Modeling of complex hat-shaped chords of cold-formed steel trusses

The search for alternate materials for residential construction in the United States has opened a vast market for cold-formed steel members and connections. Inherent in the design of cold-formed steel members, however, is the challenge to preclude or limit the negative influence of local buckling. Engineers have successfully combated local buckling by creating complex shapes that use intermediate stiffeners to enhance structural performance. Thus, complex hat shape members are commonly used for both the top and bottom chord elements of a cold-formed steel truss.; Previous attempts at experimental verification of the hat-shaped designs revealed that although the cross sections did not exhibit a local buckling tendency; rather these shapes would experience, prior to failure, a distortional buckling behavior. The AISI Specification (1996) does not adequately address the distortional buckling phenomenon. This dissertation presents findings obtained from a research study focused on investigating both the bending and axial behaviors of the complex hat shapes. Comparing the experimental results with current AISI design methods for bending and compression members yielded data that compared poorly to the strength buckling curves (local, Winter and Hancock). Upon further analysis it was determined the members had undergone yielding as a result of their complex geometry. The use of full section strength thus comparing the ratios of a member yield load (or moment) to the critical load (or moment) as determined by finite strip analysis with the ratio of the experimental load (or moment) to the yield load (or moment) have been developed and are proposed.