, it is assumed that all wave energy is concentrated in a fixed frequency. These methods are popular due to their ease of application and low computation time. With dynamically sensitive structures, however, the accuracy of these types of deterministic approach is debatable.
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The most comprehensive approach in studying the dynamic behavior of offshore structures subjected to ocean waves is stochastic irregular long-term simulation of the sea water level (SWL) and direct method of solving motion equations in a time domain. This is time consuming and computationally expensive, because a time record of the irregular wave input must be provided, while generally, one only has information about the wave spectrum available. Artificial irregular wave records can be regenerated using a wave spectrum but by choosing different random phases a seemingly endless series of time records, all with identical statistical properties, can be generated. It cannot be said which –if any– particular (artificial) time record is correct. Therefore, the time domain analyses need to be repeated many times, using different long duration irregular wave records. This can have a significant influence on the interpretation of the results, especially when extreme values are needed. The interpretation of time domain simulation results forms another difficulty in practice. A designer often needs a “design response” (an extreme dynamic internal load or displacement) with an associated (small) probability that it will be exceeded as “output” from a dynamic analysis. Any wave theory can be used in the EWA procedure. Here CNW model has been used to generate the IWTFs. This is because the irregularity and randomness of the sea surface are sensibly represented by a CNW. A CNW has a relatively short duration (say around 2 min) as compared to typical irregular waves (say 3 h). Other researchers in recent years have shown the CNW model to be a suitable alternative to long-term random time history simulations, and that it would decrease the time and calculation costs.
3.2 Intensifying Constrained NewWave Model (ICNW)
Assessment of Gaussian sea state characteristics, near a local maximum, for example highest wave crest, has been studied by Lindgren. In 1981–1982, Quasi-Deterministic (QD) theory was later suggested by Boccotti, into two versions, based on first-order Stokes expansion. The QD theories proved that in a Gaussian sea state if a local wave crest maximum (first QD theory) or a large crest-to-trough (second QD theory) occurs at some fixed time and location, the water surface elevation will be proportional to the autocovariance function. This theory provides necessary and sufficient conditions for simulating a very large irregular wave at a fixed position and time. The first version of QD formulation was similar to NewWave (NW) theory introduced by Tromans et al. The NW theory was based on statistical analysis of irregular sea surface data.
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They were the first who introduced a practical approach for representing an extreme irregular sea-surface elevation in a short-term history, which was called NewWave. Several specific characteristics included in the model, such as its spectral base and irregular shape, made the NW clearly different from deterministic regular models, for example, 5th-Stokes theory. A single profile of the NW model, however, cannot consider the stochastic nature of the ocean waves. So, this shot-term time history falls short to deliver the randomness of the sea state to an offshore structure when it is modeled under dynamic wave actions. If a NW profile is constrained to a Random Surface Elevation (RSE), an appropriate short-time random time history, with a desired wave crest elevation, can be produced. Accordingly, Taylor et al. proposed the Constrained NewWave (CNW) model. Fig. 3.2.a shows typical shapes of the NW and CNW surface elevations as a function of time.
Fig. 3.2a Typical NewWave (left) and Constrained NewWave profiles (right).
In EWA method, the CNW model has been used to generate the Intensifying Wave Train Functions (IWTFs). For this, m separate time series of stepwisely intensifying CNWs each with a duration time of td are joined together to form a single artificial time history of the sea surface. The kth CNW profile 𝜂k(t) represents the sea state k (1≤ k ≤ m) which is itself constructed based on the wave energy density spectrum Sk(ω) at a specific site. The kth CNW covers a time period of (k − 1) × td < t < k × td. The roughness of the sea state k and the power of the wave spectrum Sk(ω) stepwisely increases with a linear trend as k increases from 1 to k. The target (say design) wave height HS design and its corresponding energy density spectrum Sdesign(ω) can be located
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somewhere halfway through the sea states 1 to m. The first generation of IWTF, in which the function growth is linear, can be expressed as follows:
𝜂R1 (t) + ρ1(t) [α1 − 𝜂R1 (tc)] + ̇ 𝜂̇R1 (tc) 0 < t< td,, S1(ω)
𝜂R2 (t) + ρ2(t) [α2 − 𝜂R2 (tc)] + ̇ 𝜂̇R2 (tc) td < t<2 td,, S2(ω)
⋮
𝜂Rk (t) + ρk(t) [αk − 𝜂Rk (tc)] + ̇ 𝜂̇Rk (tc) (k-1 td < t
THE EFFECT OF TENSION STIFFENING ON THE BEHAVIOUR OF REINFORCED CONCRETE MEMBERS
1.INTRODUCTION
The bending stiffness of reinforced concrete beams under service loads is considerably smaller than the stiffness calculated on the basis of uncracked cross sections. This is because the beam contains numerous tensile cracks. Yet, at the same time, the stiffness is significantly higher than that calculated when the tensile resistance of concrete is neglected. This phenomenon, often termed tension stiffening, is attributed to the fact that concrete does not crack suddenly and completely but undergoes progressive micro cracking. Immediately after first cracking, the intact concrete between adjacent primary cracks carries considerable tensile force, mainly in the direction of the reinforcement, due to the bond between the steel and the concrete. The average tensile stress in the concrete is a significant percentage of the tensile strength of concrete. The steel stress is a maximum at a crack, where the steel carries the entire tensile force, and drops to a minimum between cracks. The bending stiffness of the member is considerably greater than that based on a fully cracked section, where concrete in tension is assumed to carry zero stress. This tension stiffening effect may be significant in the service load performance of beams.
Previous numerical studies have found that the tension stiffness of high-strength concrete (HSC) is lower than that of normal-strength concrete (NSC). Other variables such as the percentage and distribution of reinforcing steel, bar size, bond properties and shrinkage of concrete are also reported to have an effect on tension stiffening.
Tension stiffening is particularly significant in relatively lightly reinforced members, where the actual stiffness may be several times larger than the stiffness calculated on the basis of fully-cracked cross-sections, where the tensile concrete is ignored and only the embedded tensile reinforcement is considered. Tension stiffening increases with an increase in tensile stress in the concrete. Conversely tension stiffening decreases when the tensile stress in the concrete drops and, under constant load, this is caused either by cracking, by tensile creep or by a time-dependent deterioration of bond. Cracking can be caused by external loads or by restraint to imposed deformations, such as drying shrinkage. The tension stiffness of concrete plays an important role in the deformation behavior of the reinforced concrete (RC) structures in the post-cracking region of concrete.
2.MODELS FOR TENSION STIFFENING
Various methods have been proposed to account for tension stiffening in the analysis of concrete structures. These range from simple empirical estimates of the flexural rigidity of a member to assumed unloading stress-strain relationship for concrete in tension. Techniques involving an adjustment to the stiffness of the tensile steel to account for tension stiffening have also been used . An alternative approach for modelling tension stiffening is to assume that an area of concrete located at the tensile steel level is effective in providing stiffening. Figure 2.1 shows an average cross-section of a singly reinforced member. The properties of this average section are between those of the fully cracked cross section and the uncracked cross section between the primary cracks. The tensile concrete area Act, which is assumed to contribute to the beam stiffness after cracking, depends on the magnitude of the maximum applied moment M, the area of the tensile reinforcement Ast, the amount of concrete below the neutral axis, the tensile strength of the concrete (the cracking moment Mc), and the duration of sustained load. Bazant and Byung proposed a simplified equivalent transformed cross section, taken from their proposed model which is derived from the intrinsic material properties of concrete, particularly the strain-softening properties. In this simplified model, which is used in this study, the tensile resistance of concrete distributed over the tension side of the neutral axis is neglected and an equivalent tensile area Aeq and an equivalent tensile stress of concrete in this area σeq, which would yield about the same beam curvature κ, is determined. The centroid of this equivalent area coincides with that of tensile reinforcement.
Fig 2.1 Average section after cracking
The equivalent tensile area can be obtained from:
Aeq = [b(dn)2/2 + nAs1 (dn - ds1 )] x (ds2 –dn )-1 - nAs2 Eq.(2.1)
In which As1 and As2 are the area of compression and tension reinforcement, respectively, n = Es / Ec. The value of dn needed in Equation 2.1 has to be calculated from the condition of the same curvature κ. Two cases is distinguished depending on whether the tensile stress in concrete at tensile face is zero or finite1. With considering these cases, Aeq and dn could be evaluated in an iterative manner for a given bending moment. Once dn and Aeq are determined, the inertia moment of the transformed cross section can be evaluated.
3.SHORT-TERM ANALYSIS OF CROSS SECTIONS
Fig 3.1 Equivalent transformed cross section
Consider the equivalent transformed cross section in Figure 3.1. The top surface of the cross section is selected as the reference surface. The position of the centroidal axis depends on the quantity of bonded reinforcement and varies with time owing to the gradual development of creep and shrinkage in the concrete. Therefore, it is convenient to select a fixed reference point that can be used in all stages of analysis.
3.1 UNCRACKED SECTION
In Figure 3.2, the strain at a depth y below the top of the cross section is defined in terms of the top fibre strain ε0i and the initial curvature κi , as follows:
εi = ε0i + y κi Eqn (3.1)
The initial concrete stress at y below the top fibre is:
σi = Ec εi = Ec (ε0i + y κi) Eqn(3.2)
Fig 3.2 Uncracked section analysis
Integrating the stress block over the depth of the section, horizontal equilibrium requires that:
Ni = ∫ σi dA
= Ec ε0i ∫ dA + Ec κi ∫ y dA
= Ec ε0i A + Ec κi B Eqn(3.3)
where A (= ∫ dA) is the area of the transformed section and B (= ∫ ydA) is the first moment of the stress block about the top surface of the section. If the first moment of the stress block about the top fibre is integrated over the depth of the section, the resultant moment about the top surface, Mi , is found. Therefore,
Mi = ∫ σi y dA
= Ec ε0i ∫ y dA + Ec κi ∫ y2 dA
= Ec ε0i B + Ec κi Ī Eqn(3.4)
where Ī ( = ∫ y2 dA ) is the second moment of the transformed area about the top surface of the transformed section. By re arranging Eqs.3.3 and 3.4, expressions are obtained for the initial top fibre strain and curvature :
ε0i = B Mi / [Ec (B2 – A Ī )] Eqn(3.5)
κi = -A Mi / [Ec (B2 – A Ī )] Eqn(3.6)
3.2 CRACKED SECTION
The instantaneous strains and stresses on a cracked section are shown in Figure2.3
Horizontal equilibrium dictates that:
Fig 3.3 Cracked section analysis
Ts – Cc – Cs + Tc = 0 Eqn(3.7)
and moment equilibrium requires that
M = Cc dz + Cs ds1 – Tc ds2 – Ts ds2 Eqn(3.8)
where Cc , Cs , Tc , and Ts can be expressed as functions of dn and ε0i :
Cc = σ0i bdn /2 = Ec ε0i bdn /2 Eqn(3.9)
Cs = Es As1 [ε0i (dn – ds1)/dn] Eqn(3.10)
Ts = Es As2 [ε0i (ds2 – dn)/dn] Eqn(3.11)
Tc = σeq Aeq = Ec Aeq [ε0i (ds2 – dn)/dn] Eqn(3.12)
By substituting Eqs. (3.8-3.12) into Eqs. (3.7) and (3.8) and solving the simultaneous equations,
ε0i and dn are found with an iterative manner. Based on the values of ε0i and dn the curvature can be calculated:
κi = - ε0i / dn Eqn(3.13)
4.TIME-DEPENDENT ANALYSIS OF CROSS-SECTIONS
During any time period, creep and shrinkage strains develop in the concrete. The time dependent change of strain at any depth y below the top of the cross section, Δε, may be expressed in terms of the change in top fibre strain, Δε0 , and the change of curvature, Δκ :
Δε = Δε0 + y Δκ Eqn(4.1)
The increments of top fibre strain, Δε0 , and curvature, Δκ , may be obtained from the following equations :
Δε0 = (Ýe δM - Īe δN) / [Ēe (Ýe2 - Āe Īe)] Eqn(4.2)
Δκ = ( Ýe δN - Āe δM) / [Ēe (Ýe2 - Āe Īe)] Eqn(4.3)
where Āe is the area of the age-adjusted equivalent transformed section and Ýe and Īe are the first and second moments of the area of the age-adjusted equivalent transformed section about the top surface. For the determination of Āe , Ýe , and Īe the age-adjusted effective modulus Ēe is used . Therefore, the total strain and curvature may be obtained from:
ε = ε0i + Δε
κ = κi + Δκ
The deflection δ at any point along a beam can be calculated by integrating the curvature κ(x) over the length of the beam:
δ = ∫∫ κ(x) dx dx
Figure 4.1 Deflection of a typical beam
Consider the beam shown in Figure 4.1. If the variation in curvature along the member, subjected to uniform load q is parabolic, then the deflection at mid-span, δC , is given by:
δc = (κA + 10 κC + κB) L2/ 96 Eqn(4.4)
where κA and κB are the curvature at each end of the span and κC is the curvature at midspan.
5.BEHAVIOUR OF HIGH STRENGTH CONCRETE TENSION MEMBERS
Reinforced high-strength concrete (RHSC) is being increasingly used in buildings and bridges because it enables the use of smaller cross-sections, longer spans, reduction in girder height and improved durability. The recent trend of employing RHSC with a concrete strength of over 100 MPa has resulted in smaller member sizes which lead to higher tension stress in the reinforcement.
Direct tension tests were performed on reinforced high-strength concrete (RHSC) members. The test results showed not only were splitting cracks along the reinforcement more extensive, but also the transverse crack spacing became smaller. Thereby, the reduction in the tension stiffening effect in high-strength concrete (HSC) is much greater than that would be expected.
5.1 TENSION STIFFENING PREDICTION
A number of empirical relationships have been proposed for tension stiffening, where the loss of rigidity in a cracked member can be taken into account for the stress-strain response of the steel or an average stress-strain response for concrete in the post-cracking range. Collins and Mitchell considered a load-sharing concept to account for tension stiffening, where axial load N is carried by both the steel and concrete.
(εcr/ εm)0.4 Eqn(5.1)
β=[1+√500εm]-1 Eqn(5.2)
where, β is normalized stress, εcr is cracking strain, εm is axial member strain.
