Evaluation of local and distortional buckling strength of purlins with paired torsion bracing using Direct Strength Method

In this paper, the authors presented a methodology to predict the local and distortional buckling strength of purlins with paired torsion bracing using the Direct Strength Method. The procedure is based on the component stiffness method that utilizes a displacement compatibility approach to calculate the anchorage forces in a purlin system. The procedure considers the partial diaphragm restraint provided by the sheathing and incorporates both first order and approximate second order torsion effects to predict the actual distribution of stresses along the cross section of the purlin. This distribution of stresses can vary substantially from the conventionally assumed constrained bending distribution and thus, correspondingly, the predicted local and distortional buckling strength can differ substantially. To validate the methodology, the procedure has been used to predict the strength of a series of 12 base tests. The test series included 8 in. (203 mm) and 10 in. (254 mm) deep lipped Z-sections with both thin (0.057 in, 1.45 mm) and thick (0.100 in, 2.54 mm) profiles. The same standing seam deck and clips were used in all the tests. The detailed investigation into the behavior of the base test revealed several slight load imbalances that, when properly accounted for, can have an impact on the predicted strength. The comparison between the tested strength and predicted strength showed good correlation and the methodology was largely able to predict test anomalies such as failures at the brace location versus the mid-span and failures of the eave purlin versus the ridge purlin. The prediction methodology ignores the additional torsional restraint provided by the sheathing and thus generally resulted in a slightly conservative approximation of the local and distortional buckling strength.