Abstract:
Asynchronous input motion has been identified to cause unfavorable response for long structures particularly bridges. Based on observed damage patterns from previous earthquakes and many previous analytical studies, it is seen that asynchronous motion may play significant role in these observed damage patterns.
In this study, three bridge FEM models having different span lengths (75m, 100m and 125m) are subjected to different cases of both synchronous and asynchronous motions. Time history analysis for asynchronous motion with time lag of 0.01 s, 0.05 s, 0.1 s, 0.2s, 0.5s and 1.0s have been conducted. The results show that displacement at longitudinal direction, remains constant along the long direction of bridge for all the cases of synchronous and asynchronous motion. In all cases of displacement in vertical direction, asynchronous (0.5s time lag) motion governs for all the bridges. For all the cases of transverse displacement, synchronous motion is found to be maximum. 100m span module is modelled such that it resembles a 7-span module of the Bangabandhu bridge. Damping properties of seismic devices is determined such that suggested maximum displacement is achieved for earthquake load by equivalent static method. For other bridges, stiffness properties are taken proportionally.
El-Centro earthquake have also been used in this study. For this data input, asynchronous motion with 0.01s time lag governs in longitudinal displacement for 75m and 100m span module. For 125m span module synchronous motion governs as usual. For displacement in vertical direction, mostly asynchronous with time lag 1.0s was found to be governing in that case for 100m and 125m bridge. For 75m span module, asynchronous 0.5s time lag governs.
In all the cases of bridges without any seismic isolation, displacement due to asynchronous motion governs and significantly exceeds the allowable displacement. Thus, designing for situations, where seismic isolations malfunction, may not be feasible. As 100m span module is similar to that of a 7-span module of The Bangabandhu multipurpose bridge, it can be seen that present bearings are essential for proper functioning of the bridge during seismic event. If seismic isolation does not properly function during major earthquake, severe structural damage may occur due to asynchronicity of motion. Thus, maintenance of the bearings is critical for the proper seismic performance of The Bangabandhu Multipurpose bridge.
