Occurrence of large earthquakes close to cities in California is inevitable. The resulting ground shaking will subject buildings in the near-source region to large, rapid displacement pulses which are not represented in design codes. The simulated Mw7.0 earthquake on a blind-thrust fault used in this study produces peak ground displacement and velocity of 200 cm and 180 cm/sec, respectively. Over an area of several hundred square kilometers in the near-source region, flexible frame and base-isolated buildings would experience severe nonlinear behavior including the possibility of collapse at some locations. The susceptibility of welded connections to fracture significantly increases the collapse potential of steel-frame buildings under strong ground motions of the type resulting from the Mw7.0 simulation. Because collapse of a building depends on many factors which are poorly understood, the results presented here regarding collapse should be interpreted carefully.

SUMMARYRayleigh damping is commonly used to provide a source of energy dissipation in analyses of structures responding to dynamic loads such as earthquake ground motions. In a ÿnite element model, the Rayleigh damping matrix consists of a mass-proportional part and a sti ness-proportional part; the latter typically uses the initial linear sti ness matrix of the structure. Under certain conditions, for example, a nonlinear analysis with softening non-linearity, the damping forces generated by such a matrix can become unrealistically large compared to the restoring forces, resulting in an analysis being unconservative. Potential problems are demonstrated in this paper through a series of examples. A remedy to these problems is proposed in which bounds are imposed on the damping forces.

High-rise flexible-frame buildings are commonly considered to be resistant to shaking from the largest earthquakes. In addition, base isolation has become increasingly popular for critical buildings that should still function after an earthquake. How will these two types of buildings perform if a large earthquake occurs beneath a metropolitan area? To answer this question, we simulated the near-source ground motions of a M(w) 7.0 thrust earthquake and then mathematically modeled the response of a 20-story steel-frame building and a 3-story base-isolated building. The synthesized ground motions were characterized by large displacement pulses (up to 2 meters) and large ground velocities. These ground motions caused large deformation and possible collapse of the frame building, and they required exceptional measures in the design of the base-isolated building if it was to remain functional.

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