The ACI/CSA Punching Shear Model

The ACI/CSA punching shear analysis and design approach uses the ACI 318 or CSA A23.3 provisions for the basis of the implementation. A critical section is defined at d/2 from the periphery of the area of application of force. These critical sections are arranged to minimize b_od. For slab edges located within the punching check, additional critical sections will be generated by projecting perpendicular lines from the original section to the slab edges. Additionally, a set of sections will be generated for each basic slab shape (column, drop cap, etc.). Maximum overhang (from the originating shape) can be limited as a function of d as specified by the user. For ACI by default no limit is used. For CSA A23.3 the limit is 1.0d in accordance with clause 13.3.3.3.

To calculate the section stresses, an elastic distribution of stresses caused by the eccentricity between the load/reaction and the critical section centroid is superimposed with the shear stresses caused by the concentric loading to calculate a linearly varying stress distribution on the section. Where there are eccentricities in two orthogonal directions, they are considered simultaneously. γ _v for each section is calculated about the principal axes for that section. For column sections, the length/width ratios used to calculate γ _v are unmodified. For cutoff sections, the length/width ratios are modified in accordance with ACI 421.1R99.