Random vibration analysis of long-span bridges under non-uniform seismic excitations by explicit time-domain method

For long-span bridges, the spatial variability effects of seismic excitations should be taken into account. In view of the non-stationarity of seismic excitations, to improve the computational efficiency, it is necessary to carry out the random vibration analysis of long-span bridges in the time domain with non-uniform ground motion. Based on the relative motion method, the explicit time-domain expression of the dynamic response of a structure under non-uniform seismic excitation was first deduced, and an explicit time-domain method was then proposed for fast calculation of the statistical moments of structural responses under non-uniform seismic excitations. Meanwhile, using the explicit time-domain expressions of dynamic responses, an efficient Monte-Carlo simulation method was further proposed for obtaining the mean peak values of structural responses and for analyzing the dynamic reliability of the structure with non-uniform ground motion. Taking a long-span suspension bridge with a main span of 1200m as a practical example, the random vibration analysis of the bridge under longitudinal non-uniform seismic excitations was conducted in the present study. The traveling-wave effect, the incoherence effect and the local site effect on the standard deviations and mean peak values of critical responses and structural seismic dynamic reliability were studied, respectively. The results show that, for the longitudinal displacements at the mid-span section of the bridge girder and at the top sections of the main towers, the standard deviations and mean peak values of the responses under non-uniform seismic excitations are smaller than those under uniform seismic excitation. However, for the internal forces at the bottom sections of the main towers, the standard deviations and mean peak values of the responses under non-uniform seismic excitations may be larger than those under uniform seismic excitation, and the mean peak value of the bending moment and the shear force may be 21.6% and 19.5% larger, respectively. In addition, it can be observed that the spatial variability effects of seismic excitations have great influences on the failure probability of the bridge system. © 2019, Editorial Office of China Civil Engineering Journal. All right reserved.