Behavior of reinforced concrete columns under biaxial shear forces based on ACI 318

•Most design codes do not consider biaxial shear design for columns.•Traditional uniaxial shear design for biaxial loading might be unconservative if shear strength equations are accurate.•ACI 318 (19) provides new shear strength equations along with a trilinear interaction curve for biaxial shear loading.•A simple trilinear interaction curve can reasonable and conservatively, in most cases, be used for biaxial shear design. Columns that are at the intersection of frames in two different directions are subjected to biaxial shear when lateral loads are simultaneously applied in two directions. Using the resultant shear and evaluating one-way shear strength along the direction of the resultant force, as most design codes do, is unconservative especially when significant shear co-exist about both axes. The conservatism of one-way shear strength in some expressions of ACI 318 (14) balances the not consideration of biaxial shear design in the code. However, the use of more refined or accurate expressions as in ACI 318 (19) makes the consideration of biaxial shear for design a need. Experimental data from the literature is used to compare ACI 318 expressions and validate the trilinear interaction diagram approach included in ACI 318 (19) for biaxial shear loading, showing that it is a simple and reliable method for design.