Structural properties and out-of-plane stability of roller bent steel arches

Abstract

In contemporary architecture the use of steel arches has seen a significant increase. They are applied in buildings and large span bridges, combining structural design with architectural merits. For arches lacking lateral support (or freestanding arches) the out-of-plane structural stability behavior is the decisive design criterion. However, suitable methods or design rules to assess the out-of-plane structural stability resistance of arches are lacking and the collapse behavior is often unknown. Nowadays engineers have to perform laborious calculations which can lead either to conservative or nonconservative arch designs. This Ph.D. project is aimed at studying the out-of-plane structural stability behavior of steel arches, and developing design rules for these arches. The out-of-plane structural stability behavior was studied by means of geometrical and material non-linear finite element analyses including structural imperfections with ANSYS v. 11.0. The investigation was confined to wide flange circular freestanding arches which are subjected to in-plane vertical loads and manufactured by the roller bending process. The roller bending process is a manufacturing technique by which steel members are bent at ambient temperature into circular arches. It was expected that the residual stresses and mechanical properties (e.g. yield stress, ultimate tensile stress) are altered due to roller bending. Since the alteration of residual stresses and mechanical properties (imperfections) can affect the out-of-plane structural stability of freestanding steel arches, the influence of the roller bending process was studied first. Residual stress measurements and tensile tests were conducted on both straight and roller bent members to assess the influence of roller bending. In addition to the experiments, finite element simulations of the roller bending process were performed in the ANSYS v. 11.0 environment to estimate the residual stress distribution in the arches. Good agreement between the experimentally and numerically obtained residual stresses in roller bent arches was observed. Based on the experimental and numerical studies of the imperfections in roller bent arches a residual stress model and distribution of mechanical properties across the steel bent section were proposed which serve as the initial state of a roller bent arch when assessing its structural performance by means of non-linear finite element simulations. Numerical analyses showed that the residual stresses in roller bent arches have a minor influence on the load carrying capacity. However, the alterations of the mechanical properties can result in a significant reduction of the arch strength The existing column curve formulations as given in EC3 were adapted to include out-of-plane buckling of arches by altering the form of the imperfection parameter. The column curves can give an accurate prediction of the out-of-plane buckling load provided an appropriate imperfection parameter is selected and the non-dimensional slenderness is known. Based on numerous finite element calculations an imperfection parameter curve was derived which was substituted into the column curve formulation rendering a column curve for roller bent arches failing by out-of-plane buckling. Finite element results showed that multiple column curves were necessary to capture the out-of-plane buckling response of arches for various load cases and steel grades. The column curves require the determination of the non-dimensional slenderness represented by the in-plane plastic capacity and out-of-plane elastic buckling load. Since any closed form equations are lacking to approximate these buckling parameters finite element techniques were adopted. For future research it is recommended that closed-form solutions or design graphs should be derived with mechanical models to obtain the elastic-plastic buckling load of freestanding roller bent arches without using finite element analyses.