ACI 318-14 Wall Minimum Reinforcement - Basement Walls

[Jerehmy]

I've been pouring through the provisions for the better part of a day, and I keep getting hung up on 11.6.1, specifically where it says "... These limits need not be satisfied if adequate strength and stability can be demonstrated by structural analysis".

So, for my basement wall, I can have less than 0.0012 vertical reinforcement ratio as long as it works structurally? Because that's how it sounds.

The way it is now, we were originally going to have a 10" basement wall, but will likely do 8" cause we'll need less flexural rebar in addition to less concrete.

PCA notes for older ACI mention this condition may be satisfied by plain concrete design, but if that is what 11.6.1 is for, why wouldn't they explicitly state it?

Say I design the wall as plain concrete and add rebar anyways to make myself rest easy at night, does this rebar have to abide by the 18" spacing limitations?

Why even have a concrete foundation wall when you can do CMU with half the rebar? What am I missing here?

Then you have the IRC provision with concrete basement walls with rebar spaced 48"

 

[BridgeSmith]

Forgive my ignorance in this area, but why would you use a 10" wall if an 8" wall would work? Same question for 8" vs 6" wall?

 

[Jerehmy]

Boss wants 10" wall. 8' clear in basement, 10" wall seems reasonable. I'd rather have the thicker wall with the same vertical rebar instead of thicker and more rebar.

 

[BridgeSmith]

Seems like even with a 10" wall, if it's 8'+ in height, the soil pressure would require something more than #4 at 16", but maybe not. With as little as rebar costs, #4s at 12" seems like a really cheap insurance policy, but that's just me, I guess.

Edit: Running some rough numbers, if the top is supported (braced) before backfilling and the rebar is on the inside face, it seems #4s at 16" would be more than adequate.

 

[jayrod12]

#4s at 16" o/c is my typical go to. We get some kickback from the residential contractors that it's a ton of extra rebar when compared to what's specified in our local residential codes, but I stick to my guns. Most homeowners once they hear of the cost difference are comfortable paying the slight extra.

 

[Jerehmy]

We keep getting jobs for commercial buildings but they're essentially a house. Except they need to be designed per IBC not IRC, so the contractor's and homeowner's just whine about how everything is overdesigned bla bla bla. Basement foundation walls are incredibly irksome. Especially the sill plate anchorage. If I did it correctly I'd have 3/4" anchors at 15" on center. I'd be crucified. But that topic has been beaten to death on this site although it never really gets resolved.

Its either do flat plate roark (doesn't work without returns), cantilever retaining wall (again, crucified), or just do closer than IRC anchors and live with the doubt (anything closer than 4' and they whine incessantly).

 

[JAE]

Basement walls are strange things. They are really beam-columns with two-way bending in a variety of magnitudes due to various lengths and widths of wall segments.

Getting the IRC designs to correlate with ACI based designs is a fool’s errand.

The problem with the IRC is that it’s designs work in perhaps 80% of cases but houses always seem to have that odd re-entrant corner where the IRC fails...we’ve seen it many times with 1” open vertical cracks or sometimes horizontal or diagonal cracking showing up after a wet season.

Stick to your guns on buildings under the IBC. But keep in mind that a pure vertical strip of wall bending analysis can be conservative if you don’t take into consideration the axial help and the two way action that would further reduce wall bending moments.

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