Reliability Analysis of Steel SMRF and SCBF Structures Considering the Vertical Component of Near-Fault Ground Motions

Abstract

This paper assesses the effects of the vertical component of ground motions on steel structures and evaluates the current seismic design provisions of ASCE 7 on the basis of a structural reliability outlook. Eight special moment-resisting frame (SMRF) and special concentrically braced frame braced frame (SCBF) steel structures were analyzed under two groups of near-fault (i.e., strike slip and reverse fault) ground motions. Detailed methodology for calculating the reliability index (β) of structural components is explained; β is computed for structural component actions including moment and shear in beams, axial load in braces, and axial load and moment in columns. Seismic provisions of ASCE 7 were developed with an intention of achieving target β of 1.75 for earthquake-resistant structural members. Results indicated that the implementation of current seismic load combinations, in which a load factor equal to 20% of design level short-period spectral acceleration on structural dead load is used to account for the effects of the vertical component of ground motions, results in β∼1.3 for most structural members. Application of structural system drift limits, however, resulted in increased member sizes in SMRFs by which β values larger than 1.75 were achieved. It is concluded that current seismic load combinations in ASCE 7 are inadequate to account for the effects of the vertical component of near-fault ground motions. Nevertheless, performance-based design provisions can provide a reasonable and adequate margin of safety against structural member failure.