Punching-shear FAQ

On this page:

Why do my punching-shear results differ from hand calculations?

Answer: A few thing to keep in mind during review of punching shear include:

• Large openings may affect the punching shear perimeter of nearby columns. We recommend modeling large openings by removing a portion of slab, rather than using the Areas with Opening feature.

• For corner columns in models prior to SAFE 2014 v14.1.0, results should differ from ACI and PCA formulation because SAFE incorporates the I₂₃ moment-of-inertia term. Beginning with SAFE v14.1.0, an enhancement has been made to the punching shear check of corner columns for all codes that consider linear elastic shear distribution along the punching perimeter to exclude the effect of the cross moment of inertia, I23. The inclusion of I23 for corner columns was making the checks overly conservative compared to experimental results. See also references 2 and 3 at the end of the page for additional information. 

Why is punching shear different from column reaction?

Answer: Punching-shear force (Vu) should be smaller than the column reaction, and similar to the value of column reaction minus column weight and load within the shear perimeter. Unbalanced moments (Mu2 and Mu3) should be similar to the column moments at the top of the column times the reduction factor (ACI 318 gama_v). Shear and moment values should also be factored according to the governing load combination. These values are difficult to index when using an envelope combination, which is not necessary since SAFE will automatically design for all load combinations, selecting the worst case as the governing scenario.

What is the concrete contribution to punching shear when shear reinforcement is present?

Answer: As stated in the SAFE design manual, and prescribed by ACI 318 requirements, SAFE uses 2*sqrt(f'c) for shear links (rebar) and 3*sqrt(f'c) for shear studs.

Why is the punching-shear check not displayed?

Answer: SAFE checks punching shear twice, once for column punching through the drop panel, then once for drop-panel punching through the slab. In this sequence, SAFE traces the punching perimeters required for calculation.

An assumption fundamental to the punching-shear check is that each column has one drop. When multiple columns connect to a single drop, two modeling solutions are available, described as follows:

1. Drop objects may be converted into slab objects such that punching is only checked for columns through slab.
2. Drop objects connecting to multiple columns may be divided into individual segments centered over each column.

See Also References:

1- Verification examples of punching-shear calculations per various building codes are available within SAFE through Help > Documentation.

2- MacGregor's "Reinforced Concrete Mechanics and Design-Fourth Edition". The underlined sentences explain why ACI 318 Code does not accept equations which include Ixy. 

3- "PTI Manual-Fifth Edition". This page shows punching shear equations for corner columns without Ixy.