Punching of Concrete Slabs due to Column Moment Transfer

Unbalanced moments are transferred between columns and flat plates as a result of pattern gravity loading or lateral forces due to wind and earthquakes. Shear stresses produced by moment transfer must be considered in the punching design of column-slab connections as they can be significant, particularly during earthquakes. According to the ACI 318-95 code, a fraction of the unbalanced moment is assumed to be transferred by shear stresses in the slab. The code gives an empirical equation for this fraction; the equation is applicable for the first rectangular shear critical section adjacent to faces of a rectangular interior column. Other equations, for the fraction of moment transferred by shear stresses, which are applicable for shear critical sections of any shape have been developed by earlier researchers based on linear finite-element analyses supplemented by experimental results. These equations are calibrated in the present paper using nonlinear finite-element analyses. 3D finite-elements are employed; thus flexural reinforcing bars running parallel to the slab surface as well as shear reinforcement standing in the direction of the normal to the slab surface can be modeled. The finite-element model is validated, and the analysis results indicate that equations developed by earlier researchers, for the fraction of moment transferred by slab shear stresses, are adequate.