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Effect of long duration ground motions on the structural response of RC shearwall buildings Fairhurst, Michael


This dissertation investigates the effect of ground motion duration on the seismic performance and safety of reinforced concrete (RC) shearwall buildings. Long duration ground motions are characteristic of recordings from subduction tectonic regimes – such as the Cascadia Subduction Zone off of the Pacific Northwest coast of North America. However, current North American building code provisions use design spectra to quantify seismic demands on structures - this method does not account for the effect of shaking duration. This dissertation is primarily focused on the impact ground motion duration has on the performance of RC shearwall buildings. 2-dimensional and 3-dimensional numerical models of RC shearwall structures are developed and analyzed using suites of motions with different duration characteristics. It is found that code-level performance (ground motions with a 2% in 50 year probability of exceedance) of these structures is not significantly affected by the duration of the input motions. However, the collapse capacities (ability to withstand shaking levels above the design level) of the structures are impacted by ground motion duration: short duration ground motion suites tended to require higher shaking intensity levels (~20% higher; quantified through response spectral values) to induce collapse, compared to longer duration record suites. A method was developed to develop suites of ground motion records that match a target response spectrum, as well as the variability of that spectrum. The method relies on spectral matching techniques and is a modification of the variable target spectrum (VTS) method for matching the mean of a suite of motions to a target. Using this method, suites of motions are developed for use in risk based analysis – which requires prediction of structural response and the variability of that response. Using four mean and variance matched motion suites, the reliability of current code design provisions is investigated. It is concluded that current code seismic (R, RdRo) factors should be decreased by ~1/1.25 for structures subjected to long duration subduction motions in order to achieve similar collapse risks compared to similar analyses using shorter, crustal recordings. Current component factors appear to be suitable despite the increased demand variability from the longer motion suites.

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