Thursday, May 16, 2013

TUBULAR SYSTEM IN TALL BUILDINGS

1. INTRODUCTION Advancement of technology and the development of economy of world have brought the new era of construction of high rise buildings. Many structural systems have been adopted in the design of high rise buildings. One of them is tubular structural system. The tallness of a building is relative and cannot be defined in absolute terms either in relation to height or the number of storeys. But from a structural engineer’s point of view, tall building can be defined as one, that by virtue of its height is affected by lateral forces due to wind or earthquake or both to an extent that they play an important role in the structural design. 1.1 NEED OF HIGH BUILDING The land available for buildings is becoming scarce resulting in rapid increase in the cost of land. The result is multi-storeyed buildings as they provide large floor area in a relatively small area of land in urban centers. The construction of multi-storeyed buildings is dependent on available materials, the level of construction and availability of services such as elevator necessary for use in the building. Three major factors to be considered in design of such structures are strength, rigidity and stability. Two ways to achieve these requirements are: • by increasing the size of the member to achieve strength requirement • to change the form of the structure to something more rigid and stable 2. COMMON TYPES OF STRUCTURAL SYSTEMS IN TALL BUILDINGS • Moment resisting frame system • Braced frame system • Shear wall system • Advanced structural forms- tubular systems 2.1 RIGID FRAMES It is a system that utilizes the moment resisting connection between the columns and beams throughout its perimeter to resist lateral loads applied. It may be used to provide lateral load resistance for low rise buildings. Generally it’s less stiff than other systems. This is also known as sway frames or moment frames. 2.2 BRACED FRAME To resist lateral deflections, the simplest method from theoretical standpoint is intersection of full diagonal bracing or X bracing. It works well for 20 to 60 storey height but does not give room for opening such as door and windows. To provide more flexibility for placing of doors and windows, K bracing system is preferred. If we need to provide larger openings, we can use full storey knee bracing system. It is found to be efficient in energy dissipation during earthquake loads by forming plastic hinges in beam at the point of intersection of bracings with the beams. 2.3 SHEAR WALLS The lateral loads are assumed to be concentrated at floor levels. The rigid floors spread these forces to the columns or walls in the building. Specially designed reinforced concrete walls parallel to directions of load are used to resist a large part of the lateral loads caused by winds or earthquakes by acting as deep cantilever beams fixed at foundation. These elements are called shear walls. They extend over the full length of the building. Frequently, buildings have interior concrete core walls around the elevator, stair and service wells. Such walls may be considered as shear walls. The above systems discussed are not efficient for building greater than 60 storeys 2.4 ADVANCED STRUCTURAL FORMS They are: • Framed tube structure (a) • Braced tube structure (b) • Tube in tube structure (c) • Bundled tube structures (d) (d) 3. TUBE Tube is a system where in order to resist lateral loads (wind, seismic etc.) a building is designed to act like a hollow cylinder cantilevered perpendicular to ground. This system was first introduced by Fazlur Rahman Khan. The first example of a tube’s use is 43-storey khan designed Dewitt-chestnut apartment building in Chicago. Fazlur Khan defined a framed tube structure as “a 3 dimensional space structure composed of 3,4 or possibly more frames, braced frames or shear walls joined at or near their edges to form a vertical tube like structural system capable of resisting lateral forces in any direction by cantilevering from the foundation” This laid the foundation for tube structural design of many later skyscrapers including John Hancock Centre, Wilis tower, W.T.C, Petronas tower 3.1 CONCEPT OF TUBULAR SYSTEM The main idea of tubular system is to arrange the structural elements so that the system can resist the loads imposed on the structure efficiently particularly the horizontal loads. In this arrangement several elements contribute to the system i.e. slabs, beams, girders, columns. Unlike most often, the walls and cores are used to resist the horizontal loads, in tubular system the horizontal loads are resisted by column and spandrel beams at the perimeter of the tubes. Many tall buildings have adopted this system and the very first building designed using tubular concept was sears tower. The exterior framing is designed sufficiently strong to resist all lateral loads on the building, thereby allowing the interior of the building to be simply framed for gravity loads. Interior columns are comparatively few and located at the core. The distance between the interior