Size-Effect induced by cold-forming on the Strength of a HSS Truss Joint

A scale-down of joints for experimental tests is typically based on nominal material properties without considering the effects of cold forming. In this study, the effects of enhanced strength on the scaling of joints are studied numerically with the welded K-type joints made of mild steel and of high strength steel (HSS). The joint consisted of two cold-formed tubular braces, a plate chord, and one division plate. The effects of the enhanced strength from cold forming on the scaling joint were studied for the joint in different scales. The strength enhancement was based on material tests on the tension coupon extracted from the tubular bracings in two scales. The results showed that the coldforming enhanced the yield strength and the ultimate strength at corners of smaller profiles about 14%-16% and 10%-16% more than at same locations of larger bracings. In the joint model without considering the enhanced strength of corners, the maximum load of the joint scaled by the scaling factor is about 95% to 98% of the maximum load of the full-scale joint using the material properties measured from the profiles studied. In the model considering the enhanced strength of corners, the use of the scaling factor led to the joint maximum capacity that was up to 9% lower than the maximum load received from the models with full scale dimensions. The maximum load of the joint is mainly affected by the difference in the ultimate strengths between the profiles in two scales. The observations are valid for both mild steel and HSS joints. The studies indicate that the strength of the joint scaled up by scaling factors needs to be reduced up to 10% considering the size-effects of cold forming. For the similar joints with different dimensions, further studies are necessary if the strength reduction are proposed for design use.