Study of the dynamic behabiour of the Jamuna multi-purpose bridge under different seismic and vehicular loading

Abstract

The Jamuna Multipurpose Bridge is the longest and most important bridge in Bangladesh which has established the long cherished road link between the East and West of Bangladesh. The bridge is located il} a seismically active region. Seismic pintles have be",n used for seismic protection of the bridge. The bridge was instrumented with accelerometers in order to monitor the performance of the seismic devices as well as overall dynamic behavior. An earthquake data were recorded on 17th June 2004 by sensors installed on the bridge as well as by a number of Free Field Stations. The main objective of the present study is to analyze response of a Finite Element Model (FEM) of the Jamuna Multipurpose Bridge which was developed previously, due to that earthquake and compare the modeJ's response with the actual response recorded by the bridge sensors. It is observed that the peak response in acceleration, velocity and displacement of the bridge in the FEM analysis always comes later than the recorded data on the bridge and the peak acceleration at all directions is higher in the FEM model than the actual responses. The peak acceleration in actual response is 58% of the FEM response in the longitudinal direction, 12% in the transverse direction and 30% in the vertical direction. The peak velocity in actual response is 61% of the FEM response in the longitudinal direction, 32% in the transverse direction and 52% in the vertical direction. The peak displacement in actual response is 85% of the FEM response in the longitudinal direction, 69% in the transverse direction and 60% in the vertical direction. Response from the FE model clearly shows the predominant frequency of the structure and also the predominant frequency of the earthquake, but as the underlying soil has been ignored in the model, it does not show the predominant frequency of the soil. On the other hand, the actual response as recorded on the bridge shows the predominant peak of the soil and also that of the structure but does not clearly show the predominant frequency of the earthquake because the intensity of this particular earthquake was very low compared to the effect of the ambient vibration. The prime objective of the study is to predict the response of the Jamuna Multipurpose Bridge due to different earthquake loads and compare the model's response with the results of a XX11l previous analysis using Transfer Ratio Function. The response of the FE model is much less than the response of the Transfer Function. This is because of the nonlinear behaviour of the bridge which is included in the FE model. The nonlinear behaviour of the bridge was not included in transfer function analysis. The FE model provides an idea about how much displacement may be reduced due to nonlinear behaviour. In-fact if soil-structure interaction is considered in the model, the estimated response of the bridge might be further reduced. It is also found that duration of an earthquake event is very important for the bridge response. Weight and speed of a vehicle are major concern for the bridge. Study has been conducted to observe the behavior o{ the bridge due to vehicular movement in different weight and speed limit. Train induces a lot of vibrations in the bridge when it is in higher speed. Response of the bridge due to heavily loaded truck with high speed is greater than that of the low speed train. When these kinds of trucks frequently move on the bridge their response will be a major concern for this bridge. Two weIght groups of vehicle can be noticed. One group weighs between II to 15 tons, which are buses and other group of vehicles 19to 22 tones, which are trucks. From the response between two weight groups less overlapping in acceleration is found in the longitudinal direction and less overlapping in velocity is found in the transverse direction. Thus acceleration data of the longitudinal direction and velocity data of the transverse direction may be used for detection of overweight vehicles from the bridge response. If the response of the bridge exceeds 2.05 cm/sec2 in the transverse direction then the vehicle may exceed the weight limit of 20 tons or the speed limit of 40 km/h or the both. On the other hand, if the response of the bridge exceeds 0.70 cm/sec2 in longitudinal direction then the vehicle may exceed the weight limit of 20 tons or the speed limit of 40 km/h or the both. Then the vehicle can be identified. Previously train used to move at higher speed, therefore response of bridge was higher. Later the train is restricted to run in a lower speed. Then the response of the bridge is lower than the previous response. Response due to two broad gauge rails is also studied. They both have a speed of 17 km/h. But the response of presently recorded data due to train movement is larger than the previously recorded data. This may be due to the recently occurred cracks in the bridge.