5.2 METHODOLOGY
5.2.1 Details of Materials and Specimens
Testing was carried out on ten specimens that were axially loaded. Fig. 5.1 shows the geometry and instrumentation for a typical test specimen. All of the specimens had a length of 1200 mm. A single deformed steel bar, with a minimum concrete cover of 40 mm, was provided. Tension stiffening was evaluated for NSC (40 to 60 MPa) and HSC (100 to 150 MPa) using reinforcement ratios (ρ) of 2.252% respectively. The yield strength and Young’s modulus of steel were 722 MPa and 202.5 GPa respectively. The concrete properties are shown in Table 4.1. The diameters of reinforcement bars were selected to prevent the effect of splitting cracks since they are not significant when concrete cover to bar diameter (c/db) is larger than 2.5 . Details of specimens are given in Table 5.1.
5.2.2 Loading Method
Specimens were loaded vertically through one-axial tension rods. Two linear variable displacement transducers (LVDT) were clamped to the steel reinforcing bar just outside of the concrete to measure the total elongation of the reinforced concrete specimen (Fig. 5.1). At each load stage, the cracks were measured using pi-gauges. The complete response of each specimen was described by plotting the applied tension against the average member strain.
Fig 5.1 Test Setup
Average early-age shrinkage was determined for all concretes from strain measurements on 100x100x400 mm shrinkage specimens that had the same moisture curing conditions as the tension specimen. Shrinkage was included in analysis of the member response by using the calculated shrinkage strain value from the early-age shrinkage specimens to determine the initial strain for each tension specimen (Table 5.1). The initial strain was taken as an offset strain equal to; where εsh is concrete early-age shrinkage, n is modular ratio (Es/Ec), ρ is reinforcing steel ratio, Es is Young’s modulus of steel, and Ec is elastic modulus of concrete . Shrinkage strains were assumed to be uniform over the cross section. Subtracting shrinkage strains from the member response gives an idealized concrete member strain, equivalent to a member with no shrinkage.
5.3 CRACKING BEHAVIOUR
The average tensile strength of NSC and HSC was used to gain a better understanding of cracking behavior. According to tension members’ test results, the average tensile strengths of NSC and HSC (> 100 MPa) were 0.37f’c0.5 and 0.32f’c0.5 (MPa) respectively. Also, the corresponding elastic modulus of Ec for NSC and HSC was equal to 4030f’c0.5 and 3270f’c0.5 (MPa) respectively. This results in an estimated cracking strain of 92με for NSC and 98με for HSC. However, observed cracking strains were 90με on average for NSC and 100με for HSC.
The HSC specimens exhibited a larger cracking load than NSC specimens. As the concrete strength increased roughly four-fold, from 40 MPa to 145 MPa, the transverse cracking load increased by approximately 1.3 times. However, the splitting cracking loads decreased by approximately 40%. According to the experimental results, the relative transverse cracking load in the specimens matched the increase in the splitting-tensile strength of concrete. Therefore, transverse cracks were directly affected by concrete tensile strength. However, splitting cracking strength was not affected by concrete splitting-tensile strength.
Table 5.1: Specimen properties and test results
db : Steel bar diameter
c: Concrete cover
f’c : Compressive strength of concrete
ft : Splitting tensile strength of concrete
εsh: Early age shrinkage
ε’: Offset strain
T: Transverse crack
S: Splitting crack
L: Average transverse crack spacing
As the compressive strength of concrete increased from 40 MPa to 145 MPa, the average transverse crack spacing decreased by 10-50 % in both steel bar sizes.
5.4 INFLUENCE OF CONCRETE STRENGTH ON TENSION STIFFENING
HSC specimens with a strength of 90 MPa exhibit a larger tension stiffening after cracking than the NSC specimens. This behavior is clearly attributed to the early splitting cracks and excessive progress along the rebar with the increase in load. At the concrete splitting cracks along the rebar, the bond between the bar and the concrete was diminished. Therefore, the concrete is no longer able to share the tensile force, in turn resulting in a large deformation with a small stiffening effect.
The tension stiffening reduction in HSC members can be explained using elastic theory. According to elastic theory, the bond behavior of HSC can be quantitatively drawn. Since the elastic modulus of concrete is a function of compressive strength, while that of steel remains constant, the composite structural system consisted of reinforcement and concrete is altered with concrete strength, which results in a different stress state in the interface. Furthermore, HSC is more brittle than NSC and in turn, less stress redistribution can take place at the ultimate loading stage. These two material properties in HSC change the crack spacing and tensile stiffness of tension members.
Fig.5.2 shows a comparison of the measured normalized stress (β) response with the curves predicted by Eq. (5.1) and (5.2). The predictions by Eq. (5.1) underestimates the normalized stresses of NSC members, while overestimate that of HSC (f’c>100 MPa) members. The model proposed by Collins and Mitchell also overestimates the normalized stresses of HSC members. Also, it overestimates member stresses of NSC members during the early cracking and underestimates the member stresses once cracking has stabilized. From the above results and discussion, it can be concluded that the effect of material characteristics of HSC is not accounted for by Eq. (5.1) and (5.2).
5.5PROPOSED MODEL FOR HIGH STRENGTH CONCRETE
The present experimental results show that the tension stiffness of axially loaded members is highly dependent on concrete strength. According to Fig. 8, tension stiffness of HSC members after cracking cannot be sufficiently predicted by available models. Therefore, a new model is proposed to predict normalized stress, β, of HSC tension members. A best fit to the test results is obtained by using the following prediction equation.
β = 2.5( 1- 0.135 ln εm) Eqn(5.3)
This model is valid for HSC tension members with a concrete strength over 100 MPa. Eqn (5.3) was examined by comparing with the experimental normalized stress and load-deformation curves and it can be seen that the new model provides more accurate predictions for HSC tension members.
Fig.5.2 Normalized tensile behavior of concrete specimens
6. CONCLUSION
After cracking, the concrete between the cracks carries tension and hence stiffens the response of a reinforced concrete member subjected to tension. This stiffening effect, after cracking, is refered to as tension stiffening. A simple formulation is proposed for study of short-term and long-term behaviour of reinforced concrete beams. The results of a test series of five tension specimens were analyzed and observed that the tension stiffening effect is highly dependent on concrete strength when it is greater than 100 MPa. The crack spacing between the adjacent transverse cracks becomes narrower as higher concrete strength is used, and a reduction in crack spacing was also observed when the compressive strength of concrete increased. The present tension stiffening prediction equations by Belarbi and Hsu, Collins and Mitchell are not accurate enough and needs to be modified. A more accurate tension stiffening prediction equation is suggested for the design of RHSC members with steel bar diameter of 25mm and concrete strength of over 100 MPa.
REFERENCES
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[3] Fields K. and Bischoff P.H., “Tension Stiffening and Cracking of High-Strength Reinforced Concrete Tension Members,” ACI Structural Journal, Vol. 101, pp. 447-456, 2004.
[4] Mitchell D. and Abrishami H.H, ““Influence of splitting cracks on tension stiffening,”ACI Structural Journal, Vol. 93, pp. 703-710, 1996
[5] Mutsuyoshi H. and Perera S.V.T.J., “Tension Stiffening Behaviour of High Strength Concrete Tension Members,” Annual Research Journal of SLSAJ, Vol.11, pp. 10-18, 2011.
[6] Mutsuyoshi H., Perera S.V.T.J., Takeda R. and Asamoto S., “Shear Behaviour of High Strength Concrete Beams,” Proc. of Japan Concrete Institute, Vol.32, pp. 685-690, 2010.
[7] Victor G., Darius B., Aleksandr S., Kaklauskas G., Idnurn S., “Tension-Stiffening Model Based on Test Data of RC Beams,” in Tenth International Conference on Modern Building Materials, Stuctures and Techniques, Vilnius, Lithuania, 2010.
[8] Viktor G., Rokas G., Kaklauskas G., “Average stress average strain tension-stiffening relationships based on provision of design codes,” Journal of Zhejiang University, Vol.12, pp.731-736, 2011.
[9] Woo K., Lee K.Y. and Hwan S.Y., “Tension Stiffening Effect of High Strength Concrete in Axially Loaded members,” Journal of the Korea Concrete Institute, Vol 15, pp. 915- 923, 2003.
[10] Yuichi S. and Frank J.V., ““Tension Stiffening Effect of High Strength Concrete in Axially Loaded members,” Journal of Structural Engineering, Vol. 129, pp. 717-724, 2003.
.
Thursday, January 19, 2012
Structural Glazing
STRUCTURAL GLAZING IN BUILDING–MODERN GLASS
Modern glass is not only a transparent beauty, but also has unmatched utility. Today, glass is an important building element, which not only offers aesthetics but also proves economical in the long term on account of very low maintenance expense.
Amongst the types of glass made in India,
(a) Float glass
(b) Sheet glass
(c) Plate glass
Float glass is a 100% distortion free glass, manufactured indigenously by three companies in India. Float glass is a recent development that produces a fire polished, nearly optically flat surface that does not have to be ground or mechanically polished.
Sheet glass, including the machine drawn cylinder process simulating hand-blown glass, was known for its wavy irregularities.
Plate glass, first introduced into the U.K. in the mid seventeenth century, was cast and then polished flat.
Undergraduate student, Department of Civil Engineering, NSS College of Engineering, Palakkad-08,Kerala
2.0 PROPERTIES OF GLASS
2.1 Transmission Properties
Transmission through glass can be modified by improving or modifying the
Chemical make-up of the glass to increase the absorption factor. Heat absorbing glasses are designed to absorb as much of the solar infrared radiation as possible while maintaining high transmission in the visible region. Some are also designed to absorb
visible radiation to reduce glare, which further reduces heat transmission. However, due to unavoidable impurities in the soda-lime-silica mix, typical window glass does absorb some radiation that might otherwise pass through.
2.2 Material Composition
The main components of clear glass are silica sand (73%), Calcium oxide (9%), Soda (13%) and Magnesium (4%). The amount and type of additional impurities within the glass affect the wavelength of the absorbed light and the amount of impurities determines how much it will be affected. Manufacturers often purposely add their own impurities such as ion oxides to manipulate absorption effects.
2.3 Thermal Conductivity
The thermal conductivity (K) of a soda lime glass between 5 and 7 Btu/hr Sq.ft F/in. is higher than that of insulating materials, but much lower than that of most metals. Glass of normal thickness offers negligible resistance to heat transfer between the inside and outside of a building and the thermal resistance is predominantly due to the surface films.
2.4 Strength of Glass
Glass is a brittle material and does not deform plastically before failure. It fails in tension regardless of the nature of loading. The potential tensile strength of glass is about 690107N/mm2, but failure occurs at average stresses far below this value because of the stress-raising effect of surface imperfections both inherent in the glass and mechanically created. Glass is most vulnerable at its edges, with surface imperfections from cutting and handling adding to the risk of failure. Because the effect of stress raisers is indeterminate, the allowable tensile strength of glass is determined statically and a sizable safety factor included. By using the value thus established breakage can be reduced to an insignificant level but not eliminated. Glass can be greatly strengthened by development of a “stressed skin” sandwich, where both surfaces are in compression & the middle is in tension. This can be accomplished by heating the glass to near its melting point and rapidly cooling both surfaces. The contraction of the middle (of the thickness) of the sheet develops the desired stress on final cooling.
2.5 Thermal Expansion
The co-efficient of expansion for glass is 4.5x10-6. Thermal expansion and contraction of glass is of importance in the design of glazing details, but it is even more important with respect to the development of stresses within the glass from temperature differential in a pane.
3.0 TYPES OF GLASSES
3.1 Safety glass
Safety Glass is defined as glass, which has passed specific impact tests, and must either not break or break in a manner that will not cause injury. There are 3 main types.
i. Toughened Glass
Toughened glass is obtained when a glass is heated to approximately 7000C in a furnace then chilled rapidly by cold air blown onto both its surfaces. This glass is 4 to 5 times stronger than float glass and, if broken disintegrates into small fragments with dulled edges, which are unlikely to cause serious injury, and therefore it is not termed as security glass. This is a safety glass, as it has the highest impact resistance when hit on the surface, although it is vulnerable to immediate cracking when hit on the edge. It also has a very high thermal resistivity, almost 5 times that of a normal annealed glass.
ii. Laminated glass
Laminated glass is obtained when two or more thin sheets of glass separated by layers of a plastic or resin material. The principal benefit of laminated glasses is their performance under impact, the glass may fracture, but any broken fragments will remain firmly bonded to the interlayer.
There are 2 main types.
a) PVB laminated
Two or more sheets of glass are bonded together with one or more layers of polyvinyl butyral, a plastic interlayer in sheet form. The interlayer in laminated glass provides two additional benefits: sound transmittance is reduced, particularly at the higher frequencies and UV radiation is reduced by atleast 97%.
b) Resin laminated
Such glasses are manufactured by pouring liquid resin into a cavity between two sheets of glass, which are held together until the resin cures. They are principally used for decorative and acoustic purposes, where safety performance is normally of secondary importance.
iii. Wired glass
A steel wire mesh is embedded within the glass and is intended to hold it in place if cracked. The glass breaks in almost the same way as ordinary glass, into sharp dagger-like pieces, but mostly stays in place. The visual quality of the glass is impaired by the wire mesh but this has traditionally been accepted in low-cost fire resistant and roof glazing.
3.2 Security glass
There are similar to safety glass, which has passed specific impact tests and must either not break or break in a manner that will cause injury. Laminated glass is the only glass that qualifies as a security glass.
There are 3 main types
i. Anti-bandit glass
Designed to resist manual attack, such as might be made by a man armed with a hammer, crow bar or pickaxe, and to delay access to the protected space for a short period of time.
ii. Bullet resistant glass
A multi-layered variable thickness laminated glass can function as a bullet resistant glass. A laminated glass upto 80mm thickness can resist an AK-47 bullet. A glass laminated under variable thickness specification can function as a bulletproof glass.
iii. Explosion pressure resistant glass
Basically defined as glazing that affords a defined resistance against a specified explosive blast.
