Étude de la résistance en flexion des extensions en porte-à-faux de la corde supérieure de poutrelles à treillis en acier

Open web steel joists are commonly used in metal constructions. Combined with a steel deck, they have significant advantages such as being quickly and easily assembled, to allow the installation of mechanical ducts and other elements through their web, or to have floors or roofs with low dead load. The term “extension” designates the prolongation of the top chord of the joists beyond the support seat of these joists. These extensions, used to extend the floor of a mezzanine or continue a roof, then constitute cantilevered elements which must withstand gravity loads such as permanent loads from the roof or floor and overloads due to snow or to occupation. The top chord of these extensions, the one under tension, is laterally restrained by the steel deck, and it is at the height of that top chord that the load is applied, which constitutes conditions conducive to global instability. In addition, the sections used for these extensions may have only one or no axis of symmetry and be subject to local instability. Such characteristics make their flexural strength difficult to assess because these parameters, especially when combined, are not addressed in codes of practice or in the scientific literature dealing with the strength of steel flexural members. In the absence of calculation rules suitable for this type of element, the practice is to use conservative methods whose level of reliability cannot be assessed, and which generally result in uneconomic structures. In this study, the structural behavior of the joist top chord extension was established by combining experimental and analytical research.