and the exterior is spanned with beams or trusses and intentionally left column free. This maximizes the effectiveness of the perimeter tube by transferring some of the gravity loads within the structure to it and increases its ability to resist overturning due to lateral loads. Tubular structure is a structure with closed column space between two to four metres and joined by deep spandrel beam at the floor level Group of columns perpendicular to the direction of horizontal load is called flanged frame and group of columns parallel to the direction of horizontal load is called web frames. Since the columns are close to each other and the spandrel beams are deep, the structure can be considered as perforated tube and behaves as cantilevered tube. The flanged frame columns will resist the axial forces (tension and compression) and web will resist the shear forces. Taking a pure rectangular tube, as shown in figure 1. The thickness of the wall is t, the length of tube is b and width is d. The contribution of flanged frame and web to resist the horizontal load for pure rectangular tube can be obtained by calculating exact moment of inertia as When MI of flanges about their own axis is neglected, the MI of tube becomes For square tube, b=d, then the section modulus becomes Stress at extreme fiber where M is the overturning moment at the floor level and d is the between the two extreme fibers Portion of overturning moment carried by flange Portion of overturning moment carried by web Therefore, it is obvious that the largest portion of overturning moment is carried by flanges i.e. 75% of M and the remaining 25% by webs. This is due to the fact that Z of the square tube is a function of the square of the distance between the extreme fiber and width of high rise buildings is usually large. Hence, tubular system is an efficient system to resist horizontal loads. 4. FRAMED TUBE STRUCTURES Frames consist of closely spaced columns, 2 to 4 m between centers joined by deep girders. The idea is to create a tube that will act like a continuous perforated chimney or stack. The lateral resistance of framed tube structure is provided by stiff moment resisting frames that form a tube around the perimeter of the building. The gravity loading is shared between tube and the interior columns. This structural form offers an efficient, easily constructed structure appropriate for buildings having 40 to 100 storeys. W.T.C When lateral loads act, the perimeter frames aligned in the direction of loads acts as the webs of massive tube cantilever and those normal to the direction of the loading act as the flanges. Even though framed tube is a structurally efficient form, flange frames tend to suffer from shear lag. This results in the mid face flange columns being less stressed than the corner columns and therefore not contributing to their full potential lateral strength. Aesthetically the tube looks like the grid like façade as small windowed and is repetitious and hence use of fabrication in steel makes construction faster. E.g.: Aon Centre and W.T.C towers 5. BRACED TUBE STRUCTURES Further improvement of tubular structure can be made by cross bracings the frame with X-bracings over many storeys. As the diagonals of the braced tube are connected to the column at each intersection, they virtually eliminate the effects of shear lag in both flange and web frames. As a result, the structure behaves under lateral loads more like a braced frame reducing bending in the members of the frame. JOHN HANCOCK BUILDING Hence spacing’s of columns can be increased and the depth of girders will be less, thereby allowing large size windows than in conventional framed tube structures. In braced tube structures, the braces transfer axial load from more highly stressed columns to less highly stressed columns and eliminates difference between load stresses in columns. E.g.: Chicago’s John Hancock building, The Citigroup Center, Bank of China Tower 6. TUBE IN TUBE STRUCTURES This is a type of framed tube consisting of an outer-framed tube together with an internal elevator and service core. The inner tube may consist of braced frames. The outer and the inner tubes act jointly in resisting both gravity and lateral loading in steel framed buildings. However, outer tube usually plays a dominant role because of its much greater structural depth. This type of structures is also as hull and core structures. 