Modeling of steel moment frames for seismic loads

Abstract

Simple elastic models based on centerline dimensions of beams and columns are widely used for the design of steel moment resisting frames. However, for the performance prediction and evaluation of these structures, different nonlinear models are being used to better simulate their true behavior. Simple nonlinear modeling methods widely used as well as those with more detailed modeling representations are investigated and compared. A 9-story building and a 20-story building were designed for this study according to the 1997 NEHRP provisions. Different models for these structures were developed and analyzed statically and dynamically. The models investigated involved the use of centerline dimensions of elements or clear length dimensions, nonlinear springs for the beam connections, and linear or nonlinear springs for the panel zones. A second group of models also incorporated the fracturing behavior of beam connections to simulate the pre-Northridge connection behavior. Two suites of ground motions were used for the dynamic analysis: typical California and near fault ground motions. The differences in structural responses among different models for both suites of motions are investigated. According to static pushover analyses with roof displacement controlled, the benefit of the increase in capacity that results from the detailed models is consistently observed for both the 9-story and 20-story buildings. When the models were excited by different ground motions from each suite, the median responses of the more detailed models showed an increase in capacity and a decrease in demand as expected. However, due to the randomness inherent in the ground motions, variations were also observed. Overall, the model which incorporates clear length dimensions between beams and columns, panel zones and an equivalent gravity bay without composite action from the slab seems to be a practical model with appropriate accuracy.