Approximate Fundamental Period for Seismic Design of Steel Buildings Assigned to High Risk Categories

Seismic analysis provisions in ASCE/SEI 7-16 (ASCE. 2016. Minimum design loads and associated criteria for buildings and other structures. ASCE/SEI 7-16. Reston, VA: ASCE.) use an approximate fundamental period as a proxy for, or to limit, the fundamental period of a building for design. This design period does not vary as a function of the risk category assigned to a building. Fifty-four steel buildings (4-, 8-, and 16-story) with three different seismic force-resisting systems (moment frame, concentrically and eccentrically braced frame) are designed for a region of high seismicity for Risk Category II, III, and IV. The results indicate that the analytical fundamental period is affected when an importance factor greater than 1.0 is used for design, as is required for assigned higher risk categories. Moreover, the database of measured vibration data taken from steel buildings used to establish the empirical formula adopted by ASCE/SEI 7-16 to compute the approximate fundamental period did not include buildings designed with an importance factor. Consequently, if the performance objective is to achieve uniform risk within a category, then the design period should vary as a function of the risk category. In the absence of a lower-bound empirical formulation for the approximate period using a database of vibration data from buildings categorized by risk category, a straightforward modification to the current formulation is proposed that incorporates the change in system strength.