CALCULATION OF THE BUCKLING OF COLD-FORMED THIN GAUGE PURLINS CONNECTED TO SHEETING

摘要

In practical engineering, the diaphragm effect of the sheeting on a roof can significantly influence the buckling of the purlins connected to the sheeting, and contributes to their stability. In the modern sliding sheeting fixed by panel clips, such diaphragm effect significantly reduces. Evaluation of the buckling effect is therefore important for the design of the purlins. However, the stability of cold-formed thin gauge purlins evades accurate evaluation, due to the complication by lateral distortional buckling, flexural-torsional buckling and local buckling. In this work we evaluated the stability of purlins using three different codes: the China code GB 50018-2002, the Euro code EC3-1-3 and the Australian/New Zealand Standard AS/NZS 4600. GB 50018-2002 factors out the diaphragm effect of the sheeting, and consequently maybe miscalculates the buckling of the purlins. EC 3-1-3 presents a method for cold-formed purlins calculating torsional restraint stiffness of the sheeting, which applies for purlins fixed to the sheeting only by self-tapping screws. AS/NZS 4600 gives different modes of buckling calculations. The three codes give equal results when there are not neither anti-sag bar nor restraint given by sheeting on the purlin, otherwise, different results occur. The result from EC3-1-3 suggests that a small increase in the torsional restraint stiffness of the sheeting leads to significant improvement of the purlin stability when the restraint stiffness is small; such dependence quickly attenuates as the restraint stiffness increases. Once the critical lateral stiffness of purlin buckling is reached, further stiffening of the sheeting restrain makes no more difference. The torsional restraint stiffness of the sliding sheeting fixed by panel clips, albeit weak, can provide significant purlin stability. The study of the diaphragm effects of roof sheeting sheds important light to the design of the purlins.