Foundation design
I have a 24×36 ranch house to be built. It is a walk-out condition along one of the 36′ sides here in western Massachusetts. I am concerned about just having 10″ walls (requested by the building inspector) without a retaining wall design along the rear 36′ wall. Am I being overly cautious? There is about 6′ of backfill against this 8′ high rear wall and the driveway is on this side of the house. The opposite side (walk-out side) is just a frost wall with wood framing up to the first floor joists.
Concerned-
Replies
Do you mean a "counterfort" or "deadman" instead of retaining wall? A tee that comes off of the wall to offer support?
Are they the same as a butress? And are concrete wall guys ordinarily set up to form them easily?
Are they the same as a butress? And are concrete wall guys ordinarily set up to form them easily?
I'll let bluemoose answer his part, but this concrete wall guy asked his engineer about buttresses and was told they weren't worth much compared to a good footer. I routinely backfill 15' against a cast-in-place concrete wall, independent of joists or shoring. That's a serious footer down there. And a bunch of steel.
My side walls don't have the stress cracks that come from the buried and opposing walls moving together (connected by the joists) that are very common here.
I don't do block, but any retaining wall I build will have a 3' wide footer (wall on one edge) for 6' of level backfill. Then there's the substantial rebar connecting the wall to the footer. PAHS Designer/Builder- Bury it!
Any buttress we ever poured had a 4' footing with 3' verts coming out every foot of the buttress. And I agree with you that a wide footing can also accomplish alot. We have poured a lot of 4' wide footers, with the wall sitting on the inside of the footing...and lots o'steel from the footing to the wall.
Thank you for the response. I had toyed with the idea of a buttress, but think that a typical retaining wall (as you mentioned) is a better idea.
Yes, same thing as a buttress. They are a bit time consuming to form up, but I would think any reputable contractor would have the materials necessary.
I agree a guy building plywood forms should be able to do it no sweat. I was wondering about those using prefab metal forms.
The rig I see in my area is typically a 2+1/2 ton crew cab truck loaded with metal form sections towing a big trailer likewise loaded. Just how flexible can those crews be with those forms? Do you suppose butresses would be a problem?
Also, I seem to remember crews forming a key on top of footings to help lock the foundation walls in place. Is that still done?
I've mostly worked with plywood forms (Gates), but I know the aluminum forming systems come with corners, filler panels and