11.49: Lifetime seismic performance assessment of steel frame structures

Abstract

ABSTRACTSteel Concentrically Braced Frames (CBFs) are stiff, lightweight, potentially low‐cost structures that efficiently resist lateral load through diagonal bracing members. Under dissipative seismic design principles, these bracing members are allowed to behave inelastically during infrequent high‐intensity earthquakes. In Eurocode 8 based design the extent of this inelasticity is effectively controlled by the behaviour factor, q. Increasing the behaviour factor results in a greater degree of nonlinear behaviour, however doing so allows for reduced member sizes and consequently reduced initial costs.The conventional aim of seismic resistant CBF design has been solely to ensure life safety given the occurrence of an appropriate maximum earthquake event. However, like most structures, when designed in accordance with modern codes, CBFs are unlikely to suffer collapse. Yet, significant losses can still be incurred as a consequence of seismic action, primarily due to non‐structural damage caused by smaller, more frequent events. Hence, the overall suitability of a particular CBF design, compared to either other structural forms such as moment resisting frames or alternative CBF solutions, cannot be adequately assessed by examining life safety requirements alone. This paper therefore considers damage to both structural and non‐structural elements caused by seismic events across the entire hazard range. The expected damage, and the resulting losses, over the expected lifetime of the facility are calculated probabilistically, by applying a lifetime seismic performance assessment procedure. Thus, comprehensive comparisons are made between alternate frame designs.Using the OpenSees computational platform to perform non‐linear time‐history analysis and the PACT performance assessment tool to estimate expected losses, this work investigates the impact of the behaviour factor on the lifetime seismic performance of low rise CBFs designed to Eurocode 8. Alternative Moment Resisting Frame (MRF) designs are also examined. On a general level, the study demonstrates how performance assessment can be employed to improve and optimise design. More specifically to CBFs, it is demonstrated that the ideal behaviour factor is not necessarily the largest value recommended by the code. The work also highlights the significance of losses suffered by acceleration‐sensitive non‐structural components and the consequent importance of limiting floor accelerations as well as storey drifts in order to fully exploit the damage limiting potential of CBFs.