Column curves for elliptical hollow section members Farhood Nowzartash ⁎, Magdi Mohareb University of Ottawa, Department of Civil Engineering, 161 Louis Pasteur, Room A106, Room A106 Ottawa161 Louis Pasteur Room A106 Ottawa, Ottawa, ON, Canada K1N-6N5 abstract article info Article history: Received 17 November 2010 Accepted 30 March 2011 Available online 4 May 2011 Keywords: Elliptical hollow sections Plasticity Residual stresses Initial imperfection A thermo-mechanical finite element analysis model is developed to predict residual stress patterns in hot- rolled sections. The model is first verified against experimental measurements for residual stresses reported for I-sections reported in the literature. The method is then used to predict residual stresses in elliptical hollow sections. A sensitivity analysis is then conducted to assess the influence of various input parameters of the model on the predicted residual stress patterns. The effects of cross-section geometric parameters on the residual stress distribution are then investigated. A series of column curves is generated providing the compressive capacity of a column as a function of slenderness. The column curves are generated based on a) elasto-plastic geometrically nonlinear analyses, b) including the effect of residual stresses as predicted from thermo-mechanical analyses and c) incorporating initial geometric out-of straightness according to the fundamental buckling mode as predicted from an elastic buckling eigen value analysis. Generated column curves are then compared to those in current design codes. A best fit for the numeric results obtained is conducted to cast them in a format similar to that in the current codes. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction and scope Elliptical hollow sections (EHS) have emerged relatively recently as structural steel members. With their increasing popularity as a structural option among architects, recent research has focused on investigating their behaviour and developing design rules. This includes the lower bound interaction relations for EHS subject to combinations of axial force, bi-axial bending moments and torsion [1]. Also, upper bound interaction relations were developed for EHS which included the additional effects of bi-moments induced in EHS at their fixed ends [2]. The interaction relations developed were based on the plastic cross-sectional capacity resistance and do not include member stability effects. In this context, this paper reports an investigation on the stability of EHS aimed at numerically developing column curves for EHS, which account for residual stresses, slenderness effects, and out-of-straightness imperfections. The column curves developed in the present paper can be used in conjunction with the interaction relations developed in [1,2] to provide interaction relations which account slenderness effect, a necessary step towards adopting them in structural design. In addition to column slenderness, the compressive capacity of columns is known to be influenced by 1) material plasticity, 2) residual stresses and 3) initial out-of-straightness. Other factors such as material variability and load eccentricity can also play a role [3] on column strength, but will not be investigated within the present study. A methodology is described and verified against available experimental results to predict the residual stress in EHS members. Initial imperfections are determined using elastic buckling analysis. Given the residual stresses and out-of-straightness predicted a series of nonlinear load deformation FEA including material and geometric nonlinearity is to be conducted to generate points on the column curves. 2. Literature review 2.1. Residual stresses in hot rolled sections In hot rolled sections, residual stresses are induced by the uneven cooling within the different parts of the section. I-sections are the most widely members used in steel construction. Also, they are more susceptible to uneven cooling compared to closed sections. Hence, most residual stress investigations in structural steel members were devoted to I-sections. Residual stresses were measured and reported in a number of publications (e.g., [4–9]). Galambos and Ketter [10] and Young [11] proposed a simplified self-balanced residual stress patterns for I-sections based on curves conservatively fitted to measured residual stress data. Later on, Szalai and Papp [12] modified these patterns to additionally satisfy the torsional equilibrium of the cross section. The works by Weiner [13] and Landau et al. [14] are among the earliest attempts to analytically model residual stress in hot-rolled sections. Given the complexity of behaviour of steel during the cooling Journal of Constructional Steel Research 67 (2011) 1525–1536 ⁎ Corresponding author. E-mail addresses: fnowz064@uottawa.ca (F. Nowzartash), mmohareb@uottawa.ca (M. Mohareb). 0143-974X/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2011.03.026 Contents lists available at ScienceDirect Journal of Constructional Steel Research