The purpose of the study is to analyse internal forces and deformations in the roof purlins restrained by profiled decking, taking into account as many factors as possible that have a significant impact on the cross-sectional utilisation factor in terms of normal stresses, which is an important factor in efficiency and material consumption. The following assumption was confirmed in this research paper. If a profiled flooring is rigidly attached to the upper flange of a steel purlin in light-roofed frame buildings using self-tapping screws and is interconnected by rivets at the longitudinal joints, it can effectively prevent torsion and lateral bending and be used to counteract the lateral-torsional buckling of the beam. The peculiarities of checking the stability of continuous purlins as part of a roof were investigated and the effectiveness of open cross-sections of hot-rolled profiles was compared on this basis. Practical recommendations for reducing the material consumption of purlin systems by rationally selecting the type of cross-section are given. The conclusion is made on the basis of a geometrically nonlinear analysis of the stress-strain state in accordance with the full theory of beams, taking into account imperfections and stiffness of lateral bracing structures. Preference is given to purlins made of rolled I-beams, which, due to their symmetry, have smaller eccentricities of load application and internal forces by restraint, unlike channels. The analysis of the stress-strain state based on the full theory of beams and the spatial deformation model, which are in good agreement with each other, allows us to confirm the assumption that when roof purlins are restrained by profiled decking, they can be simplified in the elastic stage only for the main load in the plane of greater stiffness without the use of weights. The pitched component of the load at typical roof pitches of no more than 15º will be largely absorbed by the sheathing itself, and its value is insignificant and can be neglected in comparison with the effect of the prevailing load component in the main bending plane.

purlin, open cross-section, buckling, torsion, restraint

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