In order to determine the equivalent shear modulus and effective damping ratio between each layer of the soil, free field analysis was carried out beforehand using the Shake analysis program. Subsequently, a three-dimensional dynamic analysis was carried out using the equivalent shear modulus and effective damping as input data. A structure-free field soil interaction model. Ground motions were applied to the model to simulate the earthquake case, and loads were generated in the piles and substructure accordingly.
Although other load combinations were considered, such as ship impact and wind, these effects were not found to be critical for the substructure, and the seismic load combination dictated the design. A further model was developed to investigate the global behavior of the bridge.
The bridge is divided into six span modules, each span meters feet long, so the global analysis model examined an individual six span module and applied different levels of scour at each pier.
A scour hole may form around an individual pier, or around two or more piers. The global model looked at various combinations in order to determine the critical axial, shear and bending loads on the foundations of any particular pier.
A three-dimensional global model for a 6-span bridge module. Initial studies of the bridge were based on the deck being supported by traditional sliding bearings, with the point of fixity being the central pier of the six-span module. To avoid the fixed pier being heavily loaded during a seismic event by a longitudinal translation, shock transmission units were proposed at the free piers to ensure even load distribution between the piers.
But under this system, the loads applied to the piers were still large; therefore, as part of the value engineering process, AECOM considered alternative forms of articulation. The original seismic design strategy was to dissipate seismic energy through plastic hinges at the bottom of the piers. Further design optimization identified the benefits of seismic isolation, which allows the structure to behave elastically without damage. The application of seismic isolation has reduced the number of piles, the size of the pile caps and the size of the steel superstructure, resulting in a more cost effective design.
Seismic isolation bearings have been used worldwide to mitigate seismic response by isolating structures from seismic input. They can accommodate thermal movements with minimum resistance, but will engage under seismic excitations. In this strategy, all primary structural members remain elastic without any damage or plastic hinging.
Principles of seismic isolation for Padma Bridge. Isolation bearings contain three key elements: one to provide rigidity under service loads and lateral flexibility beyond service loads, one to provide self-centering capability, and one to provide energy dissipation. These key elements have to be properly designed and fine-tuned to achieve optimal seismic behavior. Analyses indicate that seismic forces can be greatly reduced by replacing conventional pot bearings with isolation bearings.
Friction pendulum bearings utilize the characteristics of a pendulum to lengthen the natural period of the isolated structure so as to reduce the input of earthquake forces.
The damping effect due to the sliding mechanism also helps mitigate earthquake response. An earthquake can strike a piled foundation at its most vulnerable, when the piles have lost substantial embedment through deep riverbed scour. The Padma Bridge design pursued the 'displacement based approach' for seismic resilience and implemented seismic isolation between the superstructure girder and the pier-and-foundation system.
This approach enables the principal bridge components to move relative to one another under earthquakes; thereby dissipating the large forces. Friction pendulum bearings are used to permit large relative displacements between the bridge superstructure and the bridge piers.
The two-level steeltruss bridgewill carry a four-lane highway on the upper level and a single track freight railway on a lower level. 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 m-wide and km-long bridge stretches across the Padma river, one of the three major rivers in Bangladesh. The bridge is expected to help enhance regional trade and collaboration along the Asian highway No. The project's Chinese contractor MBEC in a paper said it attached utmost importance to environment protection during building the bridge, and adopted international standards concerning health, safety and environmental assessment, including guidelines by World Health Organization and Bangladesh's environmental authorities.
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