Evaluation of Displacement-Based Analysis and Design Methods for Steel Frames with Passive Energy Dissipators

Abstract

This report investigates the use of displacement-based, or pushover methods of analysis in the design of frames incorporating passive dissipative devices. An extensive analysis and design study of 3-, 6- and 10-storey frames, both undamped moment-resisting frames (MRFs) and retrofitted with hysteretic and frictional dissipators has been performed. Frames were modelled using the finite element program Sap2000 and were analysed using both non-linear static pushover analysis and non-linear time history analysis. The principal aims were to assess the degree of improvement in performance achieved through use of the devices, and the suitability of various displacement-based analysis methods for estimating the seismic response of frames fitted with dissipative devices. It was found that both dissipative systems led to substantial improvements in frame performance, in terms of plastic hinge formation (reduced to virtually zero) and deformation (reduced by a factor of more than 2). Base shears remained similar to those for the undamped MRFs. Pushover analyses were found to be a useful design tool for the unretrofitted frames, giving good estimates of the overall displacement demands, base shears and plastic hinge formation. However, the various pushover approaches proved less successful at estimating the performance of the dissipative frames, where they appeared to underestimate the beneficial effects of energy dissipation. Of the various pushover methods assessed, the FEMA 356 approach appears to offer the most accurate and realistic estimate of seismic performance, with the exception of the inter-storey drift distribution. For the 6- and 10-storey frames (both ductile MRFs and dissipative frames), pushover methods using fixed, single load patterns gave rather poor estimates of the distribution of inter-storey drift with height. Far better drift estimates were obtained using the modal pushover method, in which pushover results obtained using force distributions based on the first three modes are combined by the SRSS method