3.3 Solar control glass
This type of glass reduces the amount of heat and light from the sun transmitted through the glass. This is usually done by increasing the amount energy either absorbed within the glass reflected from its surface. There are 2 different ways of producing a solar control glass.
i. Body Tinted glass
This is a mild coloured glass, available in blue, bronze and grey colour tints. They are produced by the addition of metal oxides to the raw materials during float glass manufacture. It not only restricts transparency in glass but also most importantly restricts the sunlight glare, and thereby partially insulating heat transmitted via the sunlight.
ii. Coated Glass
Solar control is achieved by applying a very thin transparent reflective metallic coating to the surface of clear or body tinted glasses. There are two different coating techniques:
Pyrolitic coating: This coating reduces the proportion of solar energy transmitted through the glass mainly by increasing the amount reflected.
Sputtered coating: The coating enables the glass to reduce the proportion of solar energy transmitted by increasing the amount absorbed and reflected.
3.4 Low-E coatings
Low-emissivity coatings, called Low-E for short, act to reduce surface emissivity of glass. These glasses provide excellent energy conservation by reflecting majority of the heat and allowing more of visible light thus reducing the heat gain without having to sacrifice much light transmission resulting in dual benefit of less energy requirement to cool the interior of the building as well as less energy for lighting requirement which in turn will help to reduce air-conditioning and lighting cost.
3.5 Spectrally selective glazing
The aim of this glass is to screen out as much ultraviolet and short wave infrared radiation as possible whilst allowing through the maximum amount of visible light. Because this type of glazing can have a virtually clear appearance, they admit more day light and permit much brighter views to outside, whilst still providing order similar to that of dark, reflective tinted glasses
.
4.0 GLASS APPLICATIONS
All the above varieties of processed glasses have ensured safe and functional use of glass in many areas of human life. Amongst them, a few special examples are
4.1Glass skylite/Glass dome
From small to very large spans of ceilings can be covered with glass allowing transmission of natural light permanently. Here, although glass is above our head, there is no fear as these are designed safely. A laminated glass is an ideal concept for glass skylites. While a glass roof does transmit heat faster during the day it also has the benefit of losing heat faster during the night.
4.2 Glass flooring
Today it is possible to walk on glass and be absolutely safe and secure. Proper structural support and adequate cushioning between glasses as well as glass and structure are inevitable. Glasses can be surface treated to prevent skidding. If used carefully, a glass floor does not involve major maintenance.
4.3 Front façade
For every architect or designer, the front facade of any building is very critical. Glass has opened wide avenues, daring opportunities for providing a grand awesome look to the building. Clear view with minimum obstructions, while the inside is cut from outside with regard to noise and pollution, yet due to transparency two worlds are merged.
Toughened or ideally toughened laminated glasses are used here.
4.4 Glass wash basin units
A clean, vibrant, novel, beautiful appearance is derived from a glass washbasin unit.
4.5 Glass furniture
With the advent of skilled workmanship and unique designing techniques, glass can speak different language in versatile styles as an element of furniture. Fusing, blasting, blowing, etching, staining, glue chipping, sticking, colouring, sculpting, chipping are some of the known techniques for beautifying transparent glass into a piece of art. Coffee tables, dining tables, T.V.Stands, corner tables, glass chairs, partitions, lampshades etc. are few of the endless forms of glass interiors.
5.0 GLASS INSTALLATION
There are five main techniques for single glazing.
1. Into rebates without beads
The glass sits in a rebate, held in place by a suitable glazing material, such as putty or a plastic-based compound, which then provides weather proof seal.
2. Into rebates with beads
As before the glass sits in a rebate, but is held in place by the beading. The glazing compound provides the weather seal.
3. Into grooves or channels.
It is usually used for glazing into concrete, store or timber frames with a rebate and bead at the sill, but it can also be modified to suit frames that have beads at the head and sill and grooves at the jambs.
4. Using structural gaskets.
Gaskets are used to hold the glass in a groove or rebate and to provide a weather seal. They can include H-shaped, Y-shaped or single-sided gaskets.
5. Using non-structural gaskets
The glass is held in place by a combination of pressure beads and gaskets.
Check that the clearance is appropriate for the thickness and type of glass. If the glass is in direct contact with the framing materials, it could lead to breakage.
Frameless glass glazing
The use of glass without using wooden or aluminum frames has become a necessity in the present and future concept with the change in the methods of use of thicken float glass in spacious buildings like airports, seaports, multistory buildings, banks, showrooms and spacious offices etc. It is therefore important that the development of such a glass glazing should be accurate and well designed, in accordance with the requirement of the situation and the architectural layout. It is also an important factor that such glass glazing should be fully supported by the hardware used in it with a capacity to sustain wind pressure and structural strength safety based on the necessary material and technique.
6.0 MAINTENANCE OF GLASS
Site Clean up
It is essential that all external glass be thoroughly washed with clean water to eliminate all abrasive and chemical laden dust. Excess glazing compounds and sealants should be carefully removed from the glass and frame surrounds, taking care not to scratch the finished surfaces with tools or abrasives. A solvent such as white spirit or professional glass cleaner may be used to remove any glazing compound, finger marks or grease. Frequent washing is required whilst construction continues on site, since chemicals in dust and particularly in cement may be activated by rain and cause permanent corrosion of the glass surface.
Normal Cleaning
Use a mild liquid detergent solution, then rinse the glass well with clean water and dry off. Under no circumstances may abrasive cleaning products or contaminated cloths be used. If rainwater coming from cement mortar contaminates the surface of the glass, frequent cleaning is necessary to prevent permanent staining.
Regular maintenance
It is essential that all installations are inspected and maintained during the lifetime of the building at regular intervals as recommended by the sealant and framing manufacturers.
7.0 PROTECTION OF GLASS
Glass can be a dangerous material .When standard annealed glass breaks, it forms potentially lethal shards and splinters. Glass manufactures have developed a range of safety glasses adding strength and integrity to this beautiful building material and allowing glass to be used in areas where safety is critical in unprecedented situation. Safety glass is defined as glass, which must have passed an impact test (as per BS 6206: 1981). There are 3 levels of impact: C, B and A, ‘A’ being the highest. Each involves the glass being impacted by a leather bag containing 45kg of lead shot.
Class A - 1219 mm
Class B - 457 mm
Class C - 305 mm
All security glasses automatically qualify a class A safety glasses.
Glass in doors and side panels to doors must be at least:
# a class B safety glazing material if the smaller dimension of the glass is more
than 900 mm.
# a class C safety glazing material if the smaller dimension of the glass in less than 900 mm.
Non safety glass in small panes may be permitted under certain controlled circumstances.
7.1 Permanent screen protection
The use of annealed glass is permitted in a critical location if protected by a permanent, robust screen. The screen must prevent the passage of a 75mm diameter sphere and must not be climbable.
7.2 Bathing areas and areas of special risk
BS 6262 requires that any glazing, forming part of a bath or shower screen or adjacent to or surrounding a bathing area, swimming pool or other wet areas must be at least a class C material. Consideration should also be given to the breakage characteristics. The fragmentation of toughened glass into small dice-like particles would result in, should it enter the pool, being invisible practically impossible to remove, practically causing damage to pumps filters. This can be avoided by using a laminated safety glass an account of the glass fragments remaining adhered to the plastic interlayer. This requirement also applies to all glazing in areas of special risk such as gymnasia and other places of energetic activity. In such areas, the designer must consider whether a higher class is required, or if additional safeguards such as protective rails or screens, or manifestation are necessary.
7.3 Commercial frontage
Robust glass (non-safety thick annealed glass) when fully framed is considered suitable for use in large areas in non-domestic applications, for eg: forming fronts to shops, showrooms, offices, factories, and public buildings.
Glass thickness / size limits for annealed glass that may be used in these locations are shown in the Table 1. Wind loads & other load must be considered when selecting the glass thickness.
Table1 Glass thickness and pane size
Normal Glass Maximum pane size
thickness (mm) (four edge supported )(mm)
8 1100 x 1100
10 2250 x 2250
12 4500 x 4500
15 or thicker No limits
7.4 Glass in furniture
Standard advises minimum BS 6206 classification, glass thickness and support details to insure the reasonable safety of flat glass having a total area of at least 0.02m2.
1. Glass in tables or trolleys
Glass that is not supported over its entire area:
The minimum horizontal area of each support shall be 36mm2. Annealed glass shall be supported for not less than 50% of its total perimeter and the support should be in at least two non-adjacent regions and shall be not more than 100mm from the edge of glass. Minimum BS 6206 classification and nominal thickness for glass that is not supported over its entire area as shown in table 2.
Table 2 Minimum BS 6206 classification and nominal thickness
Area of Nominal Thickness Minimum Nominal thickness
glass (m2) (mm) BS 6206 (mm)
annealed glass classification
Toughened glass Laminated glass
<= 0.25 >=10 Class C >=4.0 >=6.4
< 0.25 to >=10 Class C >=5.0 >=6.4
<=0.5
> 0.5 to >=12 Class C >=6.0 >=6.4
<=0.75
> 0.75 to >= 15 Class B >=8.0 >=8.4
< =1.5
>1.5 >=19 Class A >=10.0 >=10.8
Glass that is supported over its entire area.:
Glass which has an area not greater than 1.5m2 shall comply with the relevant nominal thickness given in table3 below.
Table 3 Nominal thickness of glass
Area of glass (m2) Nominal thickness (m)
Annealed
Toughened glass
Laminated glass
<=0.5 >=4.0 >=4.0 >=4.4
>=0.5 to <=1.0 >=5.0 >=4.0 >=4.4
>=1.0 to <=1.5 >=6.0 >=4.0 >=4.4
>1.5 N/A >=4.0 >=4.4
Contact of glass with other materials:
Hard materials such as other glass, metal or stone should not be allowed to come into direct contact with the edges or surface of the glass. Separation should be ensured by the use of suitable bushes and gaskets. Glass in furniture other than tables or trolleys. Horizontal glass supported over its entire area should comply with Table 3. Glass used to form the external surfaces (excluding horizontal glass supported over its entire area) and which is unbaked should comply with the requirements of table 4.
Table 4 Requirements of glass for external surfaces and unbaked conditions
Smaller dimension (width or height)
Less than 900mm
BS 6206 Class C
Minimum Thickness More than 900mm
BS 6206 Class B
Minimum Thickness
3 mm Fully framed 4 mm
4 mm Partially framed or unframed 6 mm
For sliding door and fixed glass retained in a rebate or groove, the edge cover provided by the rebate or groove shall be at least 4 mm.
7.5 Glass shelves
Glass shelves that are not fully enclosed in a cabinet shall be a class C safety glass to BS 6206 as a minimum. This requirement is most easily met by using toughened glass. Annealed glass is acceptable for use as shelves only when fully enclosed within a cabinet. The maximum evenly distributed safe load that a shelf can support is dependent on glass type, thickness, width and span of the glass between supports.
8.0 DEVELOPMENTS IN SAFETY GLAZING
8.1 Fire resistant glass products
Acceptable glazing products for fire – rated, hazardous locations include transparent glass ceramic and in tumescent glass. Glass ceramic are single glazed products that meet both impact resistance requirements and provide up to a 3-hour fire resistance rating.
In tumescent glass are double – glazed products with an inner layer of transparent sodium silicate that when exposed to high temperature, turns opaque and forms an insulating layer that can provide up to a two hour fire resistance rating. Unlike glass – ceramic, in tumescent glass becomes insulating when exposed to fire. It is also more expensive than glass ceramic.
8.2 Blast resistant glazing
When a building is subjected to bombing it is noted that 60% of the non-lethal injuries sustained in that event were due to shattered glass, shattered glass was found as far as 1 mile from the blast site.
The GSA standard defines performance condition based on the response of glazing to a specified blast event, and the level of protection provided by glazing system as shown in the table below.
Table 5 The level of protection provided by glazing system
Performance Criteria Protection Level Hazard Level Description of Window Glazing Response
1
Safe
None Glazing does not break.No visible damage to glazing or frame.
2 Very high None Glazing cracks but is retained by the frame. Dusting or verysmall fragments near sill or on floor acceptable.
3a High Very low Glazing cracks. Fragments enter space and land on the floor no further than 0.99m from the window.
3b High Low Glazing cracks. Fragments enter space and land on floor no further than 3m from the window.
4 Medium Medium Glazing cracks. Fragments enter space and land on the floor and impact a vertical witness panel at a distance of no more than 3m from the window at a height no greater than 0.6m above the floor.
5 Low High Glazing cracks and window system fails catastrophically. Fragments enter space impacting a vertical witness panel at a distance of no more than 3m from window at a height greater than 0.6m above floor.
9.0 CONCLUSION
1. Structural glass tends to influence modern architecture more and more. It defines not only modernity, but also value, richness and future technology.
2. There have been thousands of other developments in glass manufacture that make it one of the most useful and versatile materials available.
Modern glass is not only a transparent beauty, but also has unmatched utility. Today, glass is an important building element, which not only offers aesthetics but also proves economical in the long term on account of very low maintenance expense.
Amongst the types of glass made in India,
(a) Float glass
(b) Sheet glass
(c) Plate glass
Float glass is a 100% distortion free glass, manufactured indigenously by three companies in India. Float glass is a recent development that produces a fire polished, nearly optically flat surface that does not have to be ground or mechanically polished.
Sheet glass, including the machine drawn cylinder process simulating hand-blown glass, was known for its wavy irregularities.
Plate glass, first introduced into the U.K. in the mid seventeenth century, was cast and then polished flat.
Undergraduate student, Department of Civil Engineering, NSS College of Engineering, Palakkad-08,Kerala
2.0 PROPERTIES OF GLASS
2.1 Transmission Properties
Transmission through glass can be modified by improving or modifying the
Chemical make-up of the glass to increase the absorption factor. Heat absorbing glasses are designed to absorb as much of the solar infrared radiation as possible while maintaining high transmission in the visible region. Some are also designed to absorb
visible radiation to reduce glare, which further reduces heat transmission. However, due to unavoidable impurities in the soda-lime-silica mix, typical window glass does absorb some radiation that might otherwise pass through.
