The strength and stiffness characteristics of longspan composite open-web steel joists are studied. Eight full scale single span composite joists were tested to failure to evaluate their performance in both the elastic and inelastic ranges. The test specimens' spans, depths, slab depth, and amount of shear connection was varied. Analytical methods are presented to predict both the ultimate strength and the stiffness of the composite joist. A non-linear finite element model is presented to predict the behavior of the composite joist in the elastic and inelastic regions. / Master of Science