10' tall residential basement foundation walls 1

[TroyD]

A custom home builder has a client who desires 10' poured basement walls. The local BO reviewing the plans is requiring the walls be engineered, so he contacted me to assist. The IRC has design tables for wall thickness, reinforcing for walls up to 9' height. I am using RetainPro to evaluate the walls as a restrained at top (floor joist) with an at-rest heel pressure of 60 psf/ft. No soils report, but I gathered soil data from the USDA Web Soil Survey website for that location. My results are a 10" thick wall with #6 vert reinforcing 16" o.c. at the inside face of wall. Anyone have similar experience designing tall basement walls?

 

[jayrod12]

That seems not incredibly far off of what I've seen. Although I feel that I generally can get 9" walls to work at that height. What is the foundation at the bottom of the wall? What is the basement floor?

 

[Celt83]

Double check what the controlling criteria for the steel are in Retain Pro I'm coming up with #6 @ 18" only considering minimums for walls, if you use the minimum flexural steel requirements you end up with #6 @ 16".

Pay attention to how you end up transferring the top reaction into the joist/floor diaphragm the prescriptive connections won't check out in this case.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[XR250]

The wall is the easy part. Now try getting that load into the floor diaphragm and back out - especially when the joists run parallel to the wall. You are using I-joists or trusses you say, that makes is even more fun!

 

[XR250]

Looks like Celt83 beat me to it

 

[TroyD]

@ jayrod,
I sized a 24" wide x 10" deep footing (with 2-#5 rebar continuous at bottom). The soil pressure at the toe is ~2,300 psf, which exceeds the max. 1,500 psf allowed by the code when no soil investigation is available. But the USDA soil data indicates 'gravelly coarse sand, loamy sand, loamy coarse sand' so I think a higher soil pressure should be considered. (The 1,500 psf bearing capacity is very restrictive for toe pressure and keeping a reasonable footing width).

Basement floor slab is 4" thick (resists sliding forces)

 

[msquared48]

Don’t use #6 bar in residential construction unless you end up with #5’s less than 4” on center.

Mike McCann, PE, SE (WA, HI)

 

[jayrod12]

I didn't talk about the footing thickness, I was indicating the wall should be able to work even narrower than you have. I've rarely considered moment in a footing when the wall is connected to the floor structure at the top and has a slab at the bottom. All of the horizontal force is resisted by the slab in the end, that path is significantly stiffer than the footing in bending.

 

[XR250]

The North Carolina version of the IRC has tables up to 10 feet.
Again, I'd be very careful about how you are detailing the connection of the framing to the top of the wall and the floor diaphragm in general.
You have 1000 plf going into the floor system.
FWIW, in my experience, the actual soil lateral load is significantly less than the design load – otherwise we'd have a lot more foundation wall failures.

 

[dauwerda]

jayrod12, do you specify 9" thick walls? I was always taught to stick with even increments (8", 10", 12", etc.) as that is what is commonly available for concrete form wall ties.

 

[jayrod12]

I've been asked to do 9" walls often. I originally stayed to the 8, 10, 12 increments, but when it came to residential work the guys wanted to save every part yard of concrete possible.

 

[TroyD]

XR250, OK I just checked the current 2020 Minnesota IRC and their tables go up to 10' tall wall height also. Regarding the floor diaphragm, I am familiar with the tighter anchor bolt and blocking spacing requirements, and there are design tables provided.

 

[XR250]

TroyD,

Can you post a link to these tables? Honestly, with that amount of soil pressure, I would be looking at cantilevered or propped cantilevered retaining walls or trying to span the wall horizontally between jogs. 1000 plf is less than 6" O.C. anchor bolt spacing.

 

[TroyD]

Table attached.
Link here:

https://codes.iccsafe.org/content/MNRC2020P1/chapter-4-foundations

 

[XR250]

Thanks TroyD.

I'd love to see the math behind this table but I am grateful it exists.