UC Irvine

The current US seismic design codes are believed to do no justice to signify the effects of vertical component of ground motions. This study assesses the effects of the vertical component of ground motions on steel structures and evaluates the current seismic design provisions of ASCE 7 based on a structural reliability outlook. Eight SMRF and SCBF steel structures are 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 & shear in beams, axial load in braces, and axial load and moment in columns. Seismic provisions of ASCE 7 were developed with an intention to achieve target β of 1.75 for earthquake-resistant structural members. Results indicate 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, results in increased member sizes in SMRFs by which β values larger than 1.75 are 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.