CSA A23.3 Equations for Calculation of Allowable Shear Stress on the Unreinforced Section

The allowable shear stress is calculated by selecting the appropriate equation from CSA A23.3 (13-5), (13-6), (13-7), or (18-5).

Equation 13-5 controls in non-prestressed concrete zones with large column aspect ratios. As the aspect ratio of the column gets larger, the allowable punching shear stress approaches the allowable one-way shear stress.

Equation 13-6 is intended to correlate the allowable shear stress in non-prestressed zones with the ratio b_o/d. This equation generally controls in thinner slabs with large columns or at critical sections outside column caps.

Equation 13-7 is the upper bound of allowable shear stress for non-prestressed zones.

Equation 18-5 is for application to prestressed punching zones. In order to qualify as prestressed, a zone must meet the following criterion:

1. The effective prestress, f_pc at the column shall not be less than 0.8MPa. The effective prestress is calculated by averaging the precompression in all the elements within the punching check radius. This could result in non-prestressed equations being used in drop caps of prestressed slabs where the precompression drops below 0.8MPa in the cap. Additionally, if large restraining elements are used (i.e., shear walls) that divert the prestressing force in a region, the non-prestressed equations would correctly be used where the average precompression is below 0.8MPa.
2. The column must not be located near a slab edge or large opening.

If any of the above conditions are not met, equations 13-5 through 13-7 are applied.

CSA A23.3 Maximum Reinforced Section Stress

The reinforced shear stresses on the column sections are limited to a maximum of $0.75\lambda {\varphi }_c\sqrt{{f^{\prime }}_c}$ per CSA A23.3 13.3.8.2.

CSA A23.3 Calculation of Punching Resistance with SSR

Where SSR is used the punching resistance is calculated as follows:

CSA A23.3 Miscellaneous Provisions

The spacing to the first stud is calculated as 0.4d in accordance with clause 13.3.8.6. This spacing is rounded down to the nearest 5 mm.

The maximum typical stud spacing is 0.5d or 0.75d in accordance with clause 13.3.8.6.

The maximum tangential spacing of rails at the face of the column is limited to 2d. Tangential spacing requirements are not checked at locations other than the face of the column/support.

The SSR is extended until the cutoff section stresses are within the allowable limit of $0.19\lambda {\varphi }_c\sqrt{{f^{\prime }}_c}$per CSA A23.3 13.3.7.4. For cutoff sections outside the original column perimeter sections, γ_v is adjusted in accordance with ACI 421.1R-99 Appendix B.

The minimum rail length is 2d in accordance with 13.3.7.4.