Seismic Demand in Columns of Steel Frames

Keywords: Earthquake engineering; Simultaneous story yielding; Axial force demand; Column design; Wave propagation; Seismic design; Multistory buildings; Shear buildings; Pulse excitation; Earthquake excitation; Buckling restrained braced frame

Abstract: This report presents a procedure to estimate the number of simultaneously yielding stories N(SYS) in a shear building, which is then used to propose a procedure to estimate axial force demand in the columns of these structures. Concepts of wave propagation are applied to derive mathematical expressions to estimate the N(SYS) values in a shear building subjected to full-sine velocity base excitation. The estimation procedure is then extended to shear buildings subjected to earthquake base excitation by identifying the main pulse of the earthquake velocity record responsible for causing the maximum number of stories to yield simultaneously and by representing it with a full-sine velocity pulse. The application of the procedure is then verified for buckling restrained braced frames, whose degrees of freedom have been restrained to undergo shear deformation. The estimated N(SYS) value is further used to propose a procedure to estimate axial force demand in the columns by considering the axial force contribution from the yielded and non-yielded braces. Overall, the estimation procedures proposed to estimate the number of simultaneously yielding stories and axial force demand provided good estimates of the actual values, with the estimated values being on the conservative side.