Statistical evaluation of the lateral-torsional buckling resistance of steel I-beams, Part 1: Variability of the Eurocode 3 resistance model

The EN version of part 1-1 of EC3 introduces significant changes in the evaluation of the lateral-torsional buckling resistance of unrestrained beams, as well as alternative design procedures. Simultaneously, as a part of the preparation of the National Annexes of EC3 part 1-1 and the establishment of the corresponding NDP’s (Nationally Determined Parameters) it becomes necessary to define the partial safety coefficients for the bending resistance of beams when lateral-torsional buckling is a potential failure mode. In this paper the methodology for the resistance evaluation of beams subjected to instability is briefly described and the results are compared with FEM numerical results for the same elements. Subsequently, to assess the accuracy of the alternative design formulae, a statistical analysis of the results is performed on the basis of EN 1990-Annex D. A methodology is proposed for the evaluation of the design procedures allowing for the uncertainties in the resistance model. Results are presented for a wide set of beam geometries and loading cases and a proposal for the definition of the partial safety factor γ_rd is presented for the various methods, in line with the target failure probability of EN 1990.     

Effect of preload on the axial capacity of concrete-filled composite columns

Concrete-filled tubes are often preferred for the construction of high-rise buildings because of their high strength and stiffness compared to conventional reinforced concrete or steel columns. However, prior to infilling of concrete, the steel tubes are subjected to preloads from upper floors arising from construction loads and permanent loads of the building. These preloads cause initial stresses and deformations in the steel tubes which would affect the load carrying capacity of the composite columns. In this paper, a design method based on a modified Eurocode 4 approach, incorporating the effect of preload, is proposed to evaluate the axial capacity of concrete filled composite columns. Eight full-scale composite column specimens were tested. Parameters studied included the strength of the concrete infill, slenderness of the columns and the amount of preload applied on the steel tubes. Results obtained from the proposed method are compared against test results and other published data. Comparison studies show that the test results are on average 3% higher than predicted results with a standard deviation of 0.089. Finite element analyses are also performed for systematic verification, and the results are 8% higher than predicted results. It is conclude that the proposed design method is accurate and mostly conservative and can be readily used in the context of Eurocode 4: Part 1.1 for designing composite columns.     

双对称与单对称工字形截面悬臂梁弹性临界力矩的评估

所谓的3因素公式是估计具有侧向扭转屈曲的钢梁弹性临界力矩的最常用公式之一。本研究拓展了该公式在工字形截面悬臂梁中的应用范围:(Ⅰ)相等或不相等翼缘;(Ⅱ)端部完全嵌固或自由扭曲;(Ⅲ)在均布荷载或集中荷载的作用下(荷载作用在剪心或一个翼缘上)。内容包括(Ⅰ)弹性侧向扭转屈曲理论基础的讨论(Ⅱ)通过无量纲的形式,在固定参考范围情况下,对涉及形成屈曲问题的主要步骤及特别便于这项研究的特点给予描述(Ⅲ)通过瑞利-里兹方法得到的参数研究的数值结果,(Ⅳ)前述公式中出现C1、C2和C3三个因素的近似解析表达式的用途。     

Determination of a model partial safety factor for unreinforced masonry walls under buckling

The design/verification method in the Eurocodes is based on the partial safety concept. Eurocode 6 suggests a constant partial safety factor for the material γ_M for all design/verification problems, without consideration of model uncertainty in the design/verification formula. In the following, a model partial safety factor is determined for the problem of unreinforced masonry walls mainly subjected to vertical loading. For that purpose, the newly proposed formula for EC 6, annex G will be considered [1–3]. In order to cover all aspects in tests and to use the results for design purposes, several methods have been included in EN 1990 Annex D for design based on test data. In this study, the recommended methods in Annex D of EN 1990 for resistance of the material are used to extract the partial safety factors. A database including more than 119 experimental tests on unreinforced masonry shear walls is used to compare the model prediction and the test results and to determine the model partial safety factor.