Civil Engineering Association

Effects of ground motion spatial variations and random site conditions on seismic responses of bridge structures

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. 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 one-dimensional seismic wave propagation theory. 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.