Punching shear check ACI 318-14 vs ACI 318-19 2

[BAGW]

Hi,

I am looking at a spread footing design and I came across a significant change in punching shear calculation between ACI 318-14 and ACI 318-19. The punching shear capacity reduces by 50% for thick footings if AC 318-19 is used. Any thoughts on this.

 

[Agbsh]

Notice that there is an exemption from size factor for shallow foundation as there is no failure reported and they benefit from redistribution of soil pressure (based on FAQ from ACI or something similar they published before). The section 13.2.6.2 in ACI 318-19 reads:"For one-way shallow foundations, two-way isolated footings, or two-way combined footings and mat foundations, it is permissible to neglect the size effect factor specified in 22.5 for one-way shear strength and 22.6 for two-way shear strength". This will allow you to utilize more capacity, especially for deeper depth.

 

[BAGW]

Thanks. That exception provide close to factor of 2 increase in shear capacity for foundations.

 

[JoshPlumSE]

Huh, does anyone know the theoretical reasoning behind this change? Maybe concern about pockets or such in thick footings. If I were to guess, I'd wager that it has something to do with high rise construction.... I get the sense that these high profile jobs (for whatever reason) tend to drive concrete research and code changes. Which is frustrating for the majority of construction projects that don't resemble those types of structures.

 

[CDLD]

Josh, some info in this thread:

https://www.eng-tips.com/viewthread.cfm?qid=462564