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Foundation Retaining Wall Design

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May 20, 2013, 05:10 PM
Foundation Retaining Wall Design
Hello All,

I have finally got around to starting my garage project and I am struggling with code interpretation concerning foundation walls. There are a lot of factors here but here goes:

This is a one story 30 x 30 building. One wall will need to support a max of seven feet of unbalanced fill. An adjacent wall will support the seven feet tapered down to two feet. The other two walls are above grade and will be wood framed (no problem). At this point I am planning a spread footing, block wall on two sides and block stem wall on the other two with a ground supported slab.

Heres the problem: Chapter 4 of the IRC talks about retaining walls and reinforcement requirements based on height of unbalanced fill. Seven feet of fill with an 8 inch block wall needs #6 bar spaced 32 inches. So far so good. However... Chapter 6 describes the general requirements for masonry wall construction and specifies lateral support based on length or height. It seems that chapter six refers to above grade walls and does not account for soil loads. Chapter four doesn't mention lateral support at all except that we cannot backfill above four feet until the floor or roof is installed.

Now here is my problem: The way that IRC describes this structure is that the floor or roof at the top of this foundation wall is what is resisting the overturning moment imposed by the soil load. I would be much more comfortable with this arrangement if it was a floor attached directly to the wall. Heavier framing, plywood subfloor, etc. However, since this is a garage with just the roof attached to the wall, there is no guidance about sheathing or bracing the bottom chord of the roof trusses to eliminate shear and support the wall.

I can add elements to stiffen the structure (bond beam in the wall, buttress walls for lateral support, etc) but I don't want to confuse things when it comes time for permit approval.

Am I just overthinking this project too? Any thoughts would be appreciated.
May 20, 2013, 05:32 PM
General disclaimer: I understand that nobody here can give me specific answers for all the understood reasons. I'm just trying to fully understand what I am doing. Also, I'm not totally against hiring an engineer for this either. Obviously, I don't want the cost if I don't need one though. In fact I did have an engineer look at it and he recommended I submit my own drawings and see if they are accepted. He was actually trying to save my money... Imagine that Eek

Thanks again for your input.
May 21, 2013, 05:46 AM
The engineer gave you good advice. What you are doing is not a complicated thing, you are just worried about being code compliant.

Because of the load of the fill on the block walls, The IRC is assuming some form of top bracing of the wall - be it a floor structure or in your case, the roof structure of your garage. With solid fill block going to embedded foundation bolts attached to a sill plate, the 'weak link' will be attaching the truss to the sill plate. All your side load will be in compression against the bottom cord of the truss so this is where you need to make sure it's strong.

I would certainly do more than simply toe-nailing the truss to the top plate. Hurricane ties will help a lot, as any inward load on the wall would have to shear either the metal tie or the nails attaching it. You could also go to a solid lag through the truss and into the plate below. And remember, you'll be spreading all the load across all of your trusses, not just one.

One simple way to guarantee that the trusses can take the compression load from the wall would be to design the truss so that it's notched to fit over the sill. With the solid bottom cord braced against the inside of the sill it will not move, even if the notch is only a half-inch deep. In fact, you can achieve the same thing by installing a half-inch plywood ceiling. The plywood will act as a continuous brace running the full length of the wall and will also serve as cross-bracing to tie all the lower cords of the trusses together. Doing this will move the potential weak point back to how the sill is attached to the block foundation - easily made strong by enough solid fill and foundation bolts.

But back to your engineers advice: Reading the IRC is much like trying to interpret the Bible - get too literal with it and you'll wind up thinking that you must dance with a poisonous snake to build things right. All code departments have a plans review guy. Give him a call or set up a meeting and just ask what they recommend in your situation. It's free advice and whatever he tells you to do will be guaranteed to pass your local code requirements.

May 21, 2013, 02:32 PM
heh, Jaybee, the "real pros" actually kiss the viper. it must apparently work, as many of them meet their maker shortly thereafter Wink

sig: if this is a new economy, how come they still want my old-fashioned money?