Semi-rigid joint action in sway steel frames

Abstract

It is widely appreciated that if the inherent stiffuess possessed by the commonly used beam-to-column connections could be included in the design practice, the resulting structure would be more economical compared to those designed using the conventional 'simple' or 'rigid' frame assumption. Design rationale to incorporate the semi-rigid joints in non-sway frames has already been developed. Despite the fact that greater economy might be harnessed by the use of semi-rigid connections in multistoried unbraced frames, such practice has not yet been preferred by designers due to their scepticism regarding the sway behaviour of such frames. Detail investigation on the behaviour of 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 behaviour of multi-storied semi-rigid sway steel frames. Medium rise buildings ranging from five to eight story high having a wide range of connection stiffuess have been analysed with a view to conduct a behavioural study. Since it is widely believed that use of semi-rigid frame in unbraced situation would cause excessive lateral sway, the main focus in this study has been on the lateral drift of frames and on the levels of joint rotation at different positions of a semi-rigid frame. The prospect of using rigid and semi-rigid connections simultaneously at different locations of a frarne has also been looked into. This has led to the conclusion that with judicious combination of both semi-rigid and rigid joints, frame drift can be brought within the acceptable limit. The important characteristic behaviour of unbraced steel frames which are particularly helpful for the development of a simplified method of estimating sway have been observed in this study. An appropriate simplified representation of moment-rotation relationship of semirigid connections used in sway frames has been developed. It has been shown that the true moment-rotation relationship of connection can be replaced by an approximate secant connection stiffness which can be advantageously used in any attempt to develop simplified analysis technique of semi-rigid frame in sway mode. A simplified hand method of estimating the lateral drift of medium-rise multi-storied semi-rigid sway steel frames has been proposed. With the help of worked out examples the validity of the simplified method has been demonstrated.