2.2 Material Composition
The main components of clear glass are silica sand (73%), Calcium oxide (9%), Soda (13%) and Magnesium (4%). The amount and type of additional impurities within the glass affect the wavelength of the absorbed light and the amount of impurities determines how much it will be affected. Manufacturers often purposely add their own impurities such as ion oxides to manipulate absorption effects.
2.3 Thermal Conductivity
The thermal conductivity (K) of a soda lime glass between 5 and 7 Btu/hr Sq.ft F/in. is higher than that of insulating materials, but much lower than that of most metals. Glass of normal thickness offers negligible resistance to heat transfer between the inside and outside of a building and the thermal resistance is predominantly due to the surface films.
2.4 Strength of Glass
Glass is a brittle material and does not deform plastically before failure. It fails in tension regardless of the nature of loading. The potential tensile strength of glass is about 690107N/mm2, but failure occurs at average stresses far below this value because of the stress-raising effect of surface imperfections both inherent in the glass and mechanically created. Glass is most vulnerable at its edges, with surface imperfections from cutting and handling adding to the risk of failure. Because the effect of stress raisers is indeterminate, the allowable tensile strength of glass is determined statically and a sizable safety factor included. By using the value thus established breakage can be reduced to an insignificant level but not eliminated. Glass can be greatly strengthened by development of a “stressed skin” sandwich, where both surfaces are in compression & the middle is in tension. This can be accomplished by heating the glass to near its melting point and rapidly cooling both surfaces. The contraction of the middle (of the thickness) of the sheet develops the desired stress on final cooling.
2.5 Thermal Expansion
The co-efficient of expansion for glass is 4.5x10-6. Thermal expansion and contraction of glass is of importance in the design of glazing details, but it is even more important with respect to the development of stresses within the glass from temperature differential in a pane.
3.0 TYPES OF GLASSES
3.1 Safety glass
Safety Glass is defined as glass, which has passed specific impact tests, and must either not break or break in a manner that will not cause injury. There are 3 main types.
i. Toughened Glass
Toughened glass is obtained when a glass is heated to approximately 7000C in a furnace then chilled rapidly by cold air blown onto both its surfaces. This glass is 4 to 5 times stronger than float glass and, if broken disintegrates into small fragments with dulled edges, which are unlikely to cause serious injury, and therefore it is not termed as security glass. This is a safety glass, as it has the highest impact resistance when hit on the surface, although it is vulnerable to immediate cracking when hit on the edge. It also has a very high thermal resistivity, almost 5 times that of a normal annealed glass.
ii. Laminated glass
Laminated glass is obtained when two or more thin sheets of glass separated by layers of a plastic or resin material. The principal benefit of laminated glasses is their performance under impact, the glass may fracture, but any broken fragments will remain firmly bonded to the interlayer.
There are 2 main types.
a) PVB laminated
Two or more sheets of glass are bonded together with one or more layers of polyvinyl butyral, a plastic interlayer in sheet form. The interlayer in laminated glass provides two additional benefits: sound transmittance is reduced, particularly at the higher frequencies and UV radiation is reduced by atleast 97%.
b) Resin laminated
Such glasses are manufactured by pouring liquid resin into a cavity between two sheets of glass, which are held together until the resin cures. They are principally used for decorative and acoustic purposes, where safety performance is normally of secondary importance.
iii. Wired glass
A steel wire mesh is embedded within the glass and is intended to hold it in place if cracked. The glass breaks in almost the same way as ordinary glass, into sharp dagger-like pieces, but mostly stays in place. The visual quality of the glass is impaired by the wire mesh but this has traditionally been accepted in low-cost fire resistant and roof glazing.
3.2 Security glass
There are similar to safety glass, which has passed specific impact tests and must either not break or break in a manner that will cause injury. Laminated glass is the only glass that qualifies as a security glass.
There are 3 main types
i. Anti-bandit glass
Designed to resist manual attack, such as might be made by a man armed with a hammer, crow bar or pickaxe, and to delay access to the protected space for a short period of time.
ii. Bullet resistant glass
A multi-layered variable thickness laminated glass can function as a bullet resistant glass. A laminated glass upto 80mm thickness can resist an AK-47 bullet. A glass laminated under variable thickness specification can function as a bulletproof glass.
iii. Explosion pressure resistant glass
Basically defined as glazing that affords a defined resistance against a specified explosive blast.
3.3 Solar control glass
This type of glass reduces the amount of heat and light from the sun transmitted through the glass. This is usually done by increasing the amount energy either absorbed within the glass reflected from its surface. There are 2 different ways of producing a solar control glass.
i. Body Tinted glass
This is a mild coloured glass, available in blue, bronze and grey colour tints. They are produced by the addition of metal oxides to the raw materials during float glass manufacture. It not only restricts transparency in glass but also most importantly restricts the sunlight glare, and thereby partially insulating heat transmitted via the sunlight.
ii. Coated Glass
Solar control is achieved by applying a very thin transparent reflective metallic coating to the surface of clear or body tinted glasses. There are two different coating techniques:
Pyrolitic coating: This coating reduces the proportion of solar energy transmitted through the glass mainly by increasing the amount reflected.
Sputtered coating: The coating enables the glass to reduce the proportion of solar energy transmitted by increasing the amount absorbed and reflected.
3.4 Low-E coatings
Low-emissivity coatings, called Low-E for short, act to reduce surface emissivity of glass. These glasses provide excellent energy conservation by reflecting majority of the heat and allowing more of visible light thus reducing the heat gain without having to sacrifice much light transmission resulting in dual benefit of less energy requirement to cool the interior of the building as well as less energy for lighting requirement which in turn will help to reduce air-conditioning and lighting cost.
3.5 Spectrally selective glazing
The aim of this glass is to screen out as much ultraviolet and short wave infrared radiation as possible whilst allowing through the maximum amount of visible light. Because this type of glazing can have a virtually clear appearance, they admit more day light and permit much brighter views to outside, whilst still providing order similar to that of dark, reflective tinted glasses
.
4.0 GLASS APPLICATIONS
All the above varieties of processed glasses have ensured safe and functional use of glass in many areas of human life. Amongst them, a few special examples are
4.1Glass skylite/Glass dome
From small to very large spans of ceilings can be covered with glass allowing transmission of natural light permanently. Here, although glass is above our head, there is no fear as these are designed safely. A laminated glass is an ideal concept for glass skylites. While a glass roof does transmit heat faster during the day it also has the benefit of losing heat faster during the night.
4.2 Glass flooring
Today it is possible to walk on glass and be absolutely safe and secure. Proper structural support and adequate cushioning between glasses as well as glass and structure are inevitable. Glasses can be surface treated to prevent skidding. If used carefully, a glass floor does not involve major maintenance.
4.3 Front façade
For every architect or designer, the front facade of any building is very critical. Glass has opened wide avenues, daring opportunities for providing a grand awesome look to the building. Clear view with minimum obstructions, while the inside is cut from outside with regard to noise and pollution, yet due to transparency two worlds are merged.
Toughened or ideally toughened laminated glasses are used here.
4.4 Glass wash basin units
A clean, vibrant, novel, beautiful appearance is derived from a glass washbasin unit.
4.5 Glass furniture
With the advent of skilled workmanship and unique designing techniques, glass can speak different language in versatile styles as an element of furniture. Fusing, blasting, blowing, etching, staining, glue chipping, sticking, colouring, sculpting, chipping are some of the known techniques for beautifying transparent glass into a piece of art. Coffee tables, dining tables, T.V.Stands, corner tables, glass chairs, partitions, lampshades etc. are few of the endless forms of glass interiors.
5.0 GLASS INSTALLATION
There are five main techniques for single glazing.
1. Into rebates without beads
The glass sits in a rebate, held in place by a suitable glazing material, such as putty or a plastic-based compound, which then provides weather proof seal.
2. Into rebates with beads
As before the glass sits in a rebate, but is held in place by the beading. The glazing compound provides the weather seal.
3. Into grooves or channels.
It is usually used for glazing into concrete, store or timber frames with a rebate and bead at the sill, but it can also be modified to suit frames that have beads at the head and sill and grooves at the jambs.
4. Using structural gaskets.
Gaskets are used to hold the glass in a groove or rebate and to provide a weather seal. They can include H-shaped, Y-shaped or single-sided gaskets.
5. Using non-structural gaskets
The glass is held in place by a combination of pressure beads and gaskets.
Check that the clearance is appropriate for the thickness and type of glass. If the glass is in direct contact with the framing materials, it could lead to breakage.
Frameless glass glazing
The use of glass without using wooden or aluminum frames has become a necessity in the present and future concept with the change in the methods of use of thicken float glass in spacious buildings like airports, seaports, multistory buildings, banks, showrooms and spacious offices etc. It is therefore important that the development of such a glass glazing should be accurate and well designed, in accordance with the requirement of the situation and the architectural layout. It is also an important factor that such glass glazing should be fully supported by the hardware used in it with a capacity to sustain wind pressure and structural strength safety based on the necessary material and technique.
6.0 MAINTENANCE OF GLASS
Site Clean up
It is essential that all external glass be thoroughly washed with clean water to eliminate all abrasive and chemical laden dust. Excess glazing compounds and sealants should be carefully removed from the glass and frame surrounds, taking care not to scratch the finished surfaces with tools or abrasives. A solvent such as white spirit or professional glass cleaner may be used to remove any glazing compound, finger marks or grease. Frequent washing is required whilst construction continues on site, since chemicals in dust and particularly in cement may be activated by rain and cause permanent corrosion of the glass surface.
Normal Cleaning
Use a mild liquid detergent solution, then rinse the glass well with clean water and dry off. Under no circumstances may abrasive cleaning products or contaminated cloths be used. If rainwater coming from cement mortar contaminates the surface of the glass, frequent cleaning is necessary to prevent permanent staining.
Regular maintenance
It is essential that all installations are inspected and maintained during the lifetime of the building at regular intervals as recommended by the sealant and framing manufacturers.
7.0 PROTECTION OF GLASS
Glass can be a dangerous material .When standard annealed glass breaks, it forms potentially lethal shards and splinters. Glass manufactures have developed a range of safety glasses adding strength and integrity to this beautiful building material and allowing glass to be used in areas where safety is critical in unprecedented situation. Safety glass is defined as glass, which must have passed an impact test (as per BS 6206: 1981). There are 3 levels of impact: C, B and A, ‘A’ being the highest. Each involves the glass being impacted by a leather bag containing 45kg of lead shot.
Class A - 1219 mm
Class B - 457 mm
Class C - 305 mm
All security glasses automatically qualify a class A safety glasses.
Glass in doors and side panels to doors must be at least:
# a class B safety glazing material if the smaller dimension of the glass is more
than 900 mm.
# a class C safety glazing material if the smaller dimension of the glass in less than 900 mm.
Non safety glass in small panes may be permitted under certain controlled circumstances.
7.1 Permanent screen protection
The use of annealed glass is permitted in a critical location if protected by a permanent, robust screen. The screen must prevent the passage of a 75mm diameter sphere and must not be climbable.
7.2 Bathing areas and areas of special risk
BS 6262 requires that any glazing, forming part of a bath or shower screen or adjacent to or surrounding a bathing area, swimming pool or other wet areas must be at least a class C material. Consideration should also be given to the breakage characteristics. The fragmentation of toughened glass into small dice-like particles would result in, should it enter the pool, being invisible practically impossible to remove, practically causing damage to pumps filters. This can be avoided by using a laminated safety glass an account of the glass fragments remaining adhered to the plastic interlayer. This requirement also applies to all glazing in areas of special risk such as gymnasia and other places of energetic activity. In such areas, the designer must consider whether a higher class is required, or if additional safeguards such as protective rails or screens, or manifestation are necessary.
7.3 Commercial frontage
Robust glass (non-safety thick annealed glass) when fully framed is considered suitable for use in large areas in non-domestic applications, for eg: forming fronts to shops, showrooms, offices, factories, and public buildings.
Glass thickness / size limits for annealed glass that may be used in these locations are shown in the Table 1. Wind loads & other load must be considered when selecting the glass thickness.
Table1 Glass thickness and pane size
Normal Glass Maximum pane size
thickness (mm) (four edge supported )(mm)
8 1100 x 1100
10 2250 x 2250
12 4500 x 4500
15 or thicker No limits
7.4 Glass in furniture
Standard advises minimum BS 6206 classification, glass thickness and support details to insure the reasonable safety of flat glass having a total area of at least 0.02m2.
1. Glass in tables or trolleys
Glass that is not supported over its entire area:
The minimum horizontal area of each support shall be 36mm2. Annealed glass shall be supported for not less than 50% of its total perimeter and the support should be in at least two non-adjacent regions and shall be not more than 100mm from the edge of glass. Minimum BS 6206 classification and nominal thickness for glass that is not supported over its entire area as shown in table 2.
Table 2 Minimum BS 6206 classification and nominal thickness
Area of Nominal Thickness Minimum Nominal thickness
glass (m2) (mm) BS 6206 (mm)
annealed glass classification
Toughened glass Laminated glass
<= 0.25 >=10 Class C >=4.0 >=6.4
< 0.25 to >=10 Class C >=5.0 >=6.4
<=0.5
> 0.5 to >=12 Class C >=6.0 >=6.4
<=0.75
> 0.75 to >= 15 Class B >=8.0 >=8.4
< =1.5
>1.5 >=19 Class A >=10.0 >=10.8
Glass that is supported over its entire area.:
Glass which has an area not greater than 1.5m2 shall comply with the relevant nominal thickness given in table3 below.
Table 3 Nominal thickness of glass
Area of glass (m2) Nominal thickness (m)
Annealed
Toughened glass
Laminated glass
<=0.5 >=4.0 >=4.0 >=4.4
>=0.5 to <=1.0 >=5.0 >=4.0 >=4.4
>=1.0 to <=1.5 >=6.0 >=4.0 >=4.4
>1.5 N/A >=4.0 >=4.4
Contact of glass with other materials:
Hard materials such as other glass, metal or stone should not be allowed to come into direct contact with the edges or surface of the glass. Separation should be ensured by the use of suitable bushes and gaskets. Glass in furniture other than tables or trolleys. Horizontal glass supported over its entire area should comply with Table 3. Glass used to form the external surfaces (excluding horizontal glass supported over its entire area) and which is unbaked should comply with the requirements of table 4.
