r - The Hong Kong Polytechnic University

The 5th Cross-strait Conference on Structural and Geotechnical Engineering (SGE-5)
Hong Kong, China, 13-15 July 2011
SPATIAL GROUND MOTION MODELLING AND ITS EFFECT ON BRIDGE
RESPONSES
ABSTRACT
Hong Hao 1 and Kaiming Bi 2
1 School of Civil and Resource Engineering, the University of Western Australia,
35 Stirling Highway, Crawley WA 6009, Australia. Email: hao@civil.uwa.edu.au
2 School of Civil and Resource Engineering, the University of Western Australia,
35 Stirling Highway, Crawley WA 6009, Australia.
Earthquake ground motions at multiple supports of large dimensional structures inevitably vary owing to the
seismic wave propagation effect. It has been realized that this ground motion spatial variation affects structural
responses. Many researchers have studied ground motion spatial variation and its effect on structures. For
simplicity, most of previous studies assumed that the site is flat and homogeneous although it is known that
varying local site conditions will further enhance ground motion spatial variations. In this paper, an approximate
method is proposed to model and simulate spatially varying ground motions on surface of a non-uniform canyon
site. This method takes into consideration the combined wave passage effect, coherency loss effect and local site
effect, therefore leads to a more realistic modelling of spatial ground motions on non-uniform sites as compared
to the some simplified approaches in previous studies. The simulated multi-component spatially varying time
histories are then used as inputs to study the seismic pounding responses of a two-span simply-supported bridge
structure. The influence of torsional response induced eccentric poundings is highlighted in this study based on a
detailed 3D FE model. Numerical results demonstrate that the influence of local site effect on the seismic
ground motion spatial variations and torsional response induced eccentric pounding between adjacent bridge
spans, which are usually neglected in previous studies, are significant and should be considered in bridge
response analysis.
KEYWORDS
Ground motion simulation, local site effect, wave passage effect, coherency loss effect, torsional response,
eccentric pounding, 3D FEM.
INTRODUCTION
For large dimensional structures, such as long span bridges, pipelines, communication transmission systems, the
ground motions at different stations during an earthquake are inevitably different, which is known as the ground
motion spatial variation effect. There are many reasons that may result in the spatial variability in seismic
ground motions, e.g., the wave passage effect owing to the different arrival times of waves at different locations;
the loss of coherency due to seismic waves scattering in the heterogeneous medium of the ground; the site
amplification effect owing to different local soil properties. It has been proved that ground motion spatial
variations have great influence on the structural responses and in some cases might even govern the structural
responses (Saxena et al. 2000).The ground motion spatial variations are usually modelled by a
theoretical/semi-empirical power spectral density function and a coherency loss function. Many ground motion
spatial variation models have been proposed especially after the installation of the SMART-1 array in Lotung,
Taiwan. Zerva and Zervas (2002) overviewed these models. It should be noted that most of these models were
proposed based on the seismic data recorded from the relatively flat-lying sites. Taking different soil conditions
into consideration, Der Kiureghian (1996) proposed a theoretical coherency loss function, in which the ground
motion power spectral density function was represented by a site-dependent transfer function and a white noise
spectrum. Typical site-dependent parameters, i.e., the central frequency and damping ratio for three generic site
conditions, namely, firm, medium and soft site were proposed. The advantage of the model is that it can consider
different soil properties at different support locations and it is straightforward to use. The drawback is that it can
only implicitly represent the local site effects on ground motions. For example, it is well known that seismic
wave will be amplified and filtered when propagating through a layered soil site. The amplifications occur at
various vibration modes of the site. Therefore, the energy of surface motions will concentrate at a few
frequencies. The power spectral density function of the surface motion then may have multiple peaks. This
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