- Geometry Control During Construction (Deflection and Camber)
- Crack Control During Construction (Stress Check)
- Structural Safety and Stability During Construction
- Control of Stresses in Completed Structure
- Control of Strength of Completed Structure
Evolution of Concrete Bridge Geometry
Simple Span & Support
Later the concept of Continuous Span Bridges came up, where more economic section utilization was possible. To save construction time Precast Prestressed concrete sections (PCPS) are castand carried to job site from casting yard; erected over pier and finally spliced over supports. Needless to mention these sections are pretensioned. It helps to establish continuity for live load distribution to achieve longer spans (generally upto 40m). Compared to Single Span bridges, sudden change of slopes can be avoided which allows improved maintenance and quality of ride. However, potential possibilities of crack formation at the joints (non-prestressed) still exists.
To overcome the potential of crack formation, the concept of post-tensioning got introduced (PCPS with Post-Tensioning over the Supports only). It allows the engineer to achieve following advantages:
- Continuity established for live load
- Better performance of joints due to presence of precompression at the joints
- Reduction of Dead Load Moment
- Thinner, and longer spans
However, still at this stage Bridge geometry was restricted by non-technical parameters like limitation of transportation in length and width of members. Moreover, weight of sections were limited by the available crane capacity and lifting equipment. Hence the new concept got introduced when small pieces are casted and transported to job site; erected in place and hold with bracket to be spliced. Longer spans of 100 meter or more are achievable in this process and no of piers can be reduced to achieve economy.
About Segmental ConstructionAs its name implies, a segmental bridge is a bridge built in short sections (called segments), i.e. one piece at a time, as opposed to traditional methods that build a bridge in very large sections. The bridge is made of concrete that is either cast-in-place (constructed fully in its final location) or precast concrete (built at another location and then transported to their final location for placement in the full structure).
These bridges are very economical for long spans (over 100 meters), especially when access to the construction site is restricted. They are also chosen for their aesthetic appeal.
- The first segmental concrete bridge, built in 1950, was cast-in-place across the Lahn River in Balduinstein, Germany.
- The first precast segmental concrete bridge, built in 1962, crossed the Seine River in France.
- The first U.S. precast segmental concrete bridge, built in 1973, in Corpus Christi, Texas.
- The first U.S. cast-in-place segmental bridge, built in 1974, was built near San Diego, California.
- The first U.S. precast segmental oncrete arch bridge is the Natchez Trace Parkway Bridge, completed in 1993.
ConstructionThe sequence of construction is similar to traditional concrete bridge building, i.e. build the support towers (columns), build the temporary falsework, build the deck, and perform the finishing work. The principle differences are as follows:
- The support towers may be built segmentally. Often this is accomplished using “slip-form” construction, where the falsework moves (slips) upward following sequential concrete “pours.” The falsework uses the newly constructed concrete as the basis for moving upward.
- After the towers are built, a superstructure is built at top of the towers. This superstructure serves as the “launching” point for building the deck. (The deck is often built in both directions away from the tower, simultaneously.)
- The deck is now constructed sequentially, beginning at the tower, one section at a time.
- In cast-in-place bridges, the falsework is connected to the previously installed concrete and allowed to cantilever freely. Next, the permanent reinforcing steel and supports are installed. Finally, the concrete is placed and cured, freeing the falsework to be moved.
- In pre-cast bridges, the concrete segment is constructed on the ground, and then transported and hoisted into place. As the new segment is suspended in place by the crane, workers install steel reinforcing that attaches the new segment to preceding segments. Each segment of the bridge designed to accept connections from both preceding and succeeding segments.
- The process in step 3 is repeated until the span is completed.
- Guiniguada Viaduct, Madrid, Spain
- Aswan Cable Stayed bridge in Egypt span 250m
- Lane Ave Bridge, Ohio 110 m long Cable stayed bridge
- Benicia-Martinez NW Ramp, California, for CALTRANS, balanced cantilever, length 211m, 4 spans
- Ramirez de Arellano Guideway Puerto Rico for H2A Engineers, post-tensioned railroad viaduct, 161m, 5 spans
- Bayou Chico Channel over SR292 in Florida, composite construction, 168m, 3 spans
- Arch bridge with cable stay construction, Sommerset, Florida, 163m
- Arch bridge over river Beas and Viaducts at Ramshila (NH-21) on Kulu bypass in Himachal Pradesh, arch base approx 100m
- Ganga Bridge, Allahabad (NHAI Project)
- Monorail people movers in Las Vegas, Seattle and presently in Dallas
- Delhi Gurgaon Expressway, Delhi (Continuous Box Girder Bridge)
- Cantilever Bridge Over River Chenab between Bakkal and Kauri in Jammu & Kashmir
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- Presentation by Dr. Bijan O. Aalami