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Date of Award

7-1998

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Supervisor

Professor A.C. Heidebrecht

Abstract

A large number of low and medium-rise buildings have steel moment resisting frames as the primary lateral load resisting system. During the past few decades, much confidence has been placed on this type of structural system for resisting seismic loads. However, after recent earthquakes (e.g. Northridge, California, in 1994, and Kobe, Japan, in 1995) the confidence in this system was reduced as a result of various types of damage that moment resisting steel frames suffered. This resulted in a recognition of the need to evaluate the performance criteria on which current provisions are based. While there have not been any major casualties dueu to earthquakes in Canada during the past few decades, in fact there is an actual seismic hazard which affects significant regions of the country, for example, the cities of Victoria, Vancouver, Quebec City, Montreal, and Ottawa. The design peak ground motions in such regions are moderate in comparison with those in California or Japan, however the uncertainties associated with estimating the expected ground motions are such that twice or three times the seismic design level motions are likely to occur. The main objectives of this research study are: (i) to evaluate the seismic level of protection afforded to steel moment resisting frame building structures designed in accordance with the current Canadian provisions (i.e. NBCC (1995) and CAN/CSA-S16. 1-94), and (ii) to investigate the effect of the different design philosophies and seismic hazard design levels on the inelastic dynamic response of multi-storey steel frame structures. Six storey office buildings located in regions of high, intermediate, and low seismic hazard, and a ten storey office building located in a region of intermediate seismic hazard are designed in accordance with the current Canadian provisions using three design philosophies, namely strong-column weak-beam (SCWB), weak-column strong-beam (WCSB), and strong-column weak-panel zone (SCWP). The scope of the research program includes: (a) modelling of the structural elements; (b) nonlinear push over static analyses/inelastic dynamic analyses, and (c) evaluation of the damage potential associated with each design. In the study analytical models are modified and incorporated into the PC-ANSR computer program in order to perform the inelastic dynamic analyses of the frames. The inelastic models take into account the spreading of inelastic deformations in beam-column elements, connection flexibility and panel zone deformations. A cyclic model for the panel zone element is developed and introduced into PC-ANSR. The performance of the frames is evaluated both statically using monotonically increasing lateral load (nonlinear push over static analyses), and dynamically by subjecting the inelastic model to an ensemble of actual strong ground motion records (time-history analyses). The main ensemble of time-histories used in the study consists of twelve earthquake ground motion records selected on the basis of Newmark-Hall design spectra amplification factors. An additional ensemble of time-histories (twelve records) is selected based on the uniform hazard spectrum for Vancouver which describes the new seismic hazard information given by the Geological Survey of Canada. The additional ensemble is used to investigate the implication of the new seismic hazard information on the performance of the six storey frames in the intermediate seismic hazard region. The results of the inelastic dynamic analyses are presented in terms of statistical measures of the maximum response parameters determined during the time-history analyses. Also, the results of the nonlinear push over analyses are presented and compared with those of the dynamic analyses. The performance expectations of the frames are evaluated in order to assess both the overall level of protection provided to the frames and the preferred design philosophy. It is concluded from the analyses that in high and intermediate seismic hazard regions, a well-designed and detailed ductile moment resisting frame (i.e. SCWB or SCWP) can withstand ground motions of twice the design level with a very little likelihood of collapse, while an ill-conditioned designed frame (i.e. WCSB) may develop a collapse mechanism even at the design level excitation. In regions of low seismic hazard activity, the three frame design types perform satisfactorily, and can withstand twice the design level excitations with only a moderate amount of damage.

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