Energy Dissipation of Thin-Walled Cold-Formed Steel Members

Design of cold-formed steel (CFS) structures subjected to lateral seismic forces traditionally relies on the cyclic strength of subassemblages such as strapped/sheathed shear walls. Little regard is paid to the behavior of the individual components, their contribution to the lateral resistance of CFS structures, or to the actual seismic behavior of the structure as a whole. Understanding the cyclic behavior at the individual component level is necessary to develop accurate and computationally efficient models, a toolbox of nonlinear elements, capable of accurately and efficiently simulating the seismic behavior of CFS members and the infinite number of possible configurations in cold-formed steel structures.

This report summarizes results from a research program designed to investigate the cyclic behavior and energy dissipation of cold-formed steel C–sections structural axial and flexural framing members. Twenty four axial tests and 24 flexural tests were performed to evaluate the energy dissipation characteristics of axial and flexural members experiencing global, distortional and local buckling deformations. Specimen cross-section dimensions and lengths were selected to isolate the specific buckling modes. A cyclic loading protocol is adapted for cold-formed steel members to evaluate the energy dissipation characteristics. The protocol target displacements are defined based on elastic buckling properties.

Experimental data is utilized to calibrate a hysteretic model that represents the cyclic response of cold-formed steel C-section structural framing members. The model includes strength degradation, unloading stiffness degradation and pinching behavior of the observed experimental response. Model parameters and damage rules are calibrated for local, distortional and global buckling based on the hysteretic energy dissipated. The calibrated parameters can be utilized to develop a toolbox of nonlinear hysteretic springs to represent framing axial members in CFS structures for seismic analysis and facilitate performance based earthquake engineering of CFS structures.