The spatial variation of seismic ground motions denotes the differences in the seismic time histories at various locations on the ground surface. This text focuses on the spatial variability of the motions that is caused by the propagation of the waveforms from the earthquake source through the earth strata to the ground surface, and it brings toge

[Truncated abstract] The research carried out in this thesis concentrates on the modelling of spatial variation of seismic ground motions, and its effect on bridge structural responses. This effort brings together various aspects regarding the modelling of seismic ground motion spatial variations caused by incoherence effect, wave passage effect and local site effect, bridge structure modelling with soil-structure interaction (SSI) effect, and dynamic response modelling of pounding between different components of adjacent bridge structures. Previous studies on structural responses to spatial ground motions usually assumed homogeneous flat site conditions. It is thus reasonable to assume that the ground motion power spectral densities at various locations of the site are the same. The only variations between spatial ground motions are the loss of coherency and time delay. For a structure located on a canyon site or site of varying conditions, local site effect will amplify and filter the incoming waves and thus further alter the ground motion spatial variations. In the first part of this thesis (Chapters 2-4), a stochastic method is adopted and further developed to study the seismic responses of bridge structures located on a canyon site. In this approach, the spatially varying ground motions are modelled in two steps. Firstly, the base rock motions are assumed to have the same intensity and are modelled with a filtered Tajimi- Kanai power spectral density function and an empirical spatial ground motion coherency loss function. Then, power spectral density function of ground motion on surface of the canyon site is derived by considering the site amplification effect based on the onedimensional seismic wave propagation theory. The structural responses are formulated in the frequency domain, and mean peak responses are estimated by the standard random vibration method. The dynamic, quasi-static and total responses of a frame structure (Chapter 2) and the minimum separation distances between an abutment and the adjacent bridge deck and between two adjacent bridge decks required in the modular expansion joint (MEJ) design to preclude pounding during strong ground motion shaking are studied (Chapter 3). The influence of SSI is also examined (in Chapter 4) by modelling the soil surrounding the pile foundation as frequency-dependent springs and dashpots in the horizontal and rotational directions. A method is proposed to simulate the spatially varying earthquake ground motion time histories at a canyon site with different soil conditions. This method takes into consideration the local site effect on ground motion amplification and spatial variations. The base rock motions are modelled by a filtered Tajimi-Kanai power spectral density function or a stochastic ground motion attenuation model, and the spatial variations of seismic waves on the base rock are depicted by a coherency loss function. The power spectral density functions on the ground surfaces are derived by considering seismic wave propagations through the local site by assuming the base rock motions consisting of outof- plane SH wave and in-plane combined P and SV waves with an incident angle to the site. The spectral representation method is used to simulate the multi-component spatially varying earthquake ground motions...

July 1989.