Progression of failure in cold-formed steel diaphragms sheathed with steel deck

Light repetitively framed lateral systems are hallmarked by the high degree of redundancy in the structural system. This can manifest as complex load paths which can be difficult to distill into traditional limit state design. During a suite of recent cantilever diaphragm testing at the University of Massachusetts Amherst, load was observed to transmit from steel deck to side-lap fasteners, then to edge and support fasteners, and eventually forcing flexural torsion in the steel joists which buckled the clip angles (web of the joist-to-web of the ledger connection). Understanding this progression is critical to making informed design decisions. To confirm the experimental findings, a high-fidelity finite element model is created to match the experimental specimen geometry. The specimen is a 10'x15' diaphragm with cold-formed steel joists spaced at 2' on center. Joists frame into two ledgers as the collectors via clip angles, a standard detail in ledger-framed construction. The specimen is sheathed with steel deck. Fasteners are the connecting elements between ledgers and steel deck, joists and steel deck, and two steel decks. The diaphragm is designed to be fixed on one side, and to be loaded monotonically on the other side, representing a cold-formed steel framed floor subjected to lateral loading. The finite element model uses shell elements, and connections behavior and contacts are fully defined. The model is capable of capturing experimental progression of failure, up to and including buckling of the clip angles. With a fully validated finite element model, it is possible to better understand load path for these repetitively framed systems, and the influence of design on the progression of failure.

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