Multi-support Seismic Excitations Research Articles

A new spatial coherence model and analytical coefficients for multi-support response spectrum combination

In this paper, a new spatial coherence model of seismic ground motions is proposed by a fitting procedure. The analytical expressions of modal combination (correlation) coefficients of structural response are developed for multi-support seismic excitations. The coefficients from both the numerical integration and analytical solutions are compared to verify the accuracy of the solutions. It is shown that the analytical expressions of numerical modal combination coefficients are of high accuracy. The results of random responses of an example bridge show that the analytical modal combination coefficients developed in this paper are accurate enough to meet the requirements needed in practice. In addition, the computational efficiency of the analytical solutions of the modal combination coefficients is demonstrated by the response computation of the example bridge. It is found that the time required for the structural response analysis by using the analytical modal combination coefficients is less than 1/20 of that using numerical integral methods.

Comparison of random vibration methods for multi-support seismic excitation analysis of long-span bridges

In this paper, the spatial variability effects of ground motions on the dynamic behaviour of long-span bridges are investigated by a random vibration based spectral analysis approach and two response spectrum methods. The spatial variability of ground motions between the support points is taken into account with the coherency function, which arises from three sources: incoherence, wave-passage and site-response effects. Random vibration analyses are performed on two deck-type arch bridges and a cable-stayed bridge model. Power spectral density function and response spectrum values used in random vibration analyses are determined depending on the recordings of September 20, 1999, ChiChi, Taiwan earthquake. The results strongly imply that the filtered white noise ground motion model can be accepted as a rather convenient model to represent actual earthquake ground motions. It can be also observed that the structural responses for each random vibration analysis depend largely on the intensity and frequency contents of power spectral density functions.

Nonlinear Response of Bridges under Multisupport Excitation

The authors present the results of an extensive numerical study on the nonlinear response of bridges subjected to multisupport seismic excitation. Bridges of varying stiffness and ductility have been designed for nonsynchronous and synchronous motion, and subsequently, have been analyzed in the nonlinear range under nonsynchronous input motion. The purpose of this investigation was to assess the effects on the peak inelastic response of bridge structures and the extent of validity of the force reduction factor (q-factor) approach for the design. Findings revealed that displacement ductility demands in bridges designed by the q-factor method for a multisupport excitation were in good agreement with the chosen value of q. The bridges designed for synchronous input and then examined for nonsynchronous motion displayed an excess of strength in the central piers, whereas the opposite happened for those near the abutments, which may have displacement ductility demands larger than those requested under synchronous motion.