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This paper describes an experimental program conducted to investigate the effects of fluid-structure interaction on the modal dynamic response of three reduced-scale bridge pile foundations submerged partially or totally in water. The vibration periods of the specimens are measured for the two lateral modes and first torsional mode using ambient and forced-vibration tests. The results are presented and discussed as a function of surrounding water levels and the number and geometrical patterns of the piles. Three-dimensional (3D) finite element models of the tested specimens surrounded by different water levels are built and the results are successfully validated against the obtained experimental data. The built numerical models are used to compute 3D modal hydrodynamic pressures. A systematic analysis of the period ratios and 3D hydrodynamic loads is presented to characterize the effects of pile cap, water height and the number and geometrical pattern of the piles on dynamic response. The experimental and numerical results of this research allow a better understanding of the complex dynamically-induced fluid-structure interaction effects in the response of deep water bridge pile foundations.

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High-damping displacement spectra and corresponding damping reduction factors (η) are important ingredients for seismic design and analysis of structures equipped with seismic protection systems, as well as for displacement-based design methodologies. In this paper, we investigate η factors for three types of earthquakes characterizing seismic hazard in south-western British Columbia, Canada: (i) shallow crustal, (ii) deep inslab, and (iii) interface subduction earthquakes. We use a large and comprehensive database including records from recent relevant earthquakes, such as the 2011 Tohoku event. Our key observations are: (i) there is negligible dependence of η on soil class; (ii) there is significant dependence of η on the frequency content and duration of ground motions that characterize the different record types and (iii) η is dependent on period, particularly for inslab events. Period-dependent equations are proposed to predict η for damping ratios between 5% and 30% corresponding to the three event types. INTRODUCTIONElastic displacement spectra associated with damping levels higher than the conventional Daneshvar-2 equipped with energy dissipating and seismic isolation systems. High-damping displacement spectra are also required for displacement-based design and evaluation techniques, such as the Direct Displacement-Based Design method (Priestley and Kowalsky 2000;Priestley et al. 2007). Such displacement spectra can be determined using: (i) ground motion prediction equations (GMPEs) developed specifically for damping levels higher than 5%, or (ii) damping reduction factors, denoted hereafter by η, which are defined as the ratio between the 5%-damped displacement spectrum S d (T,5%) and displacement spectra S d (T,ξ) for higher damping levels ξ at a period T A number of GMPEs predicting spectral amplitudes at various damping levels have been proposed for different regions, e.g. Chen and Yu (2008) for western North America (WNA),and Akkar and Bommer (2007) and Cauzzi and Faccioli (2008) for Europe. These are useful in conducting probabilistic seismic hazard analysis to assess seismic hazard values for higher damping ratios. On the other hand, most guidelines and building codes adopt the approach of damping reduction factors (e.g. UBC-97, Eurocode8 2004, CHBDC 2006, ATC 2010, AASHTO 2010). An advantage of the latter approach is that these damping reduction factors can be applied directly to code-prescribed spectral amplitudes to evaluate damping effects.The main objectives of this work are: (i) to determine and characterize damping reduction factors corresponding to three event types contributing to seismic hazard in south-western British Columbia (BC), i.e. crustal, inslab, and interface events, and (ii) to propose model equations for the median of these damping reduction factors as a function of damping ratio, period, and soil class. The adopted procedure for developing such damping reduction factors for Vancouver is based on the evaluation of the damping reduction factors using various sets of ground mot...

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