Analytical and Comparative Review of Methods for Determining the Storey Stiffness

Soft story has caused the failure of many buildings in previous earthquakes. Several factors such as greater height of columns, removing some columns, lateral bracings, shear walls or infill walls due to architectural design in a story especially ground floor are the main reason of this phenomenon. Most of the seismic codes have criteria for vertical irregularity. This is usually based on the ratio of stiffness between adjacent storeys, but there is no recommendation for calculating the storey stiffness. On the other hand a great number of previous studies on stiffness have focused on approximate methods and exact methods have been almost neglected. In this study, soft storey, weak storey and approximate methods in literature are reviewed and then six exact methods for calculating the storey stiffness by using structural analysis software are investigated. This study is done on 2D reinforced concrete frames. Based on analysis results, method 3 with no modification in the model of structure and the triangular distribution of lateral forces similar to seismic forces has acceptable accuracy and desirable efficiency for designing and controlling structures.