Table 4 Requirements of glass for external surfaces and unbaked conditions
Smaller dimension (width or height)
Less than 900mm
BS 6206 Class C
Minimum Thickness More than 900mm
BS 6206 Class B
Minimum Thickness
3 mm Fully framed 4 mm
4 mm Partially framed or unframed 6 mm
For sliding door and fixed glass retained in a rebate or groove, the edge cover provided by the rebate or groove shall be at least 4 mm.
7.5 Glass shelves
Glass shelves that are not fully enclosed in a cabinet shall be a class C safety glass to BS 6206 as a minimum. This requirement is most easily met by using toughened glass. Annealed glass is acceptable for use as shelves only when fully enclosed within a cabinet. The maximum evenly distributed safe load that a shelf can support is dependent on glass type, thickness, width and span of the glass between supports.
8.0 DEVELOPMENTS IN SAFETY GLAZING
8.1 Fire resistant glass products
Acceptable glazing products for fire – rated, hazardous locations include transparent glass ceramic and in tumescent glass. Glass ceramic are single glazed products that meet both impact resistance requirements and provide up to a 3-hour fire resistance rating.
In tumescent glass are double – glazed products with an inner layer of transparent sodium silicate that when exposed to high temperature, turns opaque and forms an insulating layer that can provide up to a two hour fire resistance rating. Unlike glass – ceramic, in tumescent glass becomes insulating when exposed to fire. It is also more expensive than glass ceramic.
8.2 Blast resistant glazing
When a building is subjected to bombing it is noted that 60% of the non-lethal injuries sustained in that event were due to shattered glass, shattered glass was found as far as 1 mile from the blast site.
The GSA standard defines performance condition based on the response of glazing to a specified blast event, and the level of protection provided by glazing system as shown in the table below.
Table 5 The level of protection provided by glazing system
Performance Criteria Protection Level Hazard Level Description of Window Glazing Response
1
Safe
None Glazing does not break.No visible damage to glazing or frame.
2 Very high None Glazing cracks but is retained by the frame. Dusting or verysmall fragments near sill or on floor acceptable.
3a High Very low Glazing cracks. Fragments enter space and land on the floor no further than 0.99m from the window.
3b High Low Glazing cracks. Fragments enter space and land on floor no further than 3m from the window.
4 Medium Medium Glazing cracks. Fragments enter space and land on the floor and impact a vertical witness panel at a distance of no more than 3m from the window at a height no greater than 0.6m above the floor.
5 Low High Glazing cracks and window system fails catastrophically. Fragments enter space impacting a vertical witness panel at a distance of no more than 3m from window at a height greater than 0.6m above floor.
9.0 CONCLUSION
1. Structural glass tends to influence modern architecture more and more. It defines not only modernity, but also value, richness and future technology.
2. There have been thousands of other developments in glass manufacture that make it one of the most useful and versatile materials available.
Nature and Behaviour of Soft Clays and their improvement
The growth of Indian port city’s over the last few decades has been phenomenal
and Cochin is no exception with the increase of population, housing and construction of
various facilities have been a problem with urbanization. Having exhausted all the trouble
free hand, man is now on the look out for techniques to improve areas which were
originally considered uninhabitable. Thus studies on the nature and engineering behavior
of
soft clay’s covering long stretches of coastal line and methods to improve there geotechnical properties have been of great relevance, and is explained in the following pages.
2. REVIEW OF COCHIN MARINE CLAY
According to King (1884), 2500 years back, the sea washed up to the high ranges
of Western Ghats. The land between the Western Ghats and the present coastal line was
once well above the sea and was subsequently submerged. It was uplifted again by volcanic
action and again partially covered by sea water. The uplifted area includes the coastal belt
(10 to 30Km wide and 150 Km long) starting from Crangannore at the north to Quilon
at the south. Most of the Cochin city is situated in this belt.
3. PHYSICAL PROPERTIES OF COCHIN MARINE CLAY
Marine clay is formed by the sedimentation of clay soils in marine environments
exhibit unusual physical properties. They posses very high liquid limit and their natural
water content is close to the liquid limit values. The most striking feature of the physical
properties is the phenomenal changes that are caused by drying of sample. Some physical
properties of the Cochin marine clay is shown in the following table.
PHYSICAL PROPERTIES OF COCHIN MARINE CLAY
Sl No Physical properties (%) Moist soil Air dried soil Oven dried soil
1 Liquid limit 131.5 94.5 62.0
2 Plastic limit 53.9 45.2 40.5
3 Plasticity index 77.6 49.3 21.5
4 Shrinkage limit 18.1 18.8 19.2
5 Grain size distribution
(a) Clay
(b) Silt
(c) sand
48
34
18
33
47
20
13
55
32
4. SHEAR STRENGTH
In greater Cochin area the layers of shallow depth have their natural moisture
content quite close to their liquid limit. The clay deposits below vary widely in consisting up
to 40 to 50m where good load bearing stratum can be met with.
The shear strength of clayey soils is dependent on its water content. Any
increase in water content is necessarily accompanied by the reduction in cohesion or shear
strength. But at the same water content, different soils have different shear strength. From
the test results that were conducted at different places in Cochin, it was found that, the shear strength is very low and it is around 0.01Kg/cm² and 0.02Kg/cm².
5. CONSOLIDATION CHARACTERISTICS OF COCHIN
MARINE CLAY
Several series of consolidation tests were carried out on samples of Cochin arine clay varying different parameters like duration of loading, effect of pore fluid, effect
of washing. In the consolidation test which served as the reference test. It was found that
about 2 days were required for a complete dissipation of pore pressure and for reaching
an equilibrium void ratio for a particular loading stage. The clay deposits are highly
compressive with Cc value around 1.5 and natural moisture content
very close to liquid limit. It has been observed that almost all the structures, founded on
spread footings or raft footings invariably show actual settlement far below the computed
values.
6. PROBLEMS DUE TO THICK MARINE CLAY
Cochin City and the surroundings are covered with thick marine clay deposit
extending to a large depth (30 to 40m) below the ground level. Due to the presence of this
soft clay layer, following problems are observed.
Low bearing capacity
Higher settlement
High seepage losses
Liquefaction during earthquake
Instability of foundation excavations
Higher earth pressure on retaining structures
IS: 1904(1966) permits a maximum settlement of 40mm for isolated foundations on
sand and 65mmfor those for clay. The allowable settlement is higher for clay because
progressive settlement on clayey soils permits better strain adjustments in the structural
members. The maximum settlement for raft foundations on sand is 40mm to 65mm and that
on clay is 65mm to 100mm.
For typical industry following issues are to be addressed:
Large area is covered with thick vegetation. If the area is reclaimed without removal of the vegetation. It will result in a fill with high organic content and high compressibility. On the other hand, if the vegetation is to be removed, it requires dewatering of the area so that powered vehicle can operate on the surface and clear slush and vegetation before selected soil is placed for reclamation.
While reclaiming the area, particularly for industry requiring very large area, for example refinery, oil terminal, gas bottling plant, etc. the drainage pattern of the area needs to be planned carefully. The flow surface water which was taking place through the area prior to reclamation has to be diversified through special drains/culverts such a way that it does not create any inundation.
The area avail is low-laying and often inundated with water. Unless the area is raised by soil fill (3 to 4m thick) it is difficult to carryout any construction.
In many projects, the height of the fill required is 2 to 3m. This will require boundary wall which has a high retaining wall plus a compound wall. In the absence of retaining wall the fill material can encroach in the neighboring areas beyond the property line which may not be acceptable in many cases. Alternately, fill is placed with toe of the slop within the property line .in this case, large area with a width of 6 to 7m all along the periphery of the area remain unutilized.
The placement of fill can lead to the following foundation problems.
Negative drag force on pile foundation.
Lateral flow under surface loading leading to settlement and lateral force on the adjacent foundations.
Deep-seated slop failure for construction of embankment and heavy surface loading.
Difficulty in executing foundations requiring deep excavation.
7. GROUND IMPROVEMENT
The clay deposit also needs to be improved to meet the foundation requirements. In early days, areas having clay deposit was avoided for construction. But with scarcity of land in urban areas, we do not have choice and structures have to be built on weak deposits. Pile foundation is of course a possible approach as it by pass the weak deposit and transfer the load on next component layer. Where thickness is very large, pile foundation is uneconomical and time consuming.
The methods of ground improvement that can be adopted depend basically on nature of strata and purpose of improvement. The methods available are as follows.
8 METHODS OF GROUND IMPROVEMENT
8.1 VERTICAL DRAINS
This method is suitable for deep deposit of soft clay. The natural moisture content in this strata can brought down substantially by installing vertical drains with preloading. The pressures of vertical drains reduce the drainage path of water in the pores of the soil and thereby reduce the time required for consolidation. The spacing of drains depends upon the speed at which required improvement is to be achieved.
In earlier days, such vertical drains were installed by driving a close ended steep pipe o 100 to 200mm diameter up to the full thickness of such clay deposit. The pipe is then filled with sand and withdrawn in stage to form a vertical drain. The pipe is generally refused for installing other drains.
In the recent past, there has been number of different materials developed to replace the sand drains. These are basically plastic sections having thickness varying from 5 to 10mm and width from 100 to 150mm. The sections have channels to permit flow of water. The perimeter of the section is covered with a layer of geo-textile to prevent the entry of soil particles in to the channel. The advantages of such drain are that it results in minimum remoulding of surrounding soil during installation. The process of installation is very fast. The machine is mounted on a crane and typical drain up to 10m depth can be installed in the period of varying 1 to2 minutes including the time for shifting the machine to a new location.
The percentage of consolidation, which can be achieved by such vertical drains, can be theoretically predicted from the design charts developed based on the theory of three-dimensional consolidation. Typically, a deposit, which requires a period of over ten years for 95% consolidation, can complete the same consolidation within a short period of 3 to 6 months when vertical drains are installed.
After the drains are installed, the magnitude of preload to be placed on the required shear strength of the layer after improvement. Depending on the time available for improvement, the degree of consolidation is worked out and the effective over burden pressure (p) is computed. From the plasticity index I\of the soil, the ratio Su/p is determined. The ratios normally remain constant and therefore with increase in value of p the shear strength after treatment increases.
8.2 IN-SITU DEEP MIXING
In- site deep mixing using hydraulically operated helical augers penetrate the ground to the required depth. The hollow stem of the auger is used to inject cement / lime / any other stabilizing compound into the ground. A pair of 2 or 3 augers is operated simultaneously. After injecting the stabilizer in to the ground, the augers are rotated such that the soil along with the stabilizer is churned in-situ and mixed the stabilizer thoroughly with the soil without necessity of taking out the soil. The auger configuration can be chosen such that either a stabilized wall can be formed (to act as a barrier) or the entire area can be stabilized.
8.3 STONE COLUMN
Stone columns are cylindrical columns made below ground level which comprises of granular material of size varying from 25 to 100mm. The hole is made in the soft clay deposit by different techniques and then filled with stones in layer and compacted to form the complete column. When the structure is placed over the area treated by stone column, majority of the load (80 to 90%) is transmitted to the stone columns because of their higher stiffness. Balance 10 to 20%of the load is taken by the clay deposit. With the help of this 10%of surcharge load, the soft clay able to provide adequate confinement to the cylindrical column.
The area treated stone columns can be used to support only flexible structures such as embankment, oil storage tank, etc. because the settlement even after treatment with stone column can be large (50 to200mm). Without stone column the settlement could have been 3 to 4times higher and also the bearing capacity would have been much less.
8.4 VACUUM DEWATERING
In this technique also the layer is provided with vertical drains as mentioned above. The surface is then covered with a layer of a sand blanket 150 to 300mm thick and covered with a thin plastic sheet which is suitably anchored along the perimeter. Vacuum is then applied with a sand blanket which creates a suction pressure up to 50 to60mm height of mercury and forced the pore water out from the deposit through vertical drains. The area is also subjected to atmospheric pressure which then acts as preload. The vacuum pressure is maintained till the required improvement is achieved.
8.5 VIBROFLOTATION
Vibroflotation involves the use of a vibrating probe that can penetrate granular soil to depths of over 100feet. The vibrations of the probe cause the grain structure to collapse thereby densifying the soil surrounding the probe. To treat an area of potentially liquefiable soil, the vibroflot is raised and lowered in a grid pattern. Vibro replacement is a combination of vibroflotation with a gravel backfill resulting in stone columns, which not only increase the amount of densification, but provides a degree of reinforcement and potentially effective means of drainage.
Figure:1
VIBROFLOTATION.
SCHEMATIC DIAGRAM:1
8.6 DYNAMIC COMPACTION
In dynamic compaction, a heavy weight of 15 to 20ton is raised to a height about 20m above the ground level and freely dropped on the ground surface. The impact on the ground surface transmits compression waves, which spreads radially from the point of impact. The compression waves travels fast and creates Liquefaction of the strata, which is saturated. It is followed by shear waves reorients the soil particle which are liquefied state to denser state and compacts the configuration. The spacing between the treating point, number of impact at each point, etc. can be varied depending on the required improvement.
Figure : 2
DYNAMIC COMPACTION.
8.7 COMPACTION BY DEEP BLASTING
In this technique, a borehole is made to a depth approximately 3/4 or 2/3 of the total depth. Suitable explosive is lowered to the bottom of the borehole and the space above is back filled with compacted soil (known as stemming). The explosive is attached with cordex, which is taken up to the surface and connected to electric detonator. When the explosive is detonated, it creates a cavity, which expands instantaneously. The cavity expansion generates shock waves. Propagation of these waves densifies the soil to certain extent. After blast, a large conical depression is observed at the place of blast. The water within the pores of the deposit gets expelled out and comes up to the ground and form a large pool in the cone of depression.
The quantity of charge and spacing between the explosive can be suitably selected to achieve the required densification. As a thumb rule, about 15 to 45 grams of explosive is required for densification of every 3m of the deposit. Initial loose deposit which has relative density of about 50% and susceptible to liquefaction can be densified to a relative density of over 60% which is adequate to prevent liquefaction. The vibrations created during the blast are within permissible limits for construction purpose.
