Estimating Axial Force Demand in Columns of Shear-Type Structure Subjected to Earthquake-Base Excitation

A procedure for estimating the number of simultaneously yielding stories NSYS in a shear-type building subjected to earthquake-base excitation (previously proposed and validated) is used here for estimating axial force demand in the columns of such structures. Axial force demand of a column is obtained by summing the vertical force transferred by the NSYS simultaneously yielding stories with a square root of sum of squares (SRSS) combination of the forces that could be transferred by the remaining stories above that column, considering their full yield capacity. Axial force in the columns due to the simultaneous story yielding caused by the incident velocity wave and overlapping of the incident and reflected velocity waves occurring at the top and bottom of the building are considered. Three categories of earthquakes, namely earthquake excitations having (1) a single dominant pulse, (2) multiple distinct pulses, and (3) no distinct pulses in their velocity record, are considered. Also presented are results from a parametric study conducted by varying column stiffness and strain hardening of the brace members. The procedures for estimating the number of simultaneously yielding stories and axial force demand proposed here were found to provide good estimates of the values developed during actual earthquake excitations, especially for the first two categories of earthquakes, with the estimated values generally being on the conservative side. The procedures were also found to work well for the range of column stiffness and strain hardening ratios considered in the parametric study.