Characterizing Wall-to-Diaphragm Moment-Rotation Response in Cold-Formed Steel Systems via Fastener Limit States

Abstract

Ledger framing is currently the dominant framing system in cold-formed steel buildings and is a popular choice for its flexibility in floor joist spacing with respect to wall stud spacing. In ledger framing, the floor, which is sheathed with oriented strand board (OSB), is framed into the side of the wall via a rim track (ledger) and clip angle connection. Experimental studies on ledger-to-wall connections have demonstrated complex behavior, involving fastener pull-out through multiple plies, ledger flange buckling, and stud web crippling. This is in stark contrast to current design recommendations, which assumes the connection is a simple shear connection with no moment-rotation capacity; fastener shear capacity is the governing limit state. To improve design recommendations and understanding of load transfer mechanisms, a robust finite element model (FEM) on ledger-to-wall connections was modelled in ABAQUS. Experimentally-derived fastener behavior was adopted in the modelling approach proposed herein, and was found to accurately capture the flow of forces in the wall-diaphragm connection. The model is validated with experimental tests and is expanded to simulate the influence of metal deck on the connection stiffness and strength. In addition, effect of metal deck thickness and fastener spacing is explored. Ultimately, this work at the connection level will lead to more robust modeling and prediction capabilities for CFS diaphragms with metal deck sheathing.