Initial imperfection influences on the structural behavior of steel portal frames with variable cross sections and some sustainability considerations-Case study

摘要

Modern light gauge single storey buildings are made by pitched-roof portal frames of class 3 and class 4 tapered steel sections in accordance with stress and stiffness demand. In case of class 3 sections, when they are restrained against lateral torsional buckling, the interaction between sectional plastic buckling and overall elastic buckling of the members in compression and/or in bending is possible. 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 restrains, or when they are not enough effective, the lateral torsional mode characterizes the global behavior of frame members and, again, interaction with sectional bucking modes may occur. It is well known that the elements with slender class 3 and 4 sections are more sensitive to imperfections than more compact sections as class 1 or 2 are. The paper summarizes a systematic numerical study performed by authors on a relevant series of such type of frames applying different type of initial imperfections. A series of frames of different spans and heights have been analyzed. The frames were designed to withstand the vertical loads and satisfy the ULS and SLS criteria. The frames have pinned base connections, tapered columns, hunched rafters and a pitch roof angle of 8°. The length of the rafter hunch is 15% from the span in all the cases. Afterwards a real case of large span portal frame of 50 m, used as structural solution for an ice rink building, will be analyzed accounting for imperfections. For the same building, the present paper gives target values for utilization and operational cost. In order to achieve a viable balance between economic competitiveness, social benefit and environmental impact also some aspect of sustainable construction will be analyzed in the paper.