Instantaneous center method for shear strength assessment of bare steel deck diaphragms with different fastener types

Steel deck diaphragms transfer in-plane shear in the lateral force-resisting systems of steel buildings, and their strength is governed largely by the discrete fasteners that connect the deck to the supporting framing and the deck sheets to one another. Current design practice estimates diaphragm strength from elastic provisions in which the most highly loaded fastener is assumed to control. The instantaneous center method (ICM) is a nonlinear, mechanics-based procedure, originally developed for eccentrically loaded bolt and weld groups in steel connection design, that has recently been extended to bare steel deck diaphragms by assembling the diaphragm strength from the inelastic load-deformation response of individual fasteners under an assumed rigid-body rotation. This paper presents the current state of an ongoing study that validates and refines that extension for diaphragms with different fastener types, including arc-spot welds and side-lap screws. Fastener load–deformation behavior is represented using experimentally derived backbone curves, including digitized arc-spot weld response curves and tabulated screw backbone curves selected according to fastener diameter and sheet thickness. Predicted diaphragm strengths are compared with available experimental results from the Steel Deck Institute Diaphragm Design Manual and the Steel Diaphragm Innovation Initiative database. For welded specimens spanning four deck gauges, the method predicts strengths close to the measured values and is consistently less conservative than the elastic approach, with the best agreement for thicker decks. For mixed-fastener specimens with thinner decks, the method tends to overpredict strength, and this trend is examined in detail. Several efforts to understand the overprediction are described, including a finite-element study of distributed panel deformation relative to the rigid-body assumption, sensitivity of the prediction to the screw load-deformation model, and a code-based corner-fastener strength reduction that depends on deck thickness and span. Preliminary results indicate that the thinner-deck overprediction is associated with perimeter and corner fastener behavior and with panel flexibility rather than with deck thickness alone. The study identifies which aspects of diaphragm flexibility and fastener behavior most influence predictions for thinner decks and outlines the work required before the refinements can support design use.