Probabilistic seismic demand analysis, SMRF connection fractures, and near-source effects

Probabilistic Seismic Demand Analysis (PSDA) is an approach for computing the mean annual frequency (or annual probability) of exceeding a specified seismic demand for a given structure at a designated site. In short, PSDA combines a ground motion (e.g., spectral acceleration) hazard curve for the designated site, with demand (e.g., drift) results from nonlinear dynamic analysis of the given structure under a suite of earthquake ground motion records. PSDA is already at the core of two recent performance-based seismic guidelines, namely FEMA 350–353 for steel moment-resisting frame buildings, and the draft ISO Offshore Structures Standard. In this dissertation, PSDA is applied, extended, and used as a framework to study two topics in structural engineering that have recently received major attention, particularly since the Northridge earthquake of 1994.; Prompted by damage found after the Northridge event, the first topic of research is the effect of brittle fractures of welded beam-column connections on the seismic performance of steel moment-resisting frame (SMRF) buildings. In an effort to concisely quantify these effects across a range of ground motion intensity levels, PSDA is applied for several SMRF buildings modeled with either brittle or ductile beam-column connections. As a general rule, the additional effects of connection fractures are found to depend on the structural demand level relative to the connection capacities against fracture. An extension of PSDA that quantifies the safety of an earthquake-damaged building that has been only partially inspected for fractured connections is also developed.; Again stimulated by the Northridge earthquake, the second topic of research is how to account for the effects of near-source ground motions on nonlinear structural response in assessing the performance of a building. For several SMRF buildings, it is demonstrated that the nonlinear response to near-source ground motions can be significantly different than it is for ordinary (i.e., non-near-source) ground motions. Accounting for these differences entails certain modifications to the customary PSDA approach used in assessing structural performance. In particular, new ground motion intensity measures (IM's) and criteria for choosing between alternative IM's are introduced, which when employed in PSDA are demonstrated to ensure its accuracy at a near-fault site.