A stress ribbon bridge is a tension structure, similar in many ways to a simple suspension bridge. The stress ribbon design is rare. Few people including bridge engineers are familiar with this form and fewer than 50 have been built worldwide. The suspension cables are embedded in the deck which follows a catenary arc between supports. Unlike the simple span the ribbon is stressed in compression which adds to the stiffness of the structure. Such bridges are typically made from concrete reinforced by steel tensioning cables. They are used mainly for pedestrian and cycling traffic. Stress ribbon bridges are very economical, aesthetical and almost maintenance free structure. They require minimal quantity of materials. At present studies, on combining stress ribbon bridges with cables or arches, to build most economical stress ribbon bridges. It makes the study of features of these particular bridges as an important one.
2. FORRM OF A STRESS RIBBON BRIDGE
3. COMPARISON WITH A SIMPLE SUSPENSION BRIDGE
4. CONSTRUCTION TECHNIQUES
6. MODIFIED STRESS RIBBON BRIDGES
7. A CASE STUDY
The stress ribbon is a special type of suspension bridge where in the cables are embedded in the deck just below the walking surface. Thus the bridge follows a catenary profile and sags between supports. They use the theory of a catenary transmitting loads via tension in the deck to abutments which are anchored to the ground. This concept was first introduced by a German engineer Ulrich Finsterwalder. The first stress ribbon bridge was constructed in Switzerland in the 1960s. The new bridge at Lake Hodges is the sixth ribbon bridge in North America, with three equal spans of 330 feet is the longest of this type.
1.1 Finsterwalder’s Stress Ribbon Bridge Theory
The stress ribbon bridge combines a suspended concave span and a supported convex span. The concave span utilizes a radius of about 8200 ft while the convex span, depending on the design speed of the bridge, utilizes an approximate radius of 9800 ft (1965).
The stress ribbon itself is a reinforced concrete slab with a thickness of about 10 inches (25.4cm). This reinforcement consists of three to four layers of 1 inch (2.5cm) to 1 ¼ inch (1.2cm) diameter, high strength steel. The layers are spaced so that the prestressing pipe sleeve couplings can be used as spacers both vertically and horizontally. To resist bending moments from traffic, the slab is heavily reinforced at the top and bottom in the transverse direction.
The high strength steel tendons are stressed piece by piece during erection to produce the desired upward deflection radius of 8200 feet (2500m) under dead load of the superstructure plus the pavement. A temporary catwalk is provided to stress the first tendons. The formwork for the bridge is hung from the tendons and then removed once the concrete is cured. Concrete is placed from the middle of the freely hanging 63 suspended concave part and continues without interruption to the supports (Finsterwalder 1965).
2. FORM OF A STRESS RIBBON BRIDGE
Figure 1: Form of a stress ribbon bridge
A typical stress ribbon bridge deck consists of precast concrete planks with bearing tendons to support them during construction and separate prestressing tendons which are tensioned to create the final designed geometric form. The joints between the planks are most often sealed with in-situ concrete before stressing the deck. The prestressing tendons transfer horizontal forces in to the abutments and then to the ground most often using ground anchors. The tendons are encased in ducts which are generally grouted after tensioning in order to lock in the stress and protect them from corrosion. Since the bending in the deck is low, the depth can be minimised and results in reduction in dead load and horizontal forces in abutments.
The abutments are designed to transfer the horizontal forces from the deck cables into the ground via ground anchors. Pedestrians, wind and temperature loads can cause large changes in the bending moments in the deck close to the abutments and accordingly crack widths and fatigue in reinforcement must be considered. The ground anchors are normally tensioned in 2 stages, the first step is tensioned before the deck is erected and the rest, after the deck is complete. If stressed in one stage only, there will be a large out of balance force to be resisted by the abutments in the temporary case. The soil pressure, overturnings and sliding has to be checked for construction as well as permanent condition.
