Parameter study of the response of moment-resisting steel frame buildings to near-source ground motions

Abstract

A parameter study is performed to investigate the effects of strong near-source ground motions from moderate-size earthquakes (Mw=7) on moment-resisting steel frame buildings. Two buildings, one 6 stories and the other 20 stories, are subjected to the Olive View Hospital free-field record from the Northridge earthquake and other ground motions from three earthquake simulations (Northridge earthquake, a hypothetical Mw7.1 Elysian Park blind-thrust earthquake, and another blind-thrust earthquake of Mw7.0). Parameters examined for the buildings include material yielding, weld fracture, presence of slab, accumulation of damage from a second earthquake, and vertical ground motion. A significant fraction of the ground motions cause excessive amounts of deformations in the buildings, especially the 6-story one, even for the case where all welds are assumed to be perfect. These ground motions exceed the earthquake representation in the code, and, since they appear to be reasonable motions that should be considered in design, the implication is that the code design force levels need to be raised for locations in near-fault regions. Including weld fracture increases the displacements of the building and the potential for severe damage or collapse from column failure or excessive lateral sway. The buildings collapsed in several of the analyses. Including the floor slab increased the amount of column yielding and did not improve the behavior. A second earthquake such as a strong aftershock or subsequent main shock is a concern, especially if many welds are cracked from the initial event. However, a limited study of vertical ground motion showed it to be of minor importance. Strongly nonlinear building behavior is sensitive to many assumptions about features which are poorly understood, both structure and ground motion, and so the results need to be carefully interpreted.