Seismic Response Analysis Of Large Span Bridge With Input From Synthesized Ground Motion Field

The non-consistent inputs at multi-supports of large span cable based bridgefrom synthesized ground motion field for seismic analysis is deal with in thisdissertation. The data of synthesized motion field from an earthquake withmagnitude7.0on an active fault in Western China, and design data of a larg espan cable based bridge in another site are collected and processed. Scenarios ofthe bridge at several locations in the area covered by the motion field are set, sothat the difference among seismic responses could be analyzed and summarized.The bridge model is built by the function of finite element software ANSYS, thefree vibration characteristics of the bridge is analyzed by Block Lanczos methodof ANSYS. The result of the first20order modes shows that the stiffnessin-plane of the bridge is larger than that out-plane, the seismic effects in verticaldirection, in lengthwise and in lateral direction are contributed from the first, thesixth and the second order mode respectively. The first and the twentieth freevibration frequencies are0.06808Hz and0.54392Hz respectively.9pairs of ground points are selected from the motion field, the accelerationmotions in three directions, lengthwise, lateral and vertical, are input at the baseof each supporting tower, by means of Large Mass Method of ANSYS.Responses of the bridge are calculated from inputs consistent, traveling wave andnon-consistent at the bases of the two supporting tower of the main span.The response results of the bridge on a line parallel to the strike of sourcerupture plane at distance500m from the projected segment on ground surface ofthe upper margin of the plane show that the lengthwise displacements of the mainbeam from the traveling wave input and non-consistent input are frequentlylarger than the corresponding displacements from consistent inputs, while thelateral displacements are obviously smaller than those from consistent inputs.For example, the lateral peak displacement of the main beam at the middle ofmain span from traveling wave input with virtual velocity500m/s is just0.415time of that from consistent input. The vertical displacements from three kinds ofinputs are almost the same. The horizontal displacements at the tops of towersfrom the non-consistent inputs are a little bit bigger, while the vertical ones are alittle bit smaller than the corresponding results from the consistent inputs. The internal forces from the non-consistent inputs are larger than those from theconsistent input, as1.638times of the latter.The response results of the bridge on a line parallel to the strike of sourcerupture plane at distance2000m from the projected segment show that thelengthwise displacements of the main beam from the traveling wave input andnon-consistent input are also larger, while the lateral displacements are smallerthan those from consistent inputs. The lateral peak displacement of the mainbeam at the middle of main span from traveling wave input could be just0.538time of that from consistent input. The vertical displacements from three ki nds ofinputs are also similar. The horizontal displacements and the vertical ones at thetops of towers from the non-consistent inputs are respectively a little bit biggerand smaller than the corresponding results from the consistent input. Themaximum axial forces from the non-consistent inputs could be2.212times ofthose from consistent input.The results of the bridge on a line perpendicular to the strike of ruptureplane show quite different from the latter two that the lengthwise and the lateraldisplacements of the main beam from the traveling wave input andnon-consistent input are all smaller than those from consistent inputs. The lateralpeak displacement of the main beam at the middle of main span fromnon-consistent input could be just0.506time of that from consistent input. Thevertical displacements from three kinds of inputs are all the same. The most ofinternal forces from the non-consistent inputs are larger than those from theconsistent inputs. The maximum axial forces of the main beam at middle of themain span from the non-consistent inputs could be2.075times of those fromconsistent input.In general, the results from the non-consistent inputs at multi-supports arefrequently in between those from the traveling wave inputs with vi rtual velocities500m/s and2000m/s respectively, the latter is close to the results from theconsistent input.The displacements of the bridge from the non-consistent inputs at multi-supports with acceleration time histories synthesized at9ground poi nt pairs arequite different. For examples, the lengthwise peak displacement at the top oftower from inputs of point pair2is just0.293times of the corresponding valuefrom inputs of point pair1, one from the inputs of point pair5is3.742times ofthe corresponding value from inputs of point pair4. The internal forces of the bridge are also quite different. The bedding moment of the tower base frominputs of point pair3is0.558times of that of point pair1, one from the input ofpoint pairs5is2.761times of that of point pair4. The results in this dissertationdemonstrate the fact that the seismic response of a large span bridge near tosource rupture is strongly influenced by the spatial variation of ground motionsfrom a large earthquake.