Performance based seismic design of semi-rigid steel concrete composite multi-storey frames

Abstract

Steel-concrete composite frames offer several advantages such as efficient sections in terms of strength and stiffness, economy and speed of construction. Semirigid connections can also be easily achieved by providing additional rebars in the slab to resist hogging moments at beam ends. However, design of these frames is a complex problem as the minimum connection stiffness and strength required to satisfy seismic drift limitations are not easy to determine. Hence a comprehensive study is being undertaken with the objective of determining the most optimum connection properties for frames with various configurations such as spans and storey heights located in different seismic zones of India. In this study, regular plane frames of two, four and eight storeys are analysed under equivalent static and nonlinearstatic pushover analysis. The linear response of semi-rigid frames is characterisedin the form of ratios of roof drifts and time periods taken with respect to equivalent rigid frames. The moment-rotation curves for the semi-rigid connection are obtained from the results of an earlier study conducted by Smitha and Kumar (J Constr Steel Res 82:164-176, 2013) on stiffened flange plate composite connections.The variation of roof drifts and time periods with respect to connection stiffness is obtained. The roof drifts under equivalent static loads and time period sdecrease exponentially with increase in connection stiffness. The analyses results showed that significant reduction in design base shears can be achieved for practical semi-rigid frames as the time periods of these frames are higher than that of rigid frames. A design procedure is proposed to ensure that the frames satisfy code stipulated drift limitations. The seismic performance of a rigid and semi-rigid frame designed to satisfy current codal provisions are compared by carrying out non-linear static pushover analysis. The comparison indicates that, in low (zone II) to moderate (zone III) seismic areas, frames with semi-rigid connections can be designed to perform satisfactorily thereby reducing the overall cost of the structure.