Study On Seismic Response Of Large-Span Structures Subjected To Spatially Varying Ground Motions

The seismic properties of large-span structures, used as important public facilities, are always the focus research to be paid more attention all over the world. Due to the seismic excitations on supporting points of large-span structures are not uniform when earthquake occurrence, the effect of ground motion spatial variation on seismic response of large-span structures should be studied thoroughly. Considering the generation technique of spatially varying ground motion field still not perfect and the seismic response analysis of large-span multi-supported structures subjected to spatial ground motions are not comprehensive at present, seismic responses of several typical large-span structures, including large-span bridge structure, electrical transmission tower-line system and spatial steel structure, are analyzed under multi-support excitations based on the reasonable simulation of spatially varying ground motion field in this study. The influence of ground motion spatial variation induced by wave passage effect, coherency loss effect and local site effect on structure response are studied respectively, and the combined three effects are also studied. The main aspects of research work in this thesis are listed as follows:(1) Generation of spatially varying ground motion time histories corresponding to an uneven heterogeneous site. Based on traditional trigonometric series method, further research on generation technique of spatially varying ground motion is conducted. The ground motion spatial variations of the uneven site are simulated randomly based on an empirical coherency loss function and a filtered Kanai-Tajimi power spectral density function. The local site effect is considered by a transfer function derived from wave propagation theory. In view of heterogeneous site with multiple soil layers, the spatial ground motions of ground surface are simulated by the base rock motions propagating into the soil site, including out-of-plane wave and in-plane waves. With the trigonometric series method, the wave propagation theory and the random vibration method, the methods of simulating spatially varying ground motions corresponding to uneven homogeneous site and uneven site with multiple soil layers are performed, and the spatial ground motion time histories of the uneven site are generated. The generated spatial ground motions by this method considering ground motion spatial correlation, non-stationary property and local site effect, which can be used to analyze seismic response of large-span structures on an uneven site subjected to spatially varying ground motions. (2) The maximum relative displacement response between two adjacent bridge segments subjected to spatially varying ground motions is investigated, the relative displacement is the minimum gap that a modular expansion joint system must have to avoid pounding between bridge segments. The simulated spatially varying ground motions corresponding to four soil types are individually compatible with response spectrum defined in Chinese seismic design code, and are compatible with an empirical coherency loss function between each other. Numerical calculations of the relative displacement response between two adjacent bridge segments to spatial ground motions with different wave velocities and coherency losses are conducted, which can be used to determine the minimum gap that a expansion joint must have to avoid pounding between bridge segments on different site conditions. Moreover, the influences on maximum relative displacement response between two adjacent bridge segments induced by ground motion spatial variation, site condition and structural vibration characteristics are investigated. Numerical results indicate that the softer is the site, the larger is the relative displacement response between two adjacent structures. The effect of ground motion spatial variation on the relative displacement response is significant, especially when the vibration frequencies of two adjacent bridge segments are similar. To reduce the relative displacement and preclude pounding between adjacent structures, the design recommendation of adjusting the adjacent structures to have similar fundamental frequencies is valid only when the effect of ground motion spatial variation can be neglected.(3) Due to the large-span and high-rise flexible characteristics of a transmission tower-line system, the nonlinear responses of the structural system on an uneven site are analyzed subjected to spatially varying ground motions and comprehensive numerical simulations are carried out. A three-dimensional finite element model of the transmission tower-line system is established considering the geometric nonlinearity of transmission lines. Using the generated spatially varying ground motion time histories of the uneven heterogeneous site, seismic responses of the complex large-span structures on the uneven site under uniform excitation, wave-passage excitations and the multi-support excitations comprehensively considering ground motion spatial variation are analyzed. Discussions on the effects of the multi-component ground motions and the different local site conditions on responses of the example transmission tower-line system on uneven site with multiple soil layers are made. These investigations demonstrate the importance of considering the simultaneous multiple earthquake ground motion components and ground motion spatial variations on seismic responses of transmission tower-line system, which provide useful reference for practical seismic design of the structural system. (4) Seismic response of a large dimension steel trussed arch structure subjected to the combined spatially varying horizontal and vertical ground motions including site effect are analyzed, and the limitation span of trussed arch structure which is not required to consider the effect of ground motion spatial variation is investigated. The simulated spatial ground motions are individually compatible with the design response spectrum with 2% damping for specific site conditions defined in the Chinese seismic design code, and are compatible with an empirical coherency loss function between each other. The effects of ground motion spatial variations induced by wave propagation, coherency loss and changing site conditions on structural responses are discussed, respectively. The differences between structural responses subjected to horizontal ground motions and simultaneous vertical and horizontal ground motions are studied. Moreover, the responses of different span trussed arch structure on four site conditions defined in the design code are investigated under uniform excitation and multi-support excitations, and the internal forces of the structure induced by different excitation cases are compared. The minimum spans of trussed arch structure on four site conditions which are not required to consider the effect of ground motion spatial variation are given. The results obtained provide useful information and suggestion for practical design of steel trussed arch structure. Finally, the internal force responses of the supporting columns and upper bars in a large-span plate type truss structure induced by uniform excitation, wave-passage excitations and multi-support excitations are studied and some differences are reached. The analyzed results show that neglecting ground motion spatial variation may lead to a substantially under-estimated seismic response of large-span spatial truss structure.