Studies of imperfections in roller bending steel hollow section members

Abstract

Structurally, the action of curving a steel hollow section member makes it possible to cover significant longer spans, since the load is carried largely in compression instead of through bending action. Due to the excellent mechanical properties and geometric tolerances, the curved steel hollow section members are often employed for the construction of high load-bearing roof structures. Cold bending by means of a rolling machine with three adjustable rollers in a triangle or pyramid type is suitable for the large bends required for construction purposes. Roller bending involves progressive bending of a section by passing the member through a set of bending rolls. During the cold-bending process of steel hollow section member, with either square or rectangular section, relatively high loads are applied to achieve a permanent member curvature. These could cause imperfections such as sectional deformations and residual stresses in the member. Such loads impart large permanent local deformations in the four relatively thin hollow section member faces and sectional deformation in the cross section of the member. The sectional deformations in the cross section are caused by web crippling coupled with compression flange buckling. As a result, there is a reduction in the section moment of inertia. The permanent sectional deformation of the cross section may affect its aesthetic appeal or integrity to carry loads when used after rolling. The cold roller bending process also induces residual stress in the section walls. The residual stress magnitude and distribution could have sufficient influence on the member's stability and buckling resistance. Therefore, a certain acceptable level of sectional deformation should be established. In this paper, the experimental, numerical and analytical methods were used to investigate the relationship between the cross-sectional sectional deformation and the member's potential curvature. Furthermore, the yield line mechanism and energy minimization theory are employed to predict the sectional deformation of steel rectangular hollow section after cold roller bending. The yield line models were developed based on the actual deformation observed from experimental results and numerical simulations. Based on the theoretical analysis as well as the III experimental results, simple analytical models are proposed to predict the relationships between the bending radius of the curved member and the resulting deformations of the cross section under different deformation modes. While few studies have paid attention to the residual stress distribution caused by roller bending, a proper numerical modelling procedure is employed to simulate the rolling process and reproduce the residual stress in this paper. In addition, a small scale parametric study is conducted to investigate the effects of some key roller bending parameters, including the rolling boundary conditions, the bending ratio, the steel yield stress, the thickness ratio and the shape factor, on the resulted residual stress distribution of...