8.8 COMPACTION GROUTING
Compaction grouting is a technique where by a slow-flowing water /sand /cement mix is injected under pressure into a granular soil. The grout forms a bulb that and densifies the surrounding soil. Compaction grouting is a good option if the foundation of an existing building requires improvement, since it is possible to inject the grout from the side or at an inclined angle to reach beneath the building.
Figure:3
COMPACTION GROUTING.
SCHEMATIC DIAGRAM:2
9. CASE STUDY
PILE FOUNDATION FOR VYPEEN ISLAND BRIDGE
The new bridge connecting Ernakulam town with Bolgatti Island and Vypeen Island is supported on 1m diameter bored cast in-situ pile. The design load for each pile is 450tones. The strata comprise of soft marine clay of 20 to 30m. This is followed by alternative layers of dense sand and stiffing clay which contains decayed wood within the deposit. The SPTN value of clay with decayed wood is generally >100 and the pile length were estimated as 45 to 50m.
During excavation of the work, it was noticed that even pile length 45m was not adequate to meet the loading requirement. Due to the large depth of pile, stabilization of the borehole and cleaning of the bore before concreting was found to be difficult. The load test results revealed that the end boring resistance offered by the strata was not significant and particularly all the load was resisted by side friction. To meet the requirement of loading, the pile length had to be increased from 45 to as high as 65 to 70m depending on the strata condition. For the certain piers, before the load test results were available, the piles were terminated with depth of 45m. Additional piles were provided at these locations to meet the loading requirements. It is very rare that large diameter piles have been installed to meet large depth.
The variations in the strata conditions and uncertainty about the behavior of stiff clay with the decayed wood made the designers to adopt a conservative approach which has resulted in large depth of pile. Apart from construction of piles, the strata condition posed followed technical problems.
Reclamation of large area by dredged material on Vypeen Island.
Construction of 6 to 8m high approach embankments on soft clay deposit created problems of settlement and negative drag of the pile. To reduce this 2 structural span have been added at the Ernakulam end so that the height of the embankment get reduced and approach embankment can be supported on the available weak strata.
Figure:4
PILE FOUNDATION FOR VYPEEN ISLAND BRIDGE
10. CONCLUSIONS
Due to the increase in construction activities and urbanization the availability of suitable soils are decreasing. Cochin City and its surroundings are covered with marine clay to large depth. Which posses very low shear strength, low physical and engineering properties. Whenever the soft clay deposits are very loose sand or soft clay and are unsuitable for resting the foundation, the following options can be considered.
Adopt deep foundation such as pile foundation so that the top soil can be bypassed.
Treat the existing soil to improve the bearing capacity and reduce the settlement and place the foundation directly over the top soil.
and Cochin is no exception with the increase of population, housing and construction of
various facilities have been a problem with urbanization. Having exhausted all the trouble
free hand, man is now on the look out for techniques to improve areas which were
originally considered uninhabitable. Thus studies on the nature and engineering behavior
of
soft clay’s covering long stretches of coastal line and methods to improve there geotechnical properties have been of great relevance, and is explained in the following pages.
2. REVIEW OF COCHIN MARINE CLAY
According to King (1884), 2500 years back, the sea washed up to the high ranges
of Western Ghats. The land between the Western Ghats and the present coastal line was
once well above the sea and was subsequently submerged. It was uplifted again by volcanic
action and again partially covered by sea water. The uplifted area includes the coastal belt
(10 to 30Km wide and 150 Km long) starting from Crangannore at the north to Quilon
at the south. Most of the Cochin city is situated in this belt.
3. PHYSICAL PROPERTIES OF COCHIN MARINE CLAY
Marine clay is formed by the sedimentation of clay soils in marine environments
exhibit unusual physical properties. They posses very high liquid limit and their natural
water content is close to the liquid limit values. The most striking feature of the physical
properties is the phenomenal changes that are caused by drying of sample. Some physical
properties of the Cochin marine clay is shown in the following table.
PHYSICAL PROPERTIES OF COCHIN MARINE CLAY
Sl No Physical properties (%) Moist soil Air dried soil Oven dried soil
1 Liquid limit 131.5 94.5 62.0
2 Plastic limit 53.9 45.2 40.5
3 Plasticity index 77.6 49.3 21.5
4 Shrinkage limit 18.1 18.8 19.2
5 Grain size distribution
(a) Clay
(b) Silt
(c) sand
48
34
18
33
47
20
13
55
32
4. SHEAR STRENGTH
In greater Cochin area the layers of shallow depth have their natural moisture
content quite close to their liquid limit. The clay deposits below vary widely in consisting up
to 40 to 50m where good load bearing stratum can be met with.
The shear strength of clayey soils is dependent on its water content. Any
increase in water content is necessarily accompanied by the reduction in cohesion or shear
strength. But at the same water content, different soils have different shear strength. From
the test results that were conducted at different places in Cochin, it was found that, the shear strength is very low and it is around 0.01Kg/cm² and 0.02Kg/cm².
5. CONSOLIDATION CHARACTERISTICS OF COCHIN
MARINE CLAY
Several series of consolidation tests were carried out on samples of Cochin arine clay varying different parameters like duration of loading, effect of pore fluid, effect
of washing. In the consolidation test which served as the reference test. It was found that
about 2 days were required for a complete dissipation of pore pressure and for reaching
an equilibrium void ratio for a particular loading stage. The clay deposits are highly
compressive with Cc value around 1.5 and natural moisture content
very close to liquid limit. It has been observed that almost all the structures, founded on
spread footings or raft footings invariably show actual settlement far below the computed
values.
6. PROBLEMS DUE TO THICK MARINE CLAY
Cochin City and the surroundings are covered with thick marine clay deposit
extending to a large depth (30 to 40m) below the ground level. Due to the presence of this
soft clay layer, following problems are observed.
Low bearing capacity
Higher settlement
High seepage losses
Liquefaction during earthquake
Instability of foundation excavations
Higher earth pressure on retaining structures
IS: 1904(1966) permits a maximum settlement of 40mm for isolated foundations on
sand and 65mmfor those for clay. The allowable settlement is higher for clay because
progressive settlement on clayey soils permits better strain adjustments in the structural
members. The maximum settlement for raft foundations on sand is 40mm to 65mm and that
on clay is 65mm to 100mm.
For typical industry following issues are to be addressed:
Large area is covered with thick vegetation. If the area is reclaimed without removal of the vegetation. It will result in a fill with high organic content and high compressibility. On the other hand, if the vegetation is to be removed, it requires dewatering of the area so that powered vehicle can operate on the surface and clear slush and vegetation before selected soil is placed for reclamation.
While reclaiming the area, particularly for industry requiring very large area, for example refinery, oil terminal, gas bottling plant, etc. the drainage pattern of the area needs to be planned carefully. The flow surface water which was taking place through the area prior to reclamation has to be diversified through special drains/culverts such a way that it does not create any inundation.
The area avail is low-laying and often inundated with water. Unless the area is raised by soil fill (3 to 4m thick) it is difficult to carryout any construction.
In many projects, the height of the fill required is 2 to 3m. This will require boundary wall which has a high retaining wall plus a compound wall. In the absence of retaining wall the fill material can encroach in the neighboring areas beyond the property line which may not be acceptable in many cases. Alternately, fill is placed with toe of the slop within the property line .in this case, large area with a width of 6 to 7m all along the periphery of the area remain unutilized.
The placement of fill can lead to the following foundation problems.
Negative drag force on pile foundation.
Lateral flow under surface loading leading to settlement and lateral force on the adjacent foundations.
Deep-seated slop failure for construction of embankment and heavy surface loading.
Difficulty in executing foundations requiring deep excavation.
7. GROUND IMPROVEMENT
The clay deposit also needs to be improved to meet the foundation requirements. In early days, areas having clay deposit was avoided for construction. But with scarcity of land in urban areas, we do not have choice and structures have to be built on weak deposits. Pile foundation is of course a possible approach as it by pass the weak deposit and transfer the load on next component layer. Where thickness is very large, pile foundation is uneconomical and time consuming.
The methods of ground improvement that can be adopted depend basically on nature of strata and purpose of improvement. The methods available are as follows.
8 METHODS OF GROUND IMPROVEMENT
8.1 VERTICAL DRAINS
This method is suitable for deep deposit of soft clay. The natural moisture content in this strata can brought down substantially by installing vertical drains with preloading. The pressures of vertical drains reduce the drainage path of water in the pores of the soil and thereby reduce the time required for consolidation. The spacing of drains depends upon the speed at which required improvement is to be achieved.
In earlier days, such vertical drains were installed by driving a close ended steep pipe o 100 to 200mm diameter up to the full thickness of such clay deposit. The pipe is then filled with sand and withdrawn in stage to form a vertical drain. The pipe is generally refused for installing other drains.
In the recent past, there has been number of different materials developed to replace the sand drains. These are basically plastic sections having thickness varying from 5 to 10mm and width from 100 to 150mm. The sections have channels to permit flow of water. The perimeter of the section is covered with a layer of geo-textile to prevent the entry of soil particles in to the channel. The advantages of such drain are that it results in minimum remoulding of surrounding soil during installation. The process of installation is very fast. The machine is mounted on a crane and typical drain up to 10m depth can be installed in the period of varying 1 to2 minutes including the time for shifting the machine to a new location.
The percentage of consolidation, which can be achieved by such vertical drains, can be theoretically predicted from the design charts developed based on the theory of three-dimensional consolidation. Typically, a deposit, which requires a period of over ten years for 95% consolidation, can complete the same consolidation within a short period of 3 to 6 months when vertical drains are installed.
After the drains are installed, the magnitude of preload to be placed on the required shear strength of the layer after improvement. Depending on the time available for improvement, the degree of consolidation is worked out and the effective over burden pressure (p) is computed. From the plasticity index I\of the soil, the ratio Su/p is determined. The ratios normally remain constant and therefore with increase in value of p the shear strength after treatment increases.
8.2 IN-SITU DEEP MIXING
In- site deep mixing using hydraulically operated helical augers penetrate the ground to the required depth. The hollow stem of the auger is used to inject cement / lime / any other stabilizing compound into the ground. A pair of 2 or 3 augers is operated simultaneously. After injecting the stabilizer in to the ground, the augers are rotated such that the soil along with the stabilizer is churned in-situ and mixed the stabilizer thoroughly with the soil without necessity of taking out the soil. The auger configuration can be chosen such that either a stabilized wall can be formed (to act as a barrier) or the entire area can be stabilized.
8.3 STONE COLUMN
Stone columns are cylindrical columns made below ground level which comprises of granular material of size varying from 25 to 100mm. The hole is made in the soft clay deposit by different techniques and then filled with stones in layer and compacted to form the complete column. When the structure is placed over the area treated by stone column, majority of the load (80 to 90%) is transmitted to the stone columns because of their higher stiffness. Balance 10 to 20%of the load is taken by the clay deposit. With the help of this 10%of surcharge load, the soft clay able to provide adequate confinement to the cylindrical column.
The area treated stone columns can be used to support only flexible structures such as embankment, oil storage tank, etc. because the settlement even after treatment with stone column can be large (50 to200mm). Without stone column the settlement could have been 3 to 4times higher and also the bearing capacity would have been much less.
8.4 VACUUM DEWATERING
In this technique also the layer is provided with vertical drains as mentioned above. The surface is then covered with a layer of a sand blanket 150 to 300mm thick and covered with a thin plastic sheet which is suitably anchored along the perimeter. Vacuum is then applied with a sand blanket which creates a suction pressure up to 50 to60mm height of mercury and forced the pore water out from the deposit through vertical drains. The area is also subjected to atmospheric pressure which then acts as preload. The vacuum pressure is maintained till the required improvement is achieved.
8.5 VIBROFLOTATION
Vibroflotation involves the use of a vibrating probe that can penetrate granular soil to depths of over 100feet. The vibrations of the probe cause the grain structure to collapse thereby densifying the soil surrounding the probe. To treat an area of potentially liquefiable soil, the vibroflot is raised and lowered in a grid pattern. Vibro replacement is a combination of vibroflotation with a gravel backfill resulting in stone columns, which not only increase the amount of densification, but provides a degree of reinforcement and potentially effective means of drainage.
Figure:1
VIBROFLOTATION.
SCHEMATIC DIAGRAM:1
8.6 DYNAMIC COMPACTION
In dynamic compaction, a heavy weight of 15 to 20ton is raised to a height about 20m above the ground level and freely dropped on the ground surface. The impact on the ground surface transmits compression waves, which spreads radially from the point of impact. The compression waves travels fast and creates Liquefaction of the strata, which is saturated. It is followed by shear waves reorients the soil particle which are liquefied state to denser state and compacts the configuration. The spacing between the treating point, number of impact at each point, etc. can be varied depending on the required improvement.
Figure : 2
DYNAMIC COMPACTION.
8.7 COMPACTION BY DEEP BLASTING
In this technique, a borehole is made to a depth approximately 3/4 or 2/3 of the total depth. Suitable explosive is lowered to the bottom of the borehole and the space above is back filled with compacted soil (known as stemming). The explosive is attached with cordex, which is taken up to the surface and connected to electric detonator. When the explosive is detonated, it creates a cavity, which expands instantaneously. The cavity expansion generates shock waves. Propagation of these waves densifies the soil to certain extent. After blast, a large conical depression is observed at the place of blast. The water within the pores of the deposit gets expelled out and comes up to the ground and form a large pool in the cone of depression.
The quantity of charge and spacing between the explosive can be suitably selected to achieve the required densification. As a thumb rule, about 15 to 45 grams of explosive is required for densification of every 3m of the deposit. Initial loose deposit which has relative density of about 50% and susceptible to liquefaction can be densified to a relative density of over 60% which is adequate to prevent liquefaction. The vibrations created during the blast are within permissible limits for construction purpose.
8.8 COMPACTION GROUTING
Compaction grouting is a technique where by a slow-flowing water /sand /cement mix is injected under pressure into a granular soil. The grout forms a bulb that and densifies the surrounding soil. Compaction grouting is a good option if the foundation of an existing building requires improvement, since it is possible to inject the grout from the side or at an inclined angle to reach beneath the building.
Figure:3
COMPACTION GROUTING.
