6.15 kilometers (3.8 miles) in length, the Padma Bridge is a landmark structure and one of the longest river crossings in the world. The Padma River is the third largest river in the world, and has the largest volume of sediment transport.
During monsoon seasons, the Padma River becomes fast flowing and is susceptible to deep scour, requiring deep-pile foundations for bridge stability. The Padma Bridge site is also in an area of considerable seismic activity, resulting in significant earthquake forces being exerted on the bridge. This combination, together with other forces of nature, posed a unique challenge.
The multipurpose Padma Bridge detailed design project has been successfully completed. AECOM developed alternative concrete deck forms, including an extradosed concrete truss bridge, a concrete girder bridge and a steel truss bridge. In all cases, a two-level structure was chosen, having significant advantages over a single level structure. These included segregated highway and railway envelopes to offer enhanced safety, improved operation, inspection, maintenance, and emergency evacuation procedures, as well as efficient provisions for utilities. With the railway in the lower deck, the structural depth beneath the railway is reduced, allowing the lengths of the railway approach viaducts for tie-in at the north and south banks to be minimized. With a two-level structure the construction cost is reduced, making the structure more efficient.
Analytical models were developed for each of the bridge forms to determine member sizes and, in particular, the weight of the superstructure. The steel truss bridge was found to be the most efficient with the lightest deck. Further details of this option were developed to determine the optimum span length. Total deck weight and foundation loads were compared for span lengths of 120 meters, 150 meters and 180 meters (394 feet, 492 feet and 591 feet, respectively). From this data, a construction cost was estimated for each span length with the optimum span being 150 meters. In conclusion, the most economic and appropriate form for the bridge was found to be the steel truss bridge with a concrete top slab acting compositely.
The multipurpose bridge also has many utilities built into it, including a gas pipeline, telecommunications and a high-voltage power transmission line. Additionally, it has emergency access points in order to facilitate evacuation of a train on the lower deck.
A detailed study of seismic hazard at the site was performed to determine suitable seismic parameters for use in the design. Two levels of seismic hazard were adopted: Operating Level Earthquake and Contingency Level Earthquake. Operating Level Earthquake has a return period of 100 years with a 65 percent probability of being exceeded during that period. Contingency Level Earthquake has a return period of 475 years with a 20 percent probability of being exceeded during a 100-year bridge life period. Any damage sustained from such an earthquake would be easily detectable and capable of repair without demolition or component replacement.
In conjunction with these investigations, AECOM carried out further analysis to determine the optimum foundation design, and two pile types were investigated; large diameter (3 meter; 10 feet) raking steel tubular piles and large diameter cast-in-situ concrete bored piles. Raking piles were more efficient in resisting lateral loads resulting from earthquake motions. This type of load is resisted as axial force in the steel piles, while the lateral load is resisted by the flexural capacity of the piles for the concrete bored piles. The very large bending moments generated by a seismic event dictated that insufficient flexural capacity could be created by reinforcement alone, and a permanent steel casing would be required to enhance the capacity down to 10 meters (33 feet) below the riverbed level, which for a 100-year scour event would be -61m PWD. It would also be necessary to have more than fifteen 3-meter (10 feet) diameter vertical concrete piles, compared to eight raking steel tubular piles. The large number of piles increased the weight of the pile cap and also the local scour. All of these factors had an adverse effect on the cost and constructability of the foundations, therefore the preferred solution was recommended as being the raking steel tubular piles.
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