Study of dynamic parameters of the Jamuna multipurpose bridge

Abstract

The Jamuna Multipurpose Bridge over the mighty Jamuna River has established the long cherished road link between the East and West of Bangladesh. The bridge is expected to promote better inter-regional trade, economic and social development and play a very significant role in the socio-economic development of the country. However, the bridge is located in a seismically active region. The bridge has seismic pintles for protection against a design ground acceleration of 0.2 g. However it is important to study the dynamic properties of the bridge. For this purpose, the bridge was instrumented with sensors in order to monitor the dynamic behavior. In the present study the earthquake data recoded by the sensors are used to analyze the response of the bridge. The amplitudes of bridge response due to the application of earthquakes of different magnitudes are evaluated in this study. For this purpose, at first dynamic parameters are identified by ambient vibration assuming simplified geometry and deflection parameters of the bridge. Finally, a multi degree of freedom system is formulated to determine a Transfer Ratio function of the system. The concept of Transfer Ratio (TR) allows one to use the actual recorded data to determine the system behavior without any simplified assumption regarding the geometry of the bridge. Using the earthquake data recorded by the sensors a Transfer Ratio function is derived and used to predict possible response of the Jamuna Bridge. This Transfer Ratio is verified and also applied for the earthquakes of February 14 and August 5, 2006 recorded near the Jamuna Bridge. For case study, the earthquake of EI Centro earthquake, Imperial Valley, California, USA in 1940 and Mexico City Earthquake, September 19, 1995 have been applied on the Transfer Ratio function to investigate the response of the bridge to predict any seismic event. One of the prime objectives of the present study is to develop a detailed finite element model of the bridge considering all features of the bridge geometry such as horizontal and vertical curvature of the bridge, variable deck thickness etc. For this study eight different types of models are developed. Modal analyses of the generated models are carried out. It is observed that prestressing has no significant effect on modal periods. To make the model simple hollow section at the top of the piers can be ignored as it has a very little effect on modal frequencies. The piers can also be modeled using shell elements instead of solid elements as both the models give similar results. It is observed from the study that the model with solid elements shows slight increase in modal frequency. To model the exterior rail girder and diaphragm frame elements can also be used. It will increase the modal value insignificantly. From modal analysis it is observed that the transverse vibration occurred at the modal frequency of 1.00174 Hz in its 3'd mode which almost matches with the frequency of 1.00098 Hz of the Transfer Ratio. The goal of this study is to determine bridge behavior under VariOUSdynamic loads. A comparative study of bridge response has been carried out at various locations under various dynamic loads such as ambient vibration, traffic vibration, combined train and traffic vibration, only train vibration and earthquake. From the responses of bridge under various dynamic loading, it is observed that there is a common peak frequency at around 1.013Hz which almost mach with the predominant frequency in transverse vibration as obtained from the Finite Element Model (1.00174Hz) and the Transfer Ratio (1.00098Hz).