Vippning av stålbalkar i hallramar
Sammanfattning: The present thesis, written in Swedish, deals with the problem of lateral-torsional buckling, especially in gable frames. The aim of the work presented is to suggest an improved design method that takes into account the translational and rotational restraint produced by the roof and wall. A particular question is the behaviour of the comer region. More specific, is it a safe assumption to consider the comer as a lateral support. The study includes a review of literature and design methods, analytical solutions of some stability problems, tests on four frames and a FEM simulation. The design methods for lateral buckling usually start from the elastic critical load which defines a relative slenderness of the structure. This slenderness finally gives the predicted capacity. The lateral buckling theory has to take cross-sectional distortion into account because the rotational restraint is often provided concentratedly by purlins or wall rails. This distortion is important also for rolled beams which was not expected. Closed form solutions are presented for the column and references are given to solutions for the roof beam. In order to get some indication of the stability of the comer of a frame, the inner flanges and the stiffener were modelled as a three rod system. The slope of the roof turned out to be of no significance. The results also indicated that the stability of the comer could be questioned when the critical loads of the column and the girder were close to each other. This conclusion was also reached by studies of elasto-plastic frames carried out by Stoverink, at the RWIH, Aachen. Four frames of IPE 180, of which two were haunched, were tested. The two unhaunched frames reached their plastic capacity, and showed a ductile behaviour although their relative slenderness was above the normally accepted limit for full capacity. The two haunched frames, however, buckled before their plastic capacity was reached. This indicates that haunched frames of this type can not be expected to develop their full plastic capacity under conditions similar to those during testing. Finally, the Swedish standards are compared with experimental results, and improvements are suggested. Guidelines are also drawn up for the design of low-rise frames with respect to lateral buckling.
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