Punching Shear in Spread Footings
Punching Shear in Spread Footings
Design requirements specified in broad statements can sometimes raise questions about their applicability in specific areas. This is illustrated in the following exchange we recently had with an engineer regarding the application of two-way slab design provisions to two-way spread footings.
Q: ACI 318-19 Section 13.3.3.1 requires that the design of two-way shallow spread footings be performed in accordance with the slab design provisions of Chapters 7 and 8 as amended by the foundation design provisions of Chapter 13. I have two questions on the applicability of some minimum reinforcement provisions of Chapter 8 for shallow two-way footings:
- Is the punching shear provision of Section 8.6.1.2 for two-way slabs also applicable to shallow two-way footings or is there an exception for footings?
- Further, ACI 318-19 Section 8.6.1.1 reads "a minimum area of flexural reinforcement, As,min of 0.0018Ag or as defined in 8.6.1.2, shall be provided...". The word "or" implies the designer can choose to meet either Section 8.6.1.1 or Section 8.6.1.2 (such that, Section 8.6.1.2 would not need to be checked if 0.0018Ag is being provided). Is this the correct interpretation or do we need to check both and use the greater amount? A design example in ACI MNL-17(21) checks for minimum flexural reinforcement in a spread footing per Section 8.6.1.1 (0.0018Ag) only and does not check Section 8.6.1.2.
A: First, regarding the applicability of Section 8.6.1.2 for two-way shallow foundations, Section 13.3.3.1 does direct the designer to Chapters 7 and 8. This broad reference would imply that 8.6.1.2 does apply to shallow foundations. A related inquiry was submitted as a public comment on the draft of ACI 318-25, questioning the similarity between the behavior of a slab and that of a footing. In addition to asking if 8.6.1.2 applied, the commenter also asked about the uniform distribution of reinforcement across the width of a square footing as required by Section 13.3.3.2.
Most punching shear tests have been conducted on representative slab specimens as opposed to representative shallow foundation specimens. While the failure mode will be similar, the overall behavior is different due to soil stiffness and elevated slab continuity. A 2022 paper by Hawkins and Ospina (doi.org/10.14359/51737111) discusses this subject and includes several results where the flexure-driven punching shear failure addressed by 8.6.1.2 occurred. The paper concludes: “Evaluation of punching shear tests results of above grade two-way slabs and spread footings with low flexural reinforcement ratios shows that design recommendations in 8.6.1.2 of ACI 318-19 can be safely extended to spread footings.” Based on this work, we recommend complying with Section 8.6.1.2 if the footing thickness cannot be increased to meet the limit on shear demand. When applying Section 8.6.1.2, note that Section 13.2.6.2 permits ignoring the size effect factor. Section 8.6.1.2 further specifies, through reference to Section 8.4.2.2.3, that this reinforcement be provided over an effective width of 1.5h on either side of the column. In the response to the public comment submitted on ACI 318-25, the committee did not change 13.3.3.2 or the general reference to Chapter 8.
Second, regarding ACI 318-19 Section 8.6.1.1, the use of “or” is not intended for Section 8.6.1.2 to be an optional alternative to Section 8.6.1.1. Every design must meet Section 8.6.1.1 and provide at least 0.0018Ag. When the punching shear demand exceeds the limiting value specified in Section 8.6.1.2, the design must also provide a greater amount of reinforcement within the punching shear perimeter. The use of “or” rather than “and” was likely chosen because the amount of reinforcement required by Section 8.6.1.2 is only required when the limit on shear demand is exceeded. It may be better stated as “A minimum area of flexural reinforcement, As,min of 0.0018Ag, or as defined in 8.6.1.2, shall be provided near the tension face of the slab in the direction of the span under consideration unless a greater minimum area is required by 8.6.1.2.
We agree the design example in ACI MNL-17(21) does not check Section 8.6.1.2 and the preceding discussion is silent on whether it applies. As noted above, we recommend Section 8.6.1.2 should apply if the shear limit is exceed, which is the case for this example. Based on our calculations, the required reinforcing ratio over bslab*h would need to increase from 0.0018 (per Section 8.6.1.1) to 0.00208 (per Section 8.6.1.2), which is an increment of about 16%. The 13 No. 8 bars provided over the full footing width for flexure provide a reinforcement ratio of only 0.00198, and would need to increase to 14 No. 8 bars for a uniform spacing across the footing width.