2.3 Ground Conditions
The ideal ground condition for resisting large horizontal forces from the ribbon is a rock base. This occurs rarely but suitable foundations can be devised even if competent soils are only found at some depth below the abutments. In some cases where soil conditions do not permit the use of anchors, piles can also be used. Horizontal deformations can be significant and are considered in the design. It is also possible to use a combination of anchors and drilled shafts. Battered micropiling is another alternative which can resist the load from the ribbon because of its compression and tension capacity.
3. COMPARISON WITH A SIMPLE SUSPENSION BRIDGE
A stress ribbon bridge is a tension structure similar in many ways to a simple suspension bridge. The suspension cables are embedded in the deck which follows a catenary arc between the supports. As opposed to suspension bridges, where the cables carry the load, in stress ribbon, by tensioning the cables and the deck between abutments, the deck shares axial tension forces. Unlike the simple span the ribbon is stressed in compression, which adds to the stiffness of the structure. A simple suspension span tends to sway and bounce. The supports in turn support upward thrusting arcs that allow the grade to be changed between spans, where multiple spans are used.
Such bridges are typically made from concrete reinforced by steel tensioning cables. Where such bridges carry vehicle traffic a certain degree of stiffness is required to prevent excessive flexure of the structure, obtained by stressing the concrete in compression. Anchorage forces are unusually large since the structure is tightly tensioned.
4. CONSTRUCTION TECHNIQUES
The construction of the bridge is relatively straight forward. The abutments and piers are built first. Next the bearing cables were stretched from abutment to abutment and draped over steel saddles that rested atop the piers. The bearing tendons generally support the structure during construction, and only rarely is additional false work used. Once the bearing cables were tensioned to the specified design force, precast panels were suspended via support rods located at the four corners of each panel. At this point the bridge sagged into its catenary shape.
The next step was to place post tensioning ducts in the bridge. The ducts were placed directly above the bearing cables and support rods, which are all located in two longitudinal troughs that run the length of the bridge. After the ducts were in place, the cast-in place concrete was placed in the longitudinal troughs in small transverse closure joints. Concrete is poured in the joints between the planks and allowed to harden before the final tensioning is carried out. Retarding admixtures may be used in the concrete mix to allow all the concrete to be placed before hardening occurs. Once the final tension has been jacked into the tendons and the deflected shape is verified, the ducts containing the tendons are grouted.
After allowing the cast in place concrete to cure and achieve its full strength, the bridge was post tensioned. The post tensioning lifts each span, closes the gap between the panels, puts the entire bridge in to compression and transforms the bridge in to continuous ribbon of prestressed concrete.
5. ADVANTAGES AND APPLICATIONS
· Stress ribbon pedestrian bridges are very economical, aesthetical and almost maintenance free structures.
· They require minimal quantity of materials.
· They are erected independently from existing terrain and therefore they have a minimum impact upon the environment during construction.
· They are quick and convenient to construct if given appropriate conditions, without falsework.
· A stress ribbon bridge allows for long spans with a minimum number of piers and the piers can be shorter than those required for cable stayed or suspension bridges.
5.2 Applications of stress ribbon principle
· Ecoduct: A tunnel which was built as part of a large network of motorways outside Brno. The theory is the same as aself anchored arch but the geometry is much more complex. It is 50m wide and spans 70m a finite element programme was used in its design.
· Stuttgart trade fair hall roof: The suspended asymmetric roof comprises a regular repetition of stressed trusses with individual I-beam ribbons of steel between them. The trusses function as strut and tie A-frames based on concrete strip foundations and are tied back to the ground with anchors. The stresses in the ribbons and weight of its ‘green roof’ were used to resist wind uplift.s
6. MODIFIED STRESS RIBBON BRIDGES
One disadvantage of the traditional stress ribbon type bridges is the need to resist very large horizontal forces at the abutments. Another characteristic feature of the stress ribbon type structures, in addition to their very slender concrete decks, is that the stiffness and stability are given by the whole structural system using predominantly the geometric stiffness of the deck. At present research on the development of new structures combining classical stress-ribbon deck with arches or cables is being carried out.
