Seismic Response of non-uniform Column Heights Bridge under Spatial Variation of Ground Motion and Local Soil Condition

Document Type : Original Research

Authors
D. Darvishpour
G. Nouri
Faculty of Engineering, Kharazmi University
Abstract
It is vital to consider the spatial variations of ground motions in the design of extended structures and long bridges. In this paper, the effect of spatial variations of ground motions and local site conditions on the response of non-uniform column heights bridge is studied. To generate non-uniform accelerometers of ground motion, a simulated algorithm based on the spectrum design with unstable multivariate random process functions and a spectral density matrix is used. Accelerometers were generated with a coherence function including the effect of wave propagation and the duration of the earthquake that is consistent with the selected response spectrum. In addition, the simulation is performed in 800 time intervals with a time step of 0.025 seconds. The maximum ground acceleration is assumed 0.35 g. The response of the bridge with a length of about 242 m with 5 spans under the effect of uniform and non-uniform accelerometers was investigated by nonlinear time history analysis in OpenSees program. The local site effect was assumed by changing soil type (soil under the two piers is softer than the other piers) and apparent wave velocity under different bridge piers. The apparent velocity of the wave propagation of the soft soil assumed 1000 m/s and for the hard soil 2000 m/s. To verify the acceleration of the generated accelerograms, the generated spectrum is compared with the Eurocode design spectrum, and to validate the analysis performed on the bridge, the ratio of M/ calculated and compared with ratio that calculated by Shinozaka and Deodatis. In this paper variations of axial force, shear force and bending moments in bridge piers in different positions were studied as comparison criteria. The results showed that the simultaneity of spatial variations of ground motions and changes in the soil conditions causes a significant increase in the bridge response. Comparison of the results in the two input cases of uniform and non-uniform spatial variations of ground motions shows that the properties of spatial variations of earthquake motions can affect the response of the bridge. The results are compared based on the ratio of the maximum stress created at the base in the non-uniform excitation state to the maximum created in the uniform excitation state or the ratio of the maximum stress created at the base in the variable soil to the same soil. Based on the presented results, it was observed that the maximum bending moments in variable soil conditions can be increased to about 2.5 times to the maximum created in the same soil condition in the piers and also the maximum axial force created in the two shorter piers in the non-uniform excitation state is up to 2 times larger than in the uniform excitation state, and if the effect of different soils is applied to the two middle piers, the axial force in the middle two piers can be increased up to 3 times. Based on the obtained results, it is observed that the maximum shear force created in the direction of the transverse axis in the two middle piers occurred in a situation where non-uniform excitation coincides with the change of soil conditions under the piers and the bending moment in the direction of the transverse axis in the piers in this case has increased up to 120%.

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Modares Civil Engineering journal
Volume 21, Issue 5
November and December 2021
Pages 119-131