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Influence of lateral restraints modeling in pitched-roof single storey frames with tapered members considering initial imperfections

The portal frame is one of the most commonly used type of structural system for span ranging up to 40m and has been used for frames up to 75m span. In practice it is found that the most efficient solution is generally obtained by using class 2 or 3 flanges and class 3 or 4 webs. Whilst its economic advantages, this refinement brings additional designing challenges: the high slenderness of its resulting cross-sections together with the uncertainty of results caused by the increased difficulty of a thorough calculation of the resistance to lateral-torsional buckling of tapered Isection elements, increases this structure’s proneness to stability issues. When class 4 sections are used, which generally is the case of the rafter in the hunched part, the sectional buckling (e.g. local buckling of walls or distortion) may occur in elastic domain. If no lateral restraints, or when they are not effective enough, the lateral torsional mode characterizes the global behavior of frame members and, again, interaction with sectional bucking modes may occur. Moreover, as higher classes are more sensible to buckling, they are more sensitive to imperfections than more compact sections are. The actual design codes do not cover the practical design of this aforementioned type of structure. There are some provisions for design, but they are either too pessimistic or not cover all practical applications. This paper summarizes a numerical study performed by authors on a relevant series of such type of elements. A sophisticated nonlinear inelastic FEM model and different imperfections have been used to simulate the behavior of the elements.