IRC Prescriptive Monolithic Footings

[StrEng007]

I rarely dabble with the prescriptive code sections for foundation design. At first glance, I noticed the 2021 IRC does not have any prescriptive approach for the design of uplift on foundations. However, the Florida Building Code, Residential does. Is anyone familiar with why this limitation exists with the IRC.

For those familiar with the 2020 FBC, Residential, how have you found the prescriptive foundations sections to perform? I realize the tables are limiting for wind and don't allow you to go higher than 150 MPH or anywhere near the HVHZ for that matter.

Also, the prescriptive tables provide gravity and uplift (which is negligible in their eyes for any CMU wall construction). However, I don't see any discussion about out-of-plane lateral loads and the need to resolve the overturning of individual wall segments. With leeward winds, and uplift, there are a lot of rotations happening on an eccentric footing and manual calculations have always resulted in foundations that are ridiculous in comparison to concentric stem wall foundations.

 

[phamENG]

I'd call it less of a limitation and more of a gap. Though prescriptive, it doesn't prescribe much. It just says "you have to have an uplift load path." At braced walls there's a bit more...if the uplift load is greater than 100plf you have to use connectors until it's less than 100plf. That's it.

It effectively places the onus of designing a 'prescriptive load path' on Simpson Strong-Tie. A roll I'm sure they don't mind as it forces people to buy their products, but not the way it should be. It also creates headaches when you work in a jurisdiction where the building inspectors won't enforce it if it's not spelled out in the IRC, so contractors complain when you make them build it correctly.

 

[StrEng007]

Yes, it's difficult when the design fall outside of the prescriptive methods and you provide the analytical solution. TBH, I'm not even sure how some of those prescriptive answers exist in the first place.