Ductility Demands on CFS Structural Connections of Advanced High Strength Steel

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The objective of this paper is to investigate the ductility demand for typical cold-formed steel (CFS) connections when advanced high strength steel (AHSS) is used for the components. Although currently AHSS is mostly used in the automotive industry, the availability of AHSS sheet thicknesses directly applicable for typical CFS use, and similar forming techniques, makes AHSSs ideal candidates for developing next-generation cold-formed construction steel. Research in the last few decades has led to entire families of AHSS grades with unique combinations of strength and ductility (i.e., elongation). For the pursuit of safety and economy, it is important to determine the actual ductility demand of CFS construction so that acceptable ranges of ductility capacity and associated strength reduction factors can be established. Since connections often present the highest ductility demand for materials in CFS construction, this study attempts to bound the ductility demand by testing AHSS in lap-shear bolted connections. The testing program includes five AHSS grades (two dual-phase and three martensitic) and one mild steel grade. Four primary failure modes are studied: bearing, tilting/bearing, net section, and end tear-out. The connection strengths are compared to predictions by design equations in code and literature. The influence on connection strength and deformation resulting from using AHSS are studied. An in-depth understanding of these influences from a fracture standpoint is also explored through numerical simulations. Overall the work intends to provide the first steps towards bringing a wider class of sheet steels to CFS construction.