Experimental observations on 11 Reinforced Concrete (RC) columns tested at Nanyang Technological University (NTU), and existing experimental data of 79 shear-critical RC columns are presented. Significant arch action is found in columns of small shear span-to-depth ratio and high axial-load ratio under shear force. Utilizing the sectional method for shear strength of these types of columns, which does not consider arch action, would give a more conservative prediction. Based on the truss-arch model, an expression to predict the shear strength of shear-critical RC columns is presented, which considers both the contributions of concrete and transverse reinforcement to shear strength in the truss model as well as the contribution of arch action through compatibility of deformation. The proposed model is compared with other shear strength models using the available column test data consisting of 90 shear-critical RC columns, and the results show that the proposed model can improve the accuracy of shear strength predictions for shear-critical RC columns