Fundamental period of irregular eccentrically braced tall steel frame structures

• Investigation on the fundamental periods of eccentrically braced frames (EBF) with varying geometric irregularities.
• A new equation for the fundamental periods of EBFs which takes into account vertical and horizontal irregularities.
• The proposed equation is validated through a comparison of available measured period data for EBFs.
• Quickly estimation of the fundamental period of multi-story EBF structures by taking into account irregularities.
• The new equation yields a more reasonable estimation than the current code equation.

This paper presents the results of investigation on the fundamental periods of eccentrically braced frame (EBF) structures with varying geometric irregularities. A total of 12 EBFs are designed and analyzed. Based on the results obtained from vibration theory, equations for the approximate fundamental periods are put forth for EBFs which take into account vertical and horizontal irregularities. Through statistical comparison, it was found that a 3-variable power model which is able to account for irregularities resulted in a better fit to the Rayleigh data than equations which were dependent on height only. The proposed equations were validated through a comparison of available measured period data for EBFs. These proposed equations will allow design engineers to quickly and accurately estimate the fundamental period of EBF structures by taking into account their irregularities.