Estimating number of simultaneously yielding stories in a shear building subjected to full-sine pulse velocity base excitation

Past research has shown that seismic force demands in the columns of lateral load resisting systems depend on the number of simultaneously yielding stories (NSYS) above the column considered. Current design procedures for steel frames (e.g., those in AISC-341) typically specify that all stories are considered to be yielding. Time history analyses show that this assumption may be overly conservative for tall buildings. No systematic procedure exists for estimating this NSYS. Concepts of wave propagation theory are used here in seeking such a procedure. As a first step towards that goal, research was conducted to investigate the NSYS due to an incident wave in a shear-type building subjected to full-sine velocity base excitation, to understand the relationships between input excitation, inter-story drifts, story forces, and hence to formulate a procedure to estimate the NSYS along the building height. Story yielding capacity of a base excitation was found to depend on the magnitude of its velocity record. Accordingly, a parameter vy was defined that determines the minimum magnitude of the velocity wave required to yield a story. Mathematical expressions were derived to predict the beginning and end of story yielding due to the incident wave. With this, NSYS values at any instant as the incident wave propagates up the building could be determined. This estimation procedure was found to provide good estimates of the actual yielding for structures considered here that have vy values decreasing with height, with the estimated values being on the conservative side.