Simplified analysis of semi-rigid steel frames under sway mode

Abstract

In the conventional methods of steel frame analysis, the behaviour of joints is represented by idealised models, either as a rigid joint or a pinned joint. In reality, most practical connections are neither rigid nor pinned and possess certain amount of rotational stiffness. Design rationale to incorporate the semi-rigid joints in non-sway frames has already been developed. Despite the fact that greater economy might be hamessed by the use of semi-rigid connections in multi-storied unbraced frames, such practice has not yet been preferred by designers due to their scepticism regarding the distribution of intemaJ forces in such frames. Detail investigation on the distribution of intemal forces in semi-rigid sway frames is very much warranted to gather adequate confidence for the use of semi-rigid connections in sway frames. With the above objective an existing finite element program has been used to study the distribution of intemal forces in a multi-storied semi-rigid sway steel frame. Medium rise buildings ranging from four to nine stories high having a wide range of connection stiffness have been analysed with a view to indicate the key parameters that influence the distribution of intemal forces of semi-rigid frames. The behaviour of semi-rigid frames with certain type of beam and column sections, and equal bay width (500 cm) has been studied first. The effects of variation in bay widths, and beam and column sections have been subsequently discussed. From these studies some important observations have been made, which facilitated the development of a simplified method of analysis of semi-rigid sway steel frames. An appropriate simplified technique for representation of moment-rotation relationship of semi-rigid connections has been developed for use in the analysis of sway frames. It has been shown that the true moment-rotation relationship of connection can be replaced by an approximate secant connection stiffuess, which can be advantageously used in any attempt to develop simplified analysis technique of semi-rigid frame in sway mode. A simplified hand calculation method of estimating the intemal forces of medium-rise multi-storied semi-rigid sway steel frames subject to lateral loads has been proposed with the help of conclusions obtained from the behavioural study. The proposed method is a new approach and uses a number of graphs and tables. With the help of worked out examples the validity of the simplified method has been demonstrated.