Studies On Seismic Analysis Of Large-span Bridges Subjected To Multi-component And Multi-support Earthquake Excitations

Both theoretical research and earthquake damage analysis indicate that earthquake excitations are complex multi-component and multi-support movements. Multi-component refers to the six components of earthquake including three transiational components and three rotational components. Multi-support, also known as non-uniform excitation or spatially variability of earthquake, refers to the different excitations of different supports of the structures. It includes three aspects: wave passage effect, incoherence effect and local site effect. As to large-span bridges, it is significant to analyze the seismic performance under multi-component and multi-support excitations. The five aspects of work done in this thesis are listed as follows.(1) A wavelet-based procedure is proposed to generate artificial multi-dimensional accelerograms whose response spectra are compatible with three-dimensional target spectra. Through wavelet transform, the recorded time histories are scaled to match the target response spectra. Fast wavelet transform method are introduced to improve calculation efficiency. Then, random multi-component and multi-support model is established. The coherency between different components of different supports is analyzed using the SMART-I array accelerogram records. The results show that the coherency between different components of different supports is low, and can be assumed as a constant independent of frequency. Finally, the method to generate multi-component multi-support artificial time histories is given.(2) As to curved bridge, the method to determine the critical angle in time history analysis is investigated. The formula to decide the angle is proposed. Through two individual time history analysis, the unfavourable direction of one dimensional and two-dimensional earthquake can be determined. Then, the combination rules for orthogonal effects of time history analysis, such as the SUM rule, 100/30,100/40 percentage rules and the SRSS rule are also examined. The results show that these combination rules can't guarantee conservative results. And the paired acceleration time histories along the critical angle should be used in the analysis.(3) According to the MSRS method proposed by Kiureghian, the MATLAB program of seismic response under multi-component and multi-support excitation is established. The seismic performance of large-span cable-stayed bridge under multi-component and multi-support excitations is studied. The results show that: the amplification effect of vertical multi-support excitation is the largest, and longitudinal multi-support excitation takes second place. Transverse multi-support excitation has smallest effect on the bridge. As to wave passage effect, incoherence effect can be omitted. Pseudo-static effect has very limit effect on the total response. The coherency between different components of different and same supports should be considered in order to guarantee the safety of the bridge. Simple calculation method is recommended to account for the multi-component and multi-support effect. Finally, the method using genetic algorithm is proposed to find the maximum value of multi-component and multi-support response spectrum method.(4) The dynamic characteristics of large-span curved and linear continuous rigid frame bridge with high pier under multi-component and multi-support earthquake are compared. The influence of 'curve' to the seismic performance of the bridge type is discussed. Then, curvature, the setup of straining beam, the height of pier and the span number of the bridge to the seismic response of the bridge type are also analyzed. The results indicate that: the influence of 'curve' can amplify the seismic response of main girder, and has little effect on the seismic response of piers; the setup of straining beam can amplify the response of the bridge; Increasing the height of the bridge can decrease the seismic response of bridge; span number is not sensitive to the seismic response of the bridge piers; the effect of multi-component and multi-support can decrease the response of the pier and increase the response of the beam.(5) Semi-active control model of bridge under multi-component and multi-support earthquake is established using SIMULINK toolbox. The semi-active analysis of curved bridge under multi-component and multi-support excitation is studied. The emphasis is placed on influence of multi-component and multi-support and the excitation angle to the mitigation rate. The results indicate that the mitigation rate can be decreased by multi-component multi-support effect of earthquake, and the effect of excitation angle and multi-support excitation are coupled. In addition, based on the model of LRB isolated bridge established, the influence of spatially ground motion on the longitudinal seismic response of the system is studied using nonlinear time history analysis. The influence of wave passage effect, incoherence effect and local site effect are studied. The results indicate that the seismic response can be underestimated using the traditional assumption of uniform excitation. And the local site effect has largest detrimental influence on the bridge; wave passage effect takes the second place; incoherence effect takes the least.