Estimating the Stiffness of Eccentrically Braced Frames

Abstract

The stiffness of eccentrically braced frames (EBFs) is difficult to calculate by hand because they are indeterminate and have significant flexural, shear, and axial deformations in various members. EBF stiffness is important because it is used to compute story drifts and link rotations which have prescribed limits. This note presents an equation for the stiffness of an EBF story in terms of the design story shear, frame geometry, and beam depth. The equation is independent of specific member sizes, making it useful for determining appropriate geometry in early design. The equation is developed theoretically and validated with data from code compliant EBFs. One application of the equation is the direct calculation of the minimum beam depth required to ensure a link will satisfy inelastic rotation limits.