The Impact of Long Duration Earthquake on the Response of Multi-Storey Concentrically Braced Frame Buildings

Abstract eng:
Concentrically braced frame (CBF) buildings are widely considered in seismic zones. Although the seismic behavior of CBF systems was extensively studied under the effect of crustal ground motions, the impact of long duration mega-thrust subduction earthquake on the building response is mostly unknown. It is noted that buildings in western Canada (B.C.) are prone to crustal earthquakes and the mega-thrust Cascadia subduction earthquake with a predicted magnitude in the range of 8 to 9 and a mean recurrence period in the range of 500-600 years. From historical references, it was found that the last subduction event was in 1700. The characteristics of subduction versus crustal ground motions vary in terms of amplitude, Trifunac duration and frequency content. Thus, in order to investigate the performance of CBF buildings in Victoria, the best available proxy to a future Cascadia event are the main-shock records from the magnitude 9 Tohoku, Japan earthquake of March 11, 2011 that show several loading/unloading cycles that occured in approximately 300 s. However, the current National Building Code of Canada and Design standards do not require accounting for the number of ground motion cycles for designing earthquake-resistant buildings. Hence, multi-storey buildings designed to withstand the effects of crustal earthquakes may exhibit severe damage under the potential Cascadia subduction earthquake. In this study, the effect of mega-thrust subduction earthquake on the seismic response of moderately ductile concentrically braced frame multi-storey office buildings located on Site Class C (firm soil) in Victoria, B.C., Canada is investigated from brace yielding to brace fracture caused by low-cycle fatigue. Using data from the 2011 M9 Tohoku subduction earthquake in Japan, nonlinear dynamic analyses were performed on detailed numerical models developed in OpenSees. It was found that the effect of Trifunac duration on the nonlinear seismic response of buildings is particularly significant in terms of the strain accumulated in the fibers of hollow structural section braces, HSS, causing low-cycle fatigue fracture. Mapping the level of damage versus ranges of building height (e.g. 2-storey, 4-storey and 8-storey) it provides an image of building safety. It was found that particular attention should be given when designing low-rise buildings with a fundamental period lower than 0.35 s located in the proximity of subduction fault, such as buildings in Victoria that lie within the Cascadia subduction zone.

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