The effect of concrete slab on the behavior of connections in steel moment resisting frames

Application of composite sections in structures is increasing in recent years. This type of structure utilizes the strength and ductility of steel as well as strength and low cost of concrete. Composite action is developed when the two load carrying structural members such as concrete slab and supporting steel beam are integrally connected and deflect as a single unit. The extent to which composite action is developed depends on the provisions of shear connectors between steel and concrete. The horizontal shear that develops between the concrete slab and the steel beam during loading must be resisted so that the slip will be restrained. A fully composite section will have no slip at the concrete-steel interface. As we know, the shear connectors provide the interaction necessary for the concrete slab and steel beam parallel to the beam. One of the most important objectives in design and construction of steel moment resisting frames, is the ability of high energy dissipation due to yieldings and plastic deformations in beams in such a way that formation of plastic hinges in beams occurs prior to that of the columns. This procedure leads to fulfilling the strong column-weak beam relation. On the other hand in steel frame structures, it is common to use concrete slab in order to construct floor diaphragms. However in design codes, specially National Building Code of Iran, it is not clearly mentioned how to consider the effects of the concrete slab on connection behavior and the strong column-weak beam relation control. In this study, the behavior of bare and composite beams in steel moment frames under monotonic and cyclic loading has been investigated through numerical modeling in ABAQUS finite element software. The requirements of National Building Code of Iran regarding the ratio of bending moment strength of columns to beams ,α, have been studied. In this regard a direct connection with uniform beam section is simulated. Rotation of beams and steel columns in different sections for formation of plastic hinges are considered as a criterion to measure these relations. In order to determine the effect of the number of shear studs (percentages of composite actions) and their location specially in protected zone within a fixed length of the girder, reduced beam sections (RBS) are modeled. The results indicated that decreasing the initial distance of shear studs arrangement from the column face leads to increasing the composite action. Analytical results also showed that the effective width of concrete slab depends on the load transfer and the force distribution. Based on the results of this study, it is suggested that in calculation of strength ratio of girders and columns, the effect of the floor slabs should be taken into account to ensure that the requirement of strong column-weak beam is fulfilled, otherwise column failure may occure before girder failure. It must also be mentioned that the relative strength of columns to girders can affect the panel zone behavior in such a way that different values of α, require different shear strength of panel zone.