Modification of the ACI 318 Design Method for Slab-Column Connections Subjected to Unbalanced Moment

Abstract

In the ACT 318-11 design method for slab-column connections subjected to unbalanced moment, the contribution factors assigning the proportion of the connection unbalanced moment to be resisted by the slab flexural reinforcement and by eccentric vertical shear stresses are prescribed by only considering the aspect ratio of columns. However, the validity of the prescribed contribution factors have not been completely verified by extensive test results, and the strength prediction by the design method using the contribution factors does not agree well with existing test results. In the present study, a modified strength model was proposed without prescribing the contribution factors. In the proposed model, the contribution of flexural moment is directly calculated using the slab reinforcement ratio, and the contribution of eccentric shear is calculated based on the conventional eccentric shear stress model. The proposed method was verified by comparing its prediction with existing test results. The results showed that the proposed method predicts the test results with reasonable accuracy, and it corresponds with the fundamental mechanics of the punching shear of slab-column connections as investigated in previous studies.