lateral bracing of residential foundation walls - changes in IRC from '06 to '09

[structineer]

I'm doing a custom home build where the unbalanced retained grade measured from top of slab to top of first floor wood framing is 10'. I've reviewed all of the threads in this forum that are applicable and none really touch on this aspect. In '06, the IRC had a section that covered walls braced by wood framing and gave loads to be introduced into the framing based on wall height and soil types as well as connection requirements between the walls and the floor systems. This was removed from the '09 code altogether and remains out of it in the '12 code. After digging some more it seems clear to me that it is difficult to justify the wood floor systems bracing the walls with calculations. If you have equal walls on either side of the floor that impose the same load on the floor, you could say that the floor is a compression member, and you can check the combined stresses from axial and bending on the floor support members. However, if you have a daylight basement, there is no way the floor can brace the wall without diaphragm action. For a 10' tall wall, the load being placed into the floor is over 850 lbs/ft. Diaphragm capacities max out at around 820 lb/ft referenced in the IBC chapter 23. For TJI systems, this maxes out at 720 lb/ft. I'm using TJI floor joists, so I wouldn't be able to get the capacities that I need. I find it funny that they removed any reference of walls being braced by wood floors from the code. It seems to me that they replaced it with mention to walls being braced by slab on grade that are tied into the wall (see below for exact text). Does anyone have any information on this?

R404.1.2.2.2 Concrete foundation stem walls supporting light-frame above-grade walls.
Concrete foundation stem walls that support light-frame above-grade walls shall be designed and constructed in accordance with this section.

1. Stem walls not laterally supported at top. Concrete stem walls that are not monolithic with slabs-on-ground or are not otherwise laterally supported by slabs-on-ground and retain 48 inches (1219 mm) or less of unbalanced fill, measured from the top of the wall, shall be constructed in accordance with Section R404.1.2. Foundation stem walls that retain more than 48 inches (1219 mm) of unbalanced fill, measured from the top of the wall, shall be designed in accordance with Sections R404.1.3 and R404.4.

2. Stem walls laterally supported at top. Concrete stem walls that are monolithic with slabs-on-ground or are otherwise laterally supported by slabs-on-ground shall be constructed in accordance with Section R404.1.2. Where the unbalanced backfill retained by the stem wall is greater than 48 inches (1219 mm), the connection between the stem wall and the slab-on-ground, and the portion of the slab-on-ground providing lateral support for the wall shall be designed in accordance with PCA 100 or in accordance with accepted engineering practice.

 

[structineer]

http://www.biaw.com/documents/09 Matrix.pdf

found the link above which gives this explanation:

Deletion of these lateral
restraint provisions brings the
2009 IRC into agreement with
the 2000 & 2003 editions.
Proponents of removing the
top of foundation wall lateral
restraint provisions reasoned
that the traditional prescriptive
provisions for anchor bolts &
floor systems connections
have performed well for many
years without substantiated
problems or failures.

So, if I get in trouble in court for a wall/floor failure, can I just reference this? I doubt it. That is absolute bs.

 

[XR250]

With those kind of loads, I would be using internal or external concrete buttresses or any possible jog available and let the wall span horizontally between them. You could also try designing the wall as fixed at the base to reduce the load. This would require a larger footing and likely more reinforcing. FWIW, I have seen many instances of entire houses being racked by backfill pressures.

 

[structineer]

XR250,

Fortunately, I do have some jogs in my wall and I have a garage slab that can resist a portion of the loads on one wall. I'd love to run a finite element analysis if I have time.

 

[XR250]

ok, have fun with that