Quantifying Changes in Structural Design Needed to Account for Aftershock Hazard

Aftershocks have the potential to cause severe damage to buildings and threaten life following a major earthquake. However, their effect on seismic hazards is not explicitly accounted for in modern building design codes, nor in emerging methodologies such as performance-based seismic design. The objective of this study was to develop a methodology that can quantify the changes that would be needed in the structural design of a building to account for aftershock (AS) hazards and illustrate it using a basic nonlinear model of a building. In other words, what changes to a structural design would be needed such that the building has the same collapse probability for the combined mainshock and aftershock (MS + AS) hazard as the collapse probability for the original building, subjected to the mainshock (MS) only? The total collapse probability is computed using a combination of seismic fragility results convolved with the two types of hazard curves, namely, a typical hazard curve and an AS hazard curve. An illustrative example is presented for a two-story woodframe building and the change in structural design needed for this scenario is found to be an approximately 10% increase in both stiffness and strength for the first and second stories. Although this is illustrated on only one building, it demonstrates that further work related to consideration of AS hazards may be justified. (C) 2015 American Society of Civil Engineers.