Effects of Column-to-Beam Strength Ratio on Behaviour of Beam-to-Column Moment Joints

Abstract

Steel Moment Resisting Frames (MRFs) are believed to be very ductile structures and are highly regarded structural systems, even in areas of high seismicity. The overall ductility of a steel MRF depends on many factors, such as connection configuration, column-to-beam strength ratio, effect of joint panel zone, material and cross-sectional properties, rate of loading, etc. Seismic performance of a steel MRF depends, primarily, on the performance of beam-to-column joints in the frame. The capacity of a joint to undergo inelastic deformation determines the ductility of an MRF. In a beam-to-column moment joint, the desirable behaviour is to limit inelastic actions at beam end regions, thereby preventing irreparable damage to structures. In a simple unreinforced moment connection, this is achieved by varying the Column-to-Beam Strength Ratio (CBSR). In this paper, a minimum value of CBSR, which ensures the formation of plastic hinge at beam end region, for a simple unreinforced joint, is determined. A parametric study, using Nonlinear Finite Element Analysis (NFEA), is carried out to determine the force–deformation behaviour of ten beams to column joint subassemblages. The CBSRs are varied from 1.2 to 11 to determine the value at which inelastic actions can be limited to the beam ends. The selection of CBSRs is based on the strength of AISC standard sections and their compatibility along with the various prevalent codal provisions. Results of NFEA show that the minimum CBSR required to prevent inelastic actions in columns is close to 7.5.