Connections in higher strength Grade C450 cold formed rectangular hollow sections

Abstract

CIDECT guidelines for hollow steel joints were mainly based on research of ductile steel with yield strength up to 355 MPa. On higher strength, lower ductility steels (C450 and above) emerging, CIDECT in 2009, extended the design guidelines to C450 RHS connections also, with certain restrictions on material, geometry and class of RHS. An overall reduction factor of 0.9 in design strength and a cap on σy at 0.8 σu were also stipulated. Experimental studies were conducted at the University of Sydney to verify the need and justification for the CIDECT restrictions in C450 RHS connections. This thesis complements the experimental studies using numerical methods and some novel techniques including use of an adapted Lemaitre damage model to track fracture, tracking necking in coupon tests by using recorded engineering stress-strain data and filling in gaps in data by methods such as use of FEA and/or crosshead data, grouping variations in material test results suitably and adopting a weighted average method to depict true stress strain that would reflect plastic deformation and damage. The FE models with these features were initially benchmarked against coupon test results, then validated against RHS jointless tests and finally against 12 K gap joint tests. More than 80 parametric variations that might influence the strength and behaviour of these joints were analysed using FEA. The study led to proposals for modifications to chord plastification, punching shear, chord side wall design equations to better predict ultimate loads and fracture modes. Reduced ductility in the steel was dealt with through a modifier function that is not based on yield stress but instead recognises the reduced ultimate strains, damage parameter for fracture and the ultimate stress of the material. Another research achievement is the perfection of a methodology to numerically predict the fracture / failure behavior of RHS joints and to formulate or test, design criteria if required.