An examination of the requirements for the design of built-up compression members in the North American and European standards and specifications reveals a great variation in the allowable maximum slenderness ratio for an individual main member, and also in the determination of an equivalent slenderness ratio. The requirements of the Canadian standard with regard to the determination of the maximum allowable slenderness ratio of a main member between points of connection can be a bit confusing.This research involved a study of model built-up members that buckled about an axis perpendicular to the plane of the connectors. Twenty-four tests were conducted on model built-up members. The theoretical analysis consisted of a finite element analysis of the model built-up struts. In addition, an equivalent slenderness ratio was calculated by several methods. These equivalent slenderness ratios were then used in conjunction with the requirements of the Canadian standard to calculate a compressive resistance, which was compared with the experimental failure load.From this research on built-up members that buckle about an axis perpendicular to the plane of the connectors it was found that at least two connectors should be used, that the slenderness ratio of the main member between points of connection has a significant effect on the compressive resistance, and that Timoshenko's equivalent slenderness ratio when used in conjunction with the Canadian standard gives results that are in the best agreement with the experimental results. Key words: battens, built-up members, compressive loads, connectors, equivalent slenderness ratio.
Built-up struts that buckle about an axis perpendicular to the plane of the connectors should be treated as a "built-up" member as opposed to a "simple" member. This mode of buckling causes shear and moments in the connectors which deform the connectors. These deformations increase the lateral deformation of the member and hence affect the load-carrying capacity. To account for this effect the easiest method is to use an equivalent slenderness ratio such as the one included in the Canadian Standard. This note outlines the derivation of the equivalent slenderness ratio equation, discusses when it should and should not be used, and includes a numerical example. A rewording of the applicable clause in the Canadian Standard is suggested. Key words: battens, built-up members, connectors, slenderness ratio.
The Canadian Standard S16.1 specifies an equivalent slenderness ratio to be used when determining the compressive resistance of a built-up member when buckling occurs about an axis perpendicular to the interconnectors. This equivalent slenderness ratio is the square root of the sum of the squares of the slenderness ratio of the built-up member acting as a unit and the maximum slenderness ratio of a component part between fasteners. The Standard specifies for this second component that an effective length factor be used, the magnitude of which depends on the type of connection. An effective length factor of 0.65 is specified when welds are used. This is of concern to the authors. It is pointed out that the derivation of this equation by Bleich, and by Timoshenko and Gere, does not contain an effective length factor in the second term. The effective length factor of 0.65 comes from a paper by Duan and Chen. Results of tests on five built-up members, channels arranged in the toe-to-toe configuration, indicate that the use of an effective length factor of 0.65 gives unconservative results, especially when the member is slender. It is recommended that a factor of 1.0 be used in the second term of the equivalent slenderness ratio equation.Resum6 : La norme canadienne S 16.1 spCcifie le rapport d'Clancement Cquivalent qui doit Ctre utilisC dans le calcul de la rCsistance i la compression d'une membrure composCe lorsqu'il y a flambage sur un axe perpendiculaire aux interconnecteurs. Ce rapport est Cgal i la racine carree de la somme des carrCs du rapport d'elancement de la membrure composCe et du rapport dlClancement de la membrure composCe et du rapport d'klancement maximal d'un Cldment constituant de cette membrure entre deux attaches. I1 est Cgalement prCcisC dans cette norme qu'il faut utiliser pour ce deuxibme ClCment un facteur de longueur de flambage dont l'importance dCpend du type de raccordement. Ce facteur doit Ctre de 0,65 lorsque les ClCments sont soudCs. Cette prCcision soukve une certaine inquiCtude de la part des auteurs. Ceux-ci font en effet remarquer que la fonction dCrivCe de cette tquation par Bleich et par Timoshenko et Gere ne contient pas de facteur de longueur de flambage dans le deuxikme terme. Le facteur de 0,65 citC plus haut est tire d'un article de Duan et Chen. Des essais effectuCs sur un assemblage de cinq membrures en configuration dos i dos indiquent que '13emploi d'un facteur de 0,65 donne des rCsultats non conservateurs, particulibrement si la membrure est effilCe. I1 est recommand& d'utiliser un facteur de 1,O dans le deuxibme terme de 1'Cquation du rapport d'Clancement Cquivalent.
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