SCHEMATIC DIAGRAM:2
9. CASE STUDY
PILE FOUNDATION FOR VYPEEN ISLAND BRIDGE
The new bridge connecting Ernakulam town with Bolgatti Island and Vypeen Island is supported on 1m diameter bored cast in-situ pile. The design load for each pile is 450tones. The strata comprise of soft marine clay of 20 to 30m. This is followed by alternative layers of dense sand and stiffing clay which contains decayed wood within the deposit. The SPTN value of clay with decayed wood is generally >100 and the pile length were estimated as 45 to 50m.
During excavation of the work, it was noticed that even pile length 45m was not adequate to meet the loading requirement. Due to the large depth of pile, stabilization of the borehole and cleaning of the bore before concreting was found to be difficult. The load test results revealed that the end boring resistance offered by the strata was not significant and particularly all the load was resisted by side friction. To meet the requirement of loading, the pile length had to be increased from 45 to as high as 65 to 70m depending on the strata condition. For the certain piers, before the load test results were available, the piles were terminated with depth of 45m. Additional piles were provided at these locations to meet the loading requirements. It is very rare that large diameter piles have been installed to meet large depth.
The variations in the strata conditions and uncertainty about the behavior of stiff clay with the decayed wood made the designers to adopt a conservative approach which has resulted in large depth of pile. Apart from construction of piles, the strata condition posed followed technical problems.
Reclamation of large area by dredged material on Vypeen Island.
Construction of 6 to 8m high approach embankments on soft clay deposit created problems of settlement and negative drag of the pile. To reduce this 2 structural span have been added at the Ernakulam end so that the height of the embankment get reduced and approach embankment can be supported on the available weak strata.
Figure:4
PILE FOUNDATION FOR VYPEEN ISLAND BRIDGE
10. CONCLUSIONS
Due to the increase in construction activities and urbanization the availability of suitable soils are decreasing. Cochin City and its surroundings are covered with marine clay to large depth. Which posses very low shear strength, low physical and engineering properties. Whenever the soft clay deposits are very loose sand or soft clay and are unsuitable for resting the foundation, the following options can be considered.
Adopt deep foundation such as pile foundation so that the top soil can be bypassed.
Treat the existing soil to improve the bearing capacity and reduce the settlement and place the foundation directly over the top soil.
Marina Bay Sands
Following four years of construction in Singapore opened Hotel Marina Bay Sands. The three tower 200 meters is a huge terrace in the form of a gondola with pools and green gardens with total area of 120 thousand square meters. The main pool is located outdoors, is not visible to the rim, because of what the impression that the water's edge terminates at a height. Swimming pool stretches for 150 meters and is the largest swimming pool, built on this level. The hotel has 2,560 rooms, shopping center, conference hall, 2 theaters, a museum and several restaurants. Marina Bay Sands is one of two winning proposals for Singapore's first Integrated Resorts, the other being the Resorts World Sentosa,
Bidders were assessed based on four criteria:
• Tourism appeal and contribution
• Architectural concept and design
• Development investment
• Strength of the consortium and partners
Las Vegas Sands initially committed to invest S$3.85 billion in the project, not including the fixed S$1.2 billion cost of the 6,000,000 square feet (560,000 m2) site itself. With the escalating costs of materials, such as sand and steel, and labour shortages owing to other major infrastructure and property development in the country, Sheldon Adelson placed the total cost of the development at $8.0 billion as of July 2009.
2. DESIGN
Fig(2.1) Architectural model showing the layout of the Marina Bay Sands Integrated Resort:
The hotel and Sky Park are in and on the towers in the background, respectively; the shopping mall, theatre and casino are in the long lower building; and the ArtScience Museum is in flower-shaped building in the foreground.
The resort is designed by Moshe Safdie, who says it was initially inspired by card decks. In addition to the casino, other key components of the plan are three hotel towers with 2,560 rooms and suites, a 200,000-square-foot (19,000 m2) Art Science Museum and a convention centre with 1,200,000 square feet (111,000 m2) of space, capable of accommodating up to 45,000 people.
Conceptually, each tower is composed of two slabs of east and west-facing rooms. The double-loaded towers spread at the base forming a giant atrium at the lower levels, and converge as they rise . the tower slabs also give further character to the massing and relate to the site context: the glazed west side faces the city center while the east side is planted with lush bougainvilleas facing the botanical gardens and ocean beyond in plan, as the parcel varies in width, the cross section is decrease from one tower to the next. The three void spaces are connected by one continuous and conditioned glazed, filling the space between the towers with restaurants, retail spaces, and a public thoroughfare. Each tower slab from is also twisted slightly in relation to its pair, creating a dance-like relationship between the two parts and accentuation the slenderness of the buildings, resulting in the appearance of six towers, rather than three.
In addition to the million square meters of built space the project program also called for the development of extensive exterior gardens with swimming pools, jogging paths, and public spaces. As one of the aims of the project had been to minimize the geight of the podium buildings, seeking to reference Singapore’s pastoral hills more than its urban core, the problem emerged that the complex program left no vacant land suitable for these amenities. Creating gardens on top of the roof of the casino and the convention center was studied, however these vast spaces lacked views, overshadowed and overpowered by the adjacent hotel towers. The idea emerged to bridge between the three towers in order to reclaim exterior garden space and create a 2.5-acre park in the sky.
2.1 THE BELLY
The façade of the project which required careful consideration was thus the “belly” of the sky park. Made of more than 9000 silver-painted metal –composite panels, this skin encloses the mega trusses which bridge the buildings at level 55,as well as a multitude of back of house spaces(i.e., large mechanical rooms with water tanks supporting the pools and a network of corridors and offices for hotel operations staff). The geometry of the sky park began with a platonic torpid form, which was then further shaped to streamline the cross sections of the building. The resulting surface was then regularized and panelized using a computer script, triangulation the façade into simple shapes, the shapes were water-jet cut from flat sheet panels and shipped to the building site in containers pre-designed to be lifted to the top levels of the building.
Once on top of the building, the panels were installed via temporary aluminum frame under slung mobile gantries which traveled on a permanent track of steel rails. The gantry system track serves double day as a building maintenance unit, therefore the track was designed to be hidden within the reveal pattern of the facade panels.
3. CONSTRUCTION
Fig(3.1) When construction started in 2007.
Fig(3.2) Marina bay sands
In May of 2006, after a highly-competitive bidding process, the Singapore government selected Las Vegas sands corp. to build the country's first integrated resort property. It will be one of only two resort-casino properties in the South East Asian city-state and will operate as such for a period of 30 years. the gigantic complex complete with three 60-story hotel towers with 3 stories underground will house 2,600 hotel rooms / suites and a 4,000 car garage; topped by a two acre sky park bridging across the towers consisting of a garden, swimming pool, jogging paths, spas; ‘floating’ crystal pavilions; on the promontory a lotus-shaped museum; a grand, a multi-leveled retail arcade for international luxury brands. Celebrity chef restaurants; an outdoor event plaza; entertainment theaters and night clubs plus a Las Vegas-style casino; highly flexible exhibition halls and a convention centre that can host over 45,000 delegates.
Fig(3.3) Marina bay sands
Fig(3.4) Marina Bay Sands
Skypark is beginning to look like a park with more than 20 trees now planted 200 meters in the air.Closer to the opening, the roof top gardens will be home to 250 trees and 650 plants.
Fig(3.5) The grand arcade trave rse of the MBS hotel
The 340m-long skypark, which will house a 1,2 hectares swimming pools -
equivalent to the size of 10 olympic size swimming pools -, observation decks, gardens, restaurants, spas, and walking trails.it will be cantilevered out some 70m without any support underneath.once completed, the skypark is expected to be a leading tourist spot where one can enjoy a panoramic view of the beaches and downtown singapore.drawing by lim vong marina bay is a bay near central area in the southern part of singapore, and lies to the east of the downtown core. it is an artificial bay and was formed when land reclamation created the marina centre and marina south areas, which form a body of sheltered waters of what was once open sea. This land was reclaimed as part of the long-term strategy for singapore’s growth about 30 years ago, with a view to the downtown core eventually out-growing itself.The reclamation is about more than just increasing land area. The large area of sea which has been embraced by the reclamation has now been isolated from the larger ocean by the building of a dam across the narrow inlet so that, gradually as the singapore river discharges its fresh water load into this area over the next decade or so, it will dilute the sea-water and eventually create a fresh-water reservoir, making singapore politically less dependant on malaysia for its fresh water needs, and creating a great recreational resource in the heart of the city to boot.
Fig(3.6) Marina resort
Fengshui master says the integrated resort has many auspicious elements going for it. The sky park is curved, planes that come around will see a smile from their bird's eyes view. The towers represent three mountains or three warriors guarding the gateway to singapore. Despite fengshui being open to many interpretations, there are rules and principles to the practice, which are guided by the so-called balance of the five elements. its silvery white facade represents metal, while other elements represent fire. wood is in the landscaping and greenery, while water surrounds it.
Fig(3.7) The marina bay sands art and science museum
The lotus shaped art science museum sits at the front of the development. The building's shaped steel frame was among the most difficult parts of all the structural design. However, using software scripting it was possible to analyze this one petal at a time, altering the parameters for each subsequent part. An international team of PERI specialists from singapore and germany created a comprehensive formwork and scaffolding solution for the korean building company, ssang yong engineering & construction. in particular, the skytable large slab tables as well as PERI ACS self-climbing technology have optimised construction progress with reduced crane times. These rised steadily upwards with each floor being completed in only four days.
During construction, three iron structures were propped temporarily at places with the severest inclination beside the post-tension method. Through this, the quantity of temporary work for the main work was reduced by a great deal so that the work could be executed without interference from structures. as a result, constructors succeeded in early march 2009 in connecting at 23F above ground and a height of 70m for the east-side building and the west-side building that were inclined at 52 degrees maximum for towers I, II and III that were rising in the form.
In each case, two asymmetrically curved legs have been positioned against each other which grow together like bridge pylons to form units. although the three hotel towers are still identical with regard to the height and number of floors, the forms of the respective building elements nevertheless have considerable differences in terms of the base width, curvature radius and lateral offset dimension. Furthermore, the individual floors are also offset from one another in a longitudinal direction. Only two cranes per tower were available to the Construction crews, therefore crane independent and crane-saving formwork solutions were used to construct the core walls and floor slabs respectively.
With the help of ACS self-climbing technology and the large-area sky table slab tables, crews were able to finish a complete floor with a standard height of 3 m in only four days. Twelve elevator shafts climbed three cycles in advance and a total of 110 sky table slab tables were required for constructing two complete floors in each case.
Fig(3.8) Construction of deck
Fig(3.9) Construction of deck
Fig(3.10) Construction of deck
Fig(3.11) The marina bay sands construction site
Up to 20 m long and 5 m wide, the 100 m² sky table units could be easily moved with only one crane lift. the innovative moving method ensured that the table was always pulled out of the building in a horizontal position and the operating personnel were always standing in a safe and secure place on the slab edge. Due to the large lowering height of the multiprop props, 20 cm slab offsets and, at the same time, 50 cm parapet wall heights presented no problems. Connected to MRK frames, the tables could be additionally used for the 9 m high intermediate floors.
Fig(3.12) The marina bay sands towers
With the help of ACS, the different-sized shafts with dimensions ranging between 2.30 and 10.10 m, could be shuttered, struck and climbed without the need of a crane. Altogether, five working levels rose at the same time to the next section:
Two platforms for forming, reinforcement work and concreting the shaft walls as well as three finishing platforms for pretensioning the subsequent storey slabs. in combination with CB climbing scaffold and the vario GT 24 girder wall formwork, operational sequences on the construction site were optimized.
The technical team of ssang yong E&C applied the post-tension method that is used mainly in constructing bridges to construct a building inclined at 52 degrees at its highest, which is 10 times more inclined than the leaning tower of pisa. The team adopted a tension method whereby post-tension is installed on a 600 mm thick bearing wall and wire is stretched from inside.
Fig(3.13) Close-up to the marina bay sand middle tower
Fig(3.14) With only one crane lift, 100 square meters of slab formwork could quickly and safely moved.
The project's foundations have had to be massive because of building on marine clay. the team has had to get through the clay to make room for the underground car-parks and railway tunnels which sit beneath the facility. bored piles which go down to over 50m with large bored piles up to 2.8m in diameter have been necessary, typical of many singapore construction projects.
4. OPENING
Fig(4.1) During the 2010 Summer Youth Olympics opening ceremony
Marina Bay Sands was originally planned to be completed in a single phase in 2009, but rising construction costs and the financial crisis forced the company to open it in phases. The first phase's preview opening was further delayed until 27 April 2010, and the official opening was pushed back to 23 June 2010. The rest of the complex remained under construction and was opened after a grand opening on 17 February 2011.On 27 April 2010, Marina Bay Sands had the first of a planned 3 to 4 phase openings. The casino, parts of the conference hall, a segment of the Shoppes, 963 hotel rooms and the event plaza were opened.
The Inter-Pacific Bar Association (IPBA) held the first conference at Marina Bay Sands Convention Centre on 2–5 May 2010, but the event was marred by uncompleted facilities and power failure during a speech. IPBA withheld payment of S$300,000 and was consequently sued by Marina Bay Sands. In June IPBA counter-sued, describing the venue as a complete disaster and that its earlier payments had been imposed by duress, fear and force. An amicable settlement with undisclosed terms was announced in August. On 23 June 2010, the resort had its official opening with a 2-day celebration, this includes the Sands Sky Park, the Event Plaza along Marina Bay, more shops, additional dining options and nightlife offerings, and the rest of the hotel room
5. SPECIALITIES
5.1 CASINO
Marina Bay Sands casino is housed in its own building and offers four levels of gaming in a luxurious, spacious setting. Patrons can enjoy a wide variety of popular table games including roulette, blackjack, baccarat and sic bo, and slot machines featuring the latest games such as video poker, and electronic sic bo and roulette. Premium players and Paiza members have access to exclusive gaming and dining privileges at the casino’s upper two levels, which feature over 30 private gaming rooms.
