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Steel deck is widely used in the floors and roofs of steel framed structures to support gravity loads. In addition, it can effectively resist in-plane shear loading when properly connected to a structural steel framework and can help to brace a building against lateral forces. This feature of steel deck floors and roofs has long been recognized but, with few exceptions, designers have not been able to take advantage of it because design formulas have not been available. This investigation was undertaken for the purpose of evaluating the performance of wide, narrow, and intermediate rib decks when used as shear-resistant diaphragms in buildings. The results of over one hundred full-scale tests are reported herein. The tests were conducted with various conditions of panel properties, purlin spacing, and fastener type and arrangement. Empirical curves are developed depicting shear strength and stiffness in terms of the most significant variables. These are checked against a theoretical solution which treats one panel flute as an eccentrically loaded, elastically restrained column between purlins. Design charts are given for diaphragm strength and stiffness with modification factors to account for the most common methods of deck installation. Finally, suggestions are offered for ways in which the quality of field welding of diaphragms may be evaluated.