Seismic loaded Steel Frame with Vertical Irregularity

When an intense seism happens, the structures built in the area prone to sever damages and failure resulting in substantial financial and human loss. In construction industry, steel have been widely used since it is well-known for its high energy dissipation and quick construction process. Sometimes due to architectural demands, inclusion of irregularity is unavoidable. One type of irregularities in a building is vertical irregularity having the remarkable influence on the seismic performance.

The aim of this study was to evaluate the seismic performance of a steel frame of ten stories with the vertical irregularity known as set back resulting in different storey stiffness of 0.3, 0.5, 0.7 for top, middle and lower stories, respectively. A plan of three span with a center-to-center distance of 3200 mm in X- and Z-directions was considered for the frame. To design the frame and determine the type and dimensions of the beam and columns SAP2000 was employed. Design process led to the square hollow sections of various dimensions for different stories; one to three: , four to seven: , and eight to ten: . To figure out the variation of seismic resistance of a building with respect to irregularity, the regular frame was analyzed. With this, the variation of seismic performance of the irregular building can be well studied.

The finite element method package ABAQUS was used to perform the time-history analysis of the seismic loaded regular and irregular buildings employing incremental dynamic implicit method. The frame was modeled with wire elements. A real set of ground accelerations was applied to the base of the building, under displacement control condition, as well as dead and live loads corresponding to each storey.

The results were presented in the form of storey drifts in X- and Z-directions. It was seen that the vertical irregularity significantly affected the storey drift such that the maximum occurred in the middle of the building, stories 5 and 6.