Performance-based wind and earthquake design framework for tall steel buildings with ductile detailing

Abstract

According to the Canadian building code, wind design relies on prescriptive static methods, which lead to a conservative design. In consequence, in moderate seismic regions where both winds and earthquakes are critical, the size of ductile members resulting from seismic design, and especially those of bottom floors, may not accommodate the elastic wind demand and larger member sizes are required. In general, this is the case of rectangular buildings, where in the direction perpendicular to the largest building dimension, the base shear due to elastic wind demand is critical, while in the orthogonal direction the seismic demand governs. Hence, in one direction, the allowable base shear increases above the seismic demand, which results in reduced energy dissipation and amplification of torsion. To mitigate the incoherence in the current building code, a refined code-based multi-hazard design approach, targeting performance of tall steel buildings in Eastern Canada, is proposed. Thus, accounting for the building’s overstrength and allowing limited controlled inelasticity under high-intensity wind, a wind-related reduction factor RW is proposed to reduce the design wind load. The method is implemented for the design of two identical 15-storey steel braced frame buildings, but of different occupancy, hospital and office, respectively. Nonlinear response history analyses are performed to verify the member sections of steel braced frames, while the parameters investigated are the interstorey drift, residual interstorey drift and floor acceleration. Then, seismic and wind incremental dynamic analyses provide insight into the multi-hazard dynamic response of buildings subjected to ground motions and winds of increasing intensity. The fragility concept is used to quantify the performance of studied buildings designed with RW = 2 versus the building designed with RW = 1. Comfort criteria, as well as, the drift acceptance criteria, commonly considered in research and practice, are used to verify the effectiveness of the proposed method. The nonlinear response of the studied buildings at the design level and beyond shows that using RW = 2 for wind design does not change the response at design level, but slightly decreases the collapse capacity within the acceptable range.