The Effect of Variation of Stories’ Plan Size along Height of Relatively Tall Steel Buildings on their Seismic Behavior

In urban environments in many cases buildings with unequal story’s plan sizes are desired, resulting in non-uniform mass/stiffness distributions along the buildings’ height. In this study three groups of 12-story steel buildings with uniform, ascending and descending story plan size distribution (PSD) along their height, but having the same total architectural space, were investigated to find out how this distribution affects their seismic performance. First, three site conditions of stiff, medium and soft were considered and all buildings were designed, by considering moment frames with concentrical bracings as their lateral load bearing systems. Then, for each site class, seven appropriate set of accelerograms were selected, and by using nonlinear time history analyses, the buildings’ seismic performance levels were compared based on the formation of plastic hinges (PHs). Results show that non-uniform PSD can be quite effective on seismic performance of buildings’ structures, so that on hard and soft sites the number of PHs exceeding the collapse prevention performance level in buildings with ascending and descending PSD, respectively, decreases about 30%-45% and 20%-40% compared to the uniform buildings. Therefore, it is concluded explicitly that employing buildings with uniform plan size does not necessarily lead to a higher seismic performance level.