Date of Award

6-2013

Degree Name

MS in Civil and Environmental Engineering

Department

Civil and Environmental Engineering

Advisor

Bing Qu

Abstract

Recent research reports that steel Concentrically Braced Frames (CBFs) (even the code-compliant ones) may be susceptible to soft-story failures during strong earthquakes. Such a failure mode causes catastrophic outcomes and should be definitely avoided in practice. This thesis focuses on development and validation of a seismic retrofit strategy for low-rise and mid-rise steel CBFs vulnerable to soft-story failures. The considered retrofit strategy consists of a sufficiently stiff rocking core (RC) pinned to foundation and connected to the existing frame. For demonstration purpose, two representative benchmark steel CBF buildings, which are the three-and six-story CBFs designed forLos Angelesin the SAC Steel Project, are considered. Finite element (FE) models of the benchmark buildings are validated using the published results and explicitly take into account gusset plates, member yielding, brace buckling, brace rupture, and P-Delta effect. Eigenvalue analyses are first conducted to investigate the effect of RC on system modal properties. It is found that the added RC generally does not significantly change the fundamental period and therefore does not attract excessive earthquake force to the system. Additionally, nonlinear static pushover analyses are performed to address the beneficial contribution of RC to the system under the performance objectives including immediate occupancy, life safety, and collapse prevention. The Monte-Carlo simulation technique is used to take into account uncertainty in lateral force distribution and its effect in system seismic performance. It is found that sufficiently stiff RC creates more uniform inter-story distribution along the vertical direction in all considered scenarios. Furthermore, nonlinear dynamic analyses are conducted using three different ground motion suites. It is shown that the systems with properly selected RC can achieve the Best Safety Objective defined in FEMA 356 and ensure the collapse prevention performance under near-fault earthquakes.

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