Moment-Rotation Characterization of Cold-Formed Steel Joist-to-Ledger Connections with Variable Sheathing

Ledger framing is currently the dominant framing system in cold-formed steel construction. Chief among the advantages of using ledger framing is that the spacing of floor joist is independent of the spacing of wall studs, enabling architectural flexibility, distribution of diaphragm forces independent of axial members, and ease in construction. In this system, the ledger or rim track, typically a deep unlipped channel section, collects loads from the floor joists and transfers them to the walls studs. The ledger is installed directly on the wall such that the floor is hung from the wall (as opposed to platform framing, where the floor is installed between stories). Experimental work has demonstrated that joist-to-ledger connection behavior is complex and involves limit states such as ledger flange buckling, stud web crippling, and fastener pull-out. These limit states change based on the floor joist location with respect to the wall studs. However, current design codes for these connections assume only pure shear of the clip angle fasteners used to connect joist to ledger, and fasteners connecting the ledger to the wall studs, ignoring the experimentally-observed limit states. The work presented herein provides a robust finite element model for a joist-to-ledger connection in CFS floor diaphragm. Experimental parameters (floor joist location, and presence of OSB) are considered to capture a range of stability behavior. In addition, the influence of floor sheathing material and metal deck on the connection stiffness and strength is explored. Fasteners are treated carefully and robustly to capture complex failure modes. Pull-out behavior of the fasteners through multiple steel plies is characterized via experimental testing. Finally, this work will lead to more robust modeling and prediction capabilities for CFS diaphragms.

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