Structural Behavior in a Gypsum Roof-Deck System

The results of a study of the structural action of a gypsum slab with steel bulb tee subpurlins and steel bar joist purlins in poured-in-place gypsum concrete roof decks are presented. Present (1966) practice is to design these decks on the assumption that only the steel is effective in supporting the loads to which these decks are subjected. However, experiments have shown that the gypsum contributes significantly to the strength and rigidity of these decks. For the bulb tee, stresses and deflections are shown to be predictable by using a composite moment of inertia that includes all gypsum, both slab and formboard, and the bulb tee. The stresses and deflections of the bar joist purlins are shown to be predictable by adding to the area of the top chord of the bar joist, a transformed area of steel corresponding to the gypsum which acts in conjunction with the top chord of the bar joists. This transformed area is determined by using an effective width comparable to the effective width commonly used in the analysis of tee beams or other beams with wide flanges.