A calculation method for punching failure of slab-column connections subjected to vertical loads and unbalanced bending moments

Abstract:

According to China′s code for design of concrete structures (GB 50010—2010), the punching calculation for slabs with unbalanced bending moments is currently based on elastic theory to calculate the equivalent concentrated reaction force. This force is then used to evaluate the punching shear capacity of the plate-column joint under the action of vertical loads only. However, previous studies have revealed that the shear stress along the control perimeter exhibits plastic characteristics when the slab with unbalanced bending moments reaches its ultimate state. Therefore, to address the limitation of elastic methods in neglecting the plastic behavior of slabs, this paper presents a formula for calculating the punching of slabs with unbalanced bending moments, considering the influence of plasticity on shear stress. Initially, to assess the plastic development of shear stress along the control perimeter when slabs with unbalanced bending moments undergo shear failure, the ratio of the maximum shear stress along the control perimeter calculated by elastic methods in slabs with vertical loads and unbalanced bending moments to the ultimate shear stress along the control perimeter in slabs with vertical loads only is defined as the plastic development coefficient. Subsequently, through analysis of 76 sets of punching test data of slabs under the combined action of vertical load and unbalanced bending moment, it was observed that the plastic development coefficient decreases with the increase of concrete tensile strength, while the ratio of the eccentric shear stress induced by unbalanced bending moment to the average shear stress induced by vertical load increases. Consequently, by considering concrete tensile strength and shear stress ratio as the two horizontal coordinates and the plastic development coefficient as the vertical axis, the relationship between the three was fitted. Finally, the method for calculating the equivalent concentrated reaction force for slabs with unbalanced bending moments is revised to plastic theory, utilizing the derived formula for calculating the plastic development coefficient.