Design of Cold-Formed Steel Beams with Holes and Transverse Stiffeners in Shear

Abstract

This thesis presents a comprehensive study on shear buckling and shear strength of high strength G450 and G500 cold-formed steel sections with web holes and/or with intermediate transverse web stiffeners. The primary aim is to develop a Direct Strength Method (DSM) of design to predict the capacity of such sections. The study also suggests a new structural application, the cold-formed steel girder, which could be potentially used as heavily loaded cold-formed member such as a transfer beam. The research involved the development of two new testing apparatuses that enable the experimental shear study of cold-formed beams with an aspect ratio (shear span / web depth) greater than 1:0. The first uses a single actuator similar to that used by Basler in the 1960s. The second uses dual actuators and a sophisticated control system. The test rigs were designed in such a way that they could minimize the bending moments applied to the shear span, thus allowing shear strength close to pure shear to be achieved for shear panels with an aspect ratio of 2:0. Three test series including thirteen tests on channel sections and SupaCee® sections were conducted to validate the new test rig designs. The experimental results were also used to validate the existing Direct Strength Method (DSM) of design for shear which was derived from shear tests on beams with an aspect ratio of 1:0. It was found that the new test rigs, which generated minimal moments in the shear span, significantly enhanced the shear strength of relatively long shear panels. Further, close agreement between the DSM prediction and the experimental shear strength confirmed the viability of the DSM for shear for structures with an aspect ratio up to 2:0. A test series including twelve tests on channel sections with various circular and square holes was carried out using the dual actuator rig to study the shear strength reduction due to the occurrence of the web holes. Another six tests on channel sections with intermediate transverse web stiffeners were conducted to investigate the shear strength of transversely stiffened sections as well as the cross-section requirement of the stiffeners. The finite element package Abaqus was employed to simulate the tests. The FE models were calibrated against the experiments prior to being used to perform parametric studies. The FE analyses extended the experimental database and provided insights into the stress development and the full displacement fields. The numerical study was also used to study the influence of such factors as flange restraints and moment to shear ratios on the shear strength of cold-formed sections. The experimental and numerical results were subsequently used to validate DSM shear proposals to design beams with web holes and with intermediate transverse web stiffeners. The proposal was first developed and validated by the tests on beams with web holes with an aspect ratio of 1:0. It made use of the existing DSM design rules and introduced modifications of the shear buckling load (Vcrh) and the shear yield load (Vyh) to account for the inclusion of the web holes. The Vcrh was determined via non-dimensionalised graphs or from a simple expression derived by an artificial neural network. A practical model on the basis of a Vierendeel plastic mechanism to determine Vyh was introduced. The validation extends the applicability of the DSM proposal to perforated members with aspect ratios up to 2:0.