6.1 Stress ribbon bridges stiffened by arches
The stress ribbon deck is fixed in the side strut. Both the arches and struts are founded on the same footings. Due to the dead load the horizontal force both in the arch and in the stress ribbon have the same magnitude, but they act in opposite directions. Therefore the foundation is loaded only by vertical reactions. This self anchoring system allows a reduction in the cost of the substructure.
The arches serves as a saddle from which the stress ribbon can rise during post tensioning and during temperature drop, and where the bond can rest during a temperature rise. In the initial stage the stress ribbon behaves as a two span cable supported by the saddle that is fixed to end abutments. After post tensioning the stress ribbon with the prestressing tendons, the stress ribbon and arch behaves as one structure.
6.2 Stress ribbon bridges stiffened by cables
The second type of studied structure is a suspension structure formed by a straight or arched stress ribbon fixed at the abutments. External bearing cables stiffen the structure both in the vertical and horizontal directions. Horizontal movements caused by live load are eliminated by stoppers, which only allow horizontal movement due to temperature change and shrinkage of concrete.
Support of the deck in a horizontal direction provided by a stopper was designed and analyzed during the study and development of this structural type. This device allows horizontal movement due to the creep and shrinkage of concrete. At the same time the devices stops horizontal movement due to short term loads like a live load, wind load or earthquake. Deck deflections and bending moments are reduced to zero or very small horizontal movement. Natural frequencies and mode shapes were also determined during dynamic analysis. The influence of the aforementioned structural arrangements on frequencies and mode shapes were studied. The structure allows one to place an observation platform at midspan. But dynamic behaviour is influenced by platform positioning, weight and area. For this reason the aerodynamic stability of the structure was checked in a wind tunnel.
Figure 3: Stress Ribbon Bridge stiffened by cables
7. A CASE STUDY
Location: Over Lake Hodges, San Diego, USA
Length: 3 spans of 330 feet
This is the world’s longest stress ribbon bridge. Earlier, there was only a 9 mile road connecting the north and south sides of the lake. Bicyclists and pedestrians had to use the shoulder for travelling to and fro from work. Now this elegant structure keeps pedestrians and bicyclists of the freeway without exacting a toll on the environment or visual landscape.
The firm behind this evaluated a broad range of bridge types that might be viable for this location. They included the pre-fabricated steel truss design, various concrete girder alternatives, a laminated timber bridge in which glue is used for lamination (“glulam”) and such long-span alternatives as cable-stayed and suspension bridges. The steel ribbon concept was also considered. Steel truss, concrete and glulam have bulky super structure and long span concepts were avoided due to the very high towers. It was quite clear that the chosen bridge type had to have the following features:-
· Minimal environmental effects.
· A long span with a minimum number of piers in the lake.
· An ability to be constructed above water without false-work.
· A visual effect so minimal that the structure would blend into the landscape.
· The design should work well in both dry and wet conditions.
After considering the above options, aesthetically and functionally stress ribbon design was the perfect choice.
Stress ribbon bridges are a versatile form of bridge, the adaptable form of structure is applicable to a variety of requirements. The slender decks are visually pleasing and have a visual impact on surroundings giving a light aesthetic impression. Post tensioned concrete minimises cracking and assures durability. Bearings and expansion joints are rarely required minimising maintenance and inspections. There are also advantages in construction method, since erection using pre-cast segments does not depend on particular site condition and permits labour saving erection and a short time to delivery. Using bearing tendons can eliminate the need for site form work and large plant, contributing to fast construction programmes and preservation of the environments. There is a wide range of different topographies and soil conditions found and a number of areas which require aesthetic yet cost effective pedestrian bridges to be built: Stress ribbon bridges could provide elegant solutions to these challenges.
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4. Tyson Dinges, (2009), History of prestressed concrete: 1888 - 1963