Seismic Demands for Performance Evaluation of Steel Moment Resisting Frame Structures

The objective of this work is to improve the knowledge base on the seismic behavior of typical steel moment resisting frame structures, considering regions of different seismicity and sets of ground motions of various intensities and frequency characteristics. The emphasis is on behavior assessment and quantification of global and local force and deformation demands for different hazard levels. The research is intended to contribute to progress in the development of performance-based seismic design and evaluation of steel moment resisting frame structures.

The behavior and response of different height structures in Los Angeles, Seattle, and Boston are studied. Analytical methods and models of various complexities are utilized and evaluated for their ability to predict global and local performance. A sensitivity study is performed on the effects of analysis assumptions on demand predictions. System level behavior characteristics and seismic demands are studied from the perspective of performance at different hazard levels. Local behavior characteristics and element deformation demands are evaluated for various designs, with consideration given to subjective design decisions, variations in material properties, and different types of beam-to-column connections. A simplified procedure for estimation of global and local seismic demands is developed to facilitate decision making in the conceptual design process.

This work is concerned only with the behavior of “ductile” structures; fracturing of connections is not considered. The results from this study demonstrate that for ductile code conforming structures the global seismic demands, measured in terms of story drifts, are mostly within the range of acceptable performance at the various hazard levels – with one important exception. This exception occurs when severe ground motions drive a structure into the stability sensitive range, in which case P-delta effects constitute a potential collapse hazard. Local (element) seismic demands are found to be very sensitive to a multitude of factors, which may result in a concentration of plastic deformation demands in either the beams or the panel zones, or in sharing of demands between these two elements and possibly also the columns.