Simulation Of Earthquake Motion Based On Spatial Variable Effects And Seismic Response Of Suspension Bridge

Ground motion is a random process in both space and time. Its time-dependent propertieshave been more thoroughly studied, but the variabilities in space have been not. For commonbuildings, it is reasonable that the spatial variation of ground motion is neglected in seismicanalysis because the size of foundation along horizon is comparatively small. However, forextended structures, the spatial variation of ground motion may produce relatively great effecton the response of structures. If this variation is not considered, the calculated results will beunreliable in seismic design of structures.The seismic response spectra analysis adopted in the design criterion of bridges onlyconsiders the effect of the uniform earthquake excitation. But in real earthquakes, the realground motion inputs of every bearing points of the bridge are different because of thediversities of the seismic sources, propagation paths of earthquake waves and local siteconditions. Therefore, it's necessary to analyze the bridges under the multi-point excitations.(1) The important parameters and models used for composing time-history of groundmotion are introduced, and then the advantages and disadvantages of various models and theirapplicability are discussed.(2) Ground motion simulation. The ground motion simulation of single-point excitationgenerally used in earthquake engineering is firstly introduced, and then it begins to transfer tothe multi-point excitation. The coherence function is used to reflect the coherence of seismicsources and measured points, and the apparent velocity is used for the multi-point simulationof wave passage effect, but they can't accurately simulate the coherence and the local siteeffects. Then the equivalent power spectrum is adopted to simplify the power spectrum matrix,but the coherence of composed time-history of ground motion is not good. A new method ofsimulation is proposed. It is developed based on the ground motion simulation considering thespatial coherence. It can simulate the time-history of ground motion according with therequirement of multi-point excitation and able to reflect the nonstationarity of thetime-frequency domain of ground motion, the local site effects, partial coherence effects andwave passage effects.(3) The three methods of seismic response analysis under multi-point excitations, such asresponse spectra analysis, time-history analysis and random vibration analysis, are evaluated.Then the motion equations of dynamic time-history analysis of non-linear spatial seismic response of bridges and the relationship between excitation and response in random vibrationanalysis are deduced.(4) The seismic response analysis of suspension bridges. The present models used fordynamic analysis of suspension bridges are firstly introduced. For the finite element analysisof multi-point excitation response, the large mass method is combined with the concept ofpseudo-static displacement. The motion equation of time-history analysis of absolute structuredisplacement is deduced theoretically and its realization in numerical computation is given.Comparing with the former relative displacement method, it can obtain the absolute structuredisplacement considering the translation of the foundation and it's more convenient for theengineers to realize it in the general finite element programmers.