The effect of near‐fault directivity on building seismic collapse risk

Abstract

SUMMARY Forward directivity may cause large velocity pulses in ground motion time histories that are damaging to buildings at sites close to faults, potentially increasing seismic collapse risk. This study quantifies the effects of forward directivity on collapse risk through incremental dynamic analysis of building simulation models that are capable of capturing the key aspects of strength and stiffness degradation associated with structural collapse. The paper also describes a method for incorporating the effects of near‐fault directivity in probabilistic assessment of seismic collapse risk. The analysis is based on a suite of RC frame models that represent both past and present building code provisions, subjected to a database of near‐fault, pulse‐like ground motions with varying pulse periods. Results show that the predicted collapse capacity is strongly influenced by variations in pulse period and building ductility; pulse periods that are longer than the first‐mode elastic building period tend to be the most damaging. A detailed assessment of seismic collapse risk shows that the predicted probability of collapse in 50 years for modern concrete buildings at a representative near‐fault site is approximately 6%, which is significantly higher than the 1% probability in the far‐field region targeted by current seismic design maps in the US. Copyright © 2012 John Wiley & Sons, Ltd.

Collapse Risk Evaluation of Self-Centering Steel MRFS With Viscous Dampers in Near-Fault Regions

“…For this reason, a second set of far-field records is needed for the determination of collapse probability for sites that are not influenced by forward directivity 305 effects. Similarly to [11], the set of far-field ground motions used in this study is based on the FEMA P695 [41] far-field ground motion set, which includes 22 record pairs, each with two horizontal components for a total of 44 ground motions. Those ground motions are recorded at sites located greater than or equal to 10 km from fault rupture; event magnitudes range from M 6.5 to M 7.6 with an average magnitude of M 7.0.…”

“…According to the NEHRP Site Class, 16 sites are classified as Site Class D (stiff soil sites) and the remaining are classified as Site Class C (very stiff soil sites). The wavelet classification algorithm developed by Baker [40] indicates that nine of these ground motion records have pulses in the velocity history [11]; these pulse-like record were then removed from the far-field database. It is worth noting that the ground motion records were selected without consideration of spectral shape.…”

“…Values of β Total are taken as 0.80, β RTR is calculated based on the IDA results and the β modeling is derived though Equation (1). This value of β Total has been adopted by other researchers [11] and seems to be logical for the risk assessment of frame systems. Note that the fragility curves, as shown in Figure 4(b), have not yet been adjusted to account for the distinct spectral shape of rare ground motions, characterized by the parameter ε [43].…”

“…The risk assessment procedure of the frames, when subjected to near fault ground motions, is modified properly to take into account the effect of the pulse period of the record, T p , and is based on the approach proposed by Champion and Liel [11]. This methodology is described in the following sections.…”

“…A recent study highlighted the need to account for such higher collapse risk by showing that the probability of collapse in 50 years for reinforced concrete buildings at a representative near-fault site is approximately 6%, i.e. significantly higher than the 1% limit [11].…”