7. BUNDLED TUBE The bundled tube system can be visualized as an assemblage of individual tubes resulting in multiple cell tube. System allows for greatest height and most floor area. E.g.: Sears Tower In this system, introduction of internal webs greatly reduces the shear lag in the flanges. Hence their columns are more evenly stressed than in the single tube structures and their contribution to the lateral stiffness is greater. SEARS TOWER 8. ADVANTAGES OF TUBULAR SYSTEMS IN TALL BUILDINGS • Offers some clear advantage from materials standpoint. Designed well, tubular forms have been known to utilize the same amount of material as would have been employed for a structure that is half as large or framed conventionally. • Allows greater flexibility in planning of interior space since all the columns and lateral system is concentrated on the perimeter of structure. This allows a column free space in the interior • Regularity in the column schedule allows off-site fabrication and welding where speed can be achieved while still confronting to quality • Wind resisting system since located on the perimeter of the building meant that maximum advantage is taken of the total width of the building to resist overturning moment • Identical framing for all floors because floor members are not subjected to varying internal forces due to lateral loads 9. CRITICISM TO TUBULAR STRUCTURAL FORMS • While it allows high rise construction with greater efficiency, it significantly reduces the size of the opening in the building. So while occupants are working or living at greater heights, their view of world outside is rather obstructed. • The finite rigidity of the girders and the connections also lead to a deviation from the standard linear distribution of axial forces across the flange and web columns assumed in beam theory. This non linearity in axial force distribution is known as shear lag and is an effect that has to be accounted for in the design of tubular structures since it has an effect on overall lateral stiffness of the structure. 10. SHEAR LAG EFFECT In actual tubular structure, the distribution of axial forces along the flanged frame columns at one floor is not uniform and the distribution of shear forces along the web is not linear. This is mainly due to the flexibility of the tubular structures and is called shear lag effect Along the flanges, this non linearity can result in the corner or exterior columns experiencing greater stress than the center or interior columns. This is known as positive shear lag. However negative shear lag has been discovered to exist and this is opposite of positive shear lag and the corners are less stressed than the center columns. This deviation from traditional beam bending behaviour naturally has implications on bending stiffness of the built up structure. The presence of shear lag thus prevents the full potential in terms of rigidity of the structure to be adopted. In order to qualify the magnitude and presence of shear lag, two measures have generally been used • Involves the ratio of slopes of the stress distributions with and without shear lag for web panels • Ratio of stress between the corner and the center column for flange panels. In either case, as the ratio gets to unity, shear lag ceases to exist. 11. STRUCTURAL ANALYSIS Strength of tubular form arises from the utilization of the entire building perimeter to resist lateral loads. The goal is for the entire perimeter to function compositely and behave as a large cantilever beam anchored rigidly into the ground. In order to achieve this composite behavior, columns are closely spaced with deep spandrel beams connecting them. However due to the thinness of the tube wall and finite rigidity of spandrel beams, the assumption of classical beam bending are violated and structure cannot be accurately analyzed as pure cantilever beam bending beam. Two assumptions in traditional bending theory which are violated by the structural behavior of tubular structures are 1) Plane section remaining plane 2) Linear stress distribution in web Under lateral loading, plane sections do not remain plane due to differential elongation of columns along the flange which result from local deformation of connecting spandrels. This leads to non-linear axial stress distribution in both flange and web panel which is called shear lag. Side view of axial deformation of flange of frame’s causing shear lag effect Another reason why tubular structure cannot be readily analyzed as a cantilever beam is the occurrence of shear deformation. The finite rigidity of columns and spandrel beams give the structure a finite shear rigidity that allows shear deformation to occur. This shear mode of deformation is ignored in classical bending theory. Hence analysis of tube structures is based on 3 dimensional analysis using finite element. 12. CONCLUSION 1. The tubular systems are one of the most effective way of construction of high rise buildings 2. They are extensively used in the construction nowadays 3. The Burj Khalifa, which is the tallest building uses the bundled tube design concept 13. REFERENCES 1. Schuller. W., (1976): "High-rise building structures", John Wiley & Sons 2. Taranath. S. B., (1984): "Structural analysis and design of tall buildings", McGraw-Hill Book Company 3. Smith. B. S., and Coull. A., (1991): "Tall building structures: Analysis and Design",John Wiley & Sons. 4. Khan, F.R. (1973). Evolution of structural systems for high-rise buildings in steel and concrete. Tall Buildings in the Middle and East Europe:Proceedings of the 10th Regional Conference on Tall Buildings-Planning, Design and Construction. Bratislava: Czechoslovak Scientific and Technical Association.

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