Fundamental periods of vibration of braced steel frames for seismic design

An analytical study is performed to propose a simple expression for the fundamental period of vibration of concentrically braced steel frames. This work was carried out in the context of the development of the 2005 National Building Code of Canada (NBCC) and is mainly applicable to structures located in moderate and low seismic regions. Available field and test measurements of building periods are compared to analytical predictions. An empirical period formula is developed based on a survey of analytically computed building periods published in the literature. The influence of the 2005 NBCC design provisions on braced steel-frame periods is examined through a closed-forth period solution and an extensive parametric study. Building periods are found to vary significantly with seismic hazard levels and soil conditions. An expression is tentatively proposed that provides reasonable lower-bound period estimates for most situations in Canada.