Approximate Analysis of Semi-Rigid Steel Frames

Abstract

This chapter discusses beam-to-column connections that are used in steel frames and are flexible, ranging in stiffness from those almost rigid to some types close to pinned. Most practical connections fall in between the two idealized extremes of rigid and pinned and are referred to as semirigid connections and the resulting frame as semirigid frame. The important characteristic of such connections is the moment-rotation (M-O) curves. The stiffness of practical connection, measured as the slope of the M-O curve at appropriate level of connection rotation, causes to transmit some moment and to experience some rotation. The semirigid nature of the connections affects the frame behavior in that the distribution of internal forces and moments in the beams and columns are different from those of the idealized cases. Frame analysis neglecting the true behavior of the connection will result in unreliable prediction of frame response. Rigorous tools to analyze the semirigid frames have been available for quite some time, but lack of simple hand method remains as the primary bottleneck in treating semirigid design as a viable design alternative. This chapter develops a simplified analysis technique of semirigid frames. A finite element computer program is utilized to study the behavioral pattern of nonsway semirigid frames. By treating the semirigid connections of a range of stiffness as pinned, the simple frame design method specified in the code of practices fails to take account of the moments that are transferred to the column. The chapter addresses this limitation of the simple design adopted in the codes of practices and suggests a method of ascertaining the column moments for use in design.