5.2 SKY PARK
The Sands Sky Park is an architectural masterpiece sitting on top of the three hotel towers at Marina Bay Sands. This 1.2 hectare tropical oasis is longer than the Eiffel Tower is tall and large enough to park four-anda-half A380 jumbo jets. It extends to form the one of the world’s largest public cantilevers. It is 200 meters in the sky. Landscaped gardens are home to 250 trees and 650 plants and 12,400 square meters of space big enough to fit three football fields.
5.3 INFINITY POOL
The Infinity Pool is part of the 380 metre long Sands Sky Park, which spans across the three towers of the Marina Bay Sands complex and covers a total area of 12,000 sq mts. The length of this roof-top deck can be equated to the height of the Eiffel Tower.
The Infinity Pool is one of the world’s largest outdoor pools at three times the length of an Olympic pool and 55 storeys high. The pool creates an illusion of the water extending to the horizon while in reality, the water spills over the edge into a catchment below and is then pumped back into the pool. It has two circulation systems, with the first filtering and heating the water in the main pool and the other, filtering the water in the catch basin and returning it to the upper pool.
Fig(5.1) Infinity Pool
6. CONCLUSION
Marina bay sand is a magnificent destination for entertainment, business, and shopping.
The success of project lies in the fact that the inventiveness of the design was matched by an equally inventive and novel approach developed by engineering and constructon teams.
Delivers once in lifetime experiences.
This landmark building is situated in the heart of Singapore’s central business district.
With a luxury hotel, state of art convention, exhibition facilities, and some of the best shopping &dining in the region.
This is the place to go for world class entertainment.
Bidders were assessed based on four criteria:
• Tourism appeal and contribution
• Architectural concept and design
• Development investment
• Strength of the consortium and partners
Las Vegas Sands initially committed to invest S$3.85 billion in the project, not including the fixed S$1.2 billion cost of the 6,000,000 square feet (560,000 m2) site itself. With the escalating costs of materials, such as sand and steel, and labour shortages owing to other major infrastructure and property development in the country, Sheldon Adelson placed the total cost of the development at $8.0 billion as of July 2009.
2. DESIGN
Fig(2.1) Architectural model showing the layout of the Marina Bay Sands Integrated Resort:
The hotel and Sky Park are in and on the towers in the background, respectively; the shopping mall, theatre and casino are in the long lower building; and the ArtScience Museum is in flower-shaped building in the foreground.
The resort is designed by Moshe Safdie, who says it was initially inspired by card decks. In addition to the casino, other key components of the plan are three hotel towers with 2,560 rooms and suites, a 200,000-square-foot (19,000 m2) Art Science Museum and a convention centre with 1,200,000 square feet (111,000 m2) of space, capable of accommodating up to 45,000 people.
Conceptually, each tower is composed of two slabs of east and west-facing rooms. The double-loaded towers spread at the base forming a giant atrium at the lower levels, and converge as they rise . the tower slabs also give further character to the massing and relate to the site context: the glazed west side faces the city center while the east side is planted with lush bougainvilleas facing the botanical gardens and ocean beyond in plan, as the parcel varies in width, the cross section is decrease from one tower to the next. The three void spaces are connected by one continuous and conditioned glazed, filling the space between the towers with restaurants, retail spaces, and a public thoroughfare. Each tower slab from is also twisted slightly in relation to its pair, creating a dance-like relationship between the two parts and accentuation the slenderness of the buildings, resulting in the appearance of six towers, rather than three.
In addition to the million square meters of built space the project program also called for the development of extensive exterior gardens with swimming pools, jogging paths, and public spaces. As one of the aims of the project had been to minimize the geight of the podium buildings, seeking to reference Singapore’s pastoral hills more than its urban core, the problem emerged that the complex program left no vacant land suitable for these amenities. Creating gardens on top of the roof of the casino and the convention center was studied, however these vast spaces lacked views, overshadowed and overpowered by the adjacent hotel towers. The idea emerged to bridge between the three towers in order to reclaim exterior garden space and create a 2.5-acre park in the sky.
2.1 THE BELLY
The façade of the project which required careful consideration was thus the “belly” of the sky park. Made of more than 9000 silver-painted metal –composite panels, this skin encloses the mega trusses which bridge the buildings at level 55,as well as a multitude of back of house spaces(i.e., large mechanical rooms with water tanks supporting the pools and a network of corridors and offices for hotel operations staff). The geometry of the sky park began with a platonic torpid form, which was then further shaped to streamline the cross sections of the building. The resulting surface was then regularized and panelized using a computer script, triangulation the façade into simple shapes, the shapes were water-jet cut from flat sheet panels and shipped to the building site in containers pre-designed to be lifted to the top levels of the building.
Once on top of the building, the panels were installed via temporary aluminum frame under slung mobile gantries which traveled on a permanent track of steel rails. The gantry system track serves double day as a building maintenance unit, therefore the track was designed to be hidden within the reveal pattern of the facade panels.
3. CONSTRUCTION
Fig(3.1) When construction started in 2007.
Fig(3.2) Marina bay sands
In May of 2006, after a highly-competitive bidding process, the Singapore government selected Las Vegas sands corp. to build the country's first integrated resort property. It will be one of only two resort-casino properties in the South East Asian city-state and will operate as such for a period of 30 years. the gigantic complex complete with three 60-story hotel towers with 3 stories underground will house 2,600 hotel rooms / suites and a 4,000 car garage; topped by a two acre sky park bridging across the towers consisting of a garden, swimming pool, jogging paths, spas; ‘floating’ crystal pavilions; on the promontory a lotus-shaped museum; a grand, a multi-leveled retail arcade for international luxury brands. Celebrity chef restaurants; an outdoor event plaza; entertainment theaters and night clubs plus a Las Vegas-style casino; highly flexible exhibition halls and a convention centre that can host over 45,000 delegates.
Fig(3.3) Marina bay sands
Fig(3.4) Marina Bay Sands
Skypark is beginning to look like a park with more than 20 trees now planted 200 meters in the air.Closer to the opening, the roof top gardens will be home to 250 trees and 650 plants.
Fig(3.5) The grand arcade trave rse of the MBS hotel
The 340m-long skypark, which will house a 1,2 hectares swimming pools -
equivalent to the size of 10 olympic size swimming pools -, observation decks, gardens, restaurants, spas, and walking trails.it will be cantilevered out some 70m without any support underneath.once completed, the skypark is expected to be a leading tourist spot where one can enjoy a panoramic view of the beaches and downtown singapore.drawing by lim vong marina bay is a bay near central area in the southern part of singapore, and lies to the east of the downtown core. it is an artificial bay and was formed when land reclamation created the marina centre and marina south areas, which form a body of sheltered waters of what was once open sea. This land was reclaimed as part of the long-term strategy for singapore’s growth about 30 years ago, with a view to the downtown core eventually out-growing itself.The reclamation is about more than just increasing land area. The large area of sea which has been embraced by the reclamation has now been isolated from the larger ocean by the building of a dam across the narrow inlet so that, gradually as the singapore river discharges its fresh water load into this area over the next decade or so, it will dilute the sea-water and eventually create a fresh-water reservoir, making singapore politically less dependant on malaysia for its fresh water needs, and creating a great recreational resource in the heart of the city to boot.
Fig(3.6) Marina resort
Fengshui master says the integrated resort has many auspicious elements going for it. The sky park is curved, planes that come around will see a smile from their bird's eyes view. The towers represent three mountains or three warriors guarding the gateway to singapore. Despite fengshui being open to many interpretations, there are rules and principles to the practice, which are guided by the so-called balance of the five elements. its silvery white facade represents metal, while other elements represent fire. wood is in the landscaping and greenery, while water surrounds it.
Fig(3.7) The marina bay sands art and science museum
The lotus shaped art science museum sits at the front of the development. The building's shaped steel frame was among the most difficult parts of all the structural design. However, using software scripting it was possible to analyze this one petal at a time, altering the parameters for each subsequent part. An international team of PERI specialists from singapore and germany created a comprehensive formwork and scaffolding solution for the korean building company, ssang yong engineering & construction. in particular, the skytable large slab tables as well as PERI ACS self-climbing technology have optimised construction progress with reduced crane times. These rised steadily upwards with each floor being completed in only four days.
During construction, three iron structures were propped temporarily at places with the severest inclination beside the post-tension method. Through this, the quantity of temporary work for the main work was reduced by a great deal so that the work could be executed without interference from structures. as a result, constructors succeeded in early march 2009 in connecting at 23F above ground and a height of 70m for the east-side building and the west-side building that were inclined at 52 degrees maximum for towers I, II and III that were rising in the form.
In each case, two asymmetrically curved legs have been positioned against each other which grow together like bridge pylons to form units. although the three hotel towers are still identical with regard to the height and number of floors, the forms of the respective building elements nevertheless have considerable differences in terms of the base width, curvature radius and lateral offset dimension. Furthermore, the individual floors are also offset from one another in a longitudinal direction. Only two cranes per tower were available to the Construction crews, therefore crane independent and crane-saving formwork solutions were used to construct the core walls and floor slabs respectively.
With the help of ACS self-climbing technology and the large-area sky table slab tables, crews were able to finish a complete floor with a standard height of 3 m in only four days. Twelve elevator shafts climbed three cycles in advance and a total of 110 sky table slab tables were required for constructing two complete floors in each case.
Fig(3.8) Construction of deck
Fig(3.9) Construction of deck
Fig(3.10) Construction of deck
Fig(3.11) The marina bay sands construction site
Up to 20 m long and 5 m wide, the 100 m² sky table units could be easily moved with only one crane lift. the innovative moving method ensured that the table was always pulled out of the building in a horizontal position and the operating personnel were always standing in a safe and secure place on the slab edge. Due to the large lowering height of the multiprop props, 20 cm slab offsets and, at the same time, 50 cm parapet wall heights presented no problems. Connected to MRK frames, the tables could be additionally used for the 9 m high intermediate floors.
Fig(3.12) The marina bay sands towers
With the help of ACS, the different-sized shafts with dimensions ranging between 2.30 and 10.10 m, could be shuttered, struck and climbed without the need of a crane. Altogether, five working levels rose at the same time to the next section:
Two platforms for forming, reinforcement work and concreting the shaft walls as well as three finishing platforms for pretensioning the subsequent storey slabs. in combination with CB climbing scaffold and the vario GT 24 girder wall formwork, operational sequences on the construction site were optimized.
The technical team of ssang yong E&C applied the post-tension method that is used mainly in constructing bridges to construct a building inclined at 52 degrees at its highest, which is 10 times more inclined than the leaning tower of pisa. The team adopted a tension method whereby post-tension is installed on a 600 mm thick bearing wall and wire is stretched from inside.
Fig(3.13) Close-up to the marina bay sand middle tower
Fig(3.14) With only one crane lift, 100 square meters of slab formwork could quickly and safely moved.
The project's foundations have had to be massive because of building on marine clay. the team has had to get through the clay to make room for the underground car-parks and railway tunnels which sit beneath the facility. bored piles which go down to over 50m with large bored piles up to 2.8m in diameter have been necessary, typical of many singapore construction projects.
4. OPENING
Fig(4.1) During the 2010 Summer Youth Olympics opening ceremony
Marina Bay Sands was originally planned to be completed in a single phase in 2009, but rising construction costs and the financial crisis forced the company to open it in phases. The first phase's preview opening was further delayed until 27 April 2010, and the official opening was pushed back to 23 June 2010. The rest of the complex remained under construction and was opened after a grand opening on 17 February 2011.On 27 April 2010, Marina Bay Sands had the first of a planned 3 to 4 phase openings. The casino, parts of the conference hall, a segment of the Shoppes, 963 hotel rooms and the event plaza were opened.
The Inter-Pacific Bar Association (IPBA) held the first conference at Marina Bay Sands Convention Centre on 2–5 May 2010, but the event was marred by uncompleted facilities and power failure during a speech. IPBA withheld payment of S$300,000 and was consequently sued by Marina Bay Sands. In June IPBA counter-sued, describing the venue as a complete disaster and that its earlier payments had been imposed by duress, fear and force. An amicable settlement with undisclosed terms was announced in August. On 23 June 2010, the resort had its official opening with a 2-day celebration, this includes the Sands Sky Park, the Event Plaza along Marina Bay, more shops, additional dining options and nightlife offerings, and the rest of the hotel room
5. SPECIALITIES
5.1 CASINO
Marina Bay Sands casino is housed in its own building and offers four levels of gaming in a luxurious, spacious setting. Patrons can enjoy a wide variety of popular table games including roulette, blackjack, baccarat and sic bo, and slot machines featuring the latest games such as video poker, and electronic sic bo and roulette. Premium players and Paiza members have access to exclusive gaming and dining privileges at the casino’s upper two levels, which feature over 30 private gaming rooms.
5.2 SKY PARK
The Sands Sky Park is an architectural masterpiece sitting on top of the three hotel towers at Marina Bay Sands. This 1.2 hectare tropical oasis is longer than the Eiffel Tower is tall and large enough to park four-anda-half A380 jumbo jets. It extends to form the one of the world’s largest public cantilevers. It is 200 meters in the sky. Landscaped gardens are home to 250 trees and 650 plants and 12,400 square meters of space big enough to fit three football fields.
5.3 INFINITY POOL
The Infinity Pool is part of the 380 metre long Sands Sky Park, which spans across the three towers of the Marina Bay Sands complex and covers a total area of 12,000 sq mts. The length of this roof-top deck can be equated to the height of the Eiffel Tower.
The Infinity Pool is one of the world’s largest outdoor pools at three times the length of an Olympic pool and 55 storeys high. The pool creates an illusion of the water extending to the horizon while in reality, the water spills over the edge into a catchment below and is then pumped back into the pool. It has two circulation systems, with the first filtering and heating the water in the main pool and the other, filtering the water in the catch basin and returning it to the upper pool.
Fig(5.1) Infinity Pool
6. CONCLUSION
Marina bay sand is a magnificent destination for entertainment, business, and shopping.
The success of project lies in the fact that the inventiveness of the design was matched by an equally inventive and novel approach developed by engineering and constructon teams.
Delivers once in lifetime experiences.
This landmark building is situated in the heart of Singapore’s central business district.
With a luxury hotel, state of art convention, exhibition facilities, and some of the best shopping &dining in the region.
This is the place to go for world class entertainment.
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