Evaluation of the seismic level of protection afforded to steel moment resisting frame structures designed for different design philosophies

Steel moment resisting frames have been considered as excellent systems for resisting seismic loads. However, after recent earthquakes (e.g., Northridge, California, in 1994 and Kobe, Japan, in 1995) the confidence in this structural system was reduced as a result of various types of damage that moment resisting steel frames suffered. This paper presents the results of the evaluation of seismic level of protection afforded to steel moment resisting frame buildings designed in accordance with the National Building Code of Canada. Six- and 10-storey office buildings located in a region of intermediate seismic hazard are designed in accordance with the current Canadian code provisions. Three different design philosophies are considered, namely strong column - weak beam (SCWB), weak column - strong beam (WCSB), and strong column - weak panel zone (SCWP). The performance of these frames is evaluated dynamically by subjecting an inelastic model to an ensemble of 12 actual strong ground motion records. The model takes into account both connection flexibility and panel zone shear deformation. The results are presented in terms of response parameters determined from static pushover analyses, as well as statistical measures of the maximum response parameters determined from the inelastic dynamic analyses. The computed performance of the frames is evaluated in order to assess both the overall level of protection of the frames and the preferred design philosophy. It is concluded that a well-designed and well-detailed ductile moment resisting frame designed using either the SCWB or SCWP design philosophy can withstand ground motions of twice the design level with very little likelihood of collapse, whereas a frame designed using the WCSB approach is ill-conditioned and may develop a collapse mechanism at an excitation level well below twice the design level.Key words: seismic, ductile, steel, frame buildings, performance, design, ductility, damage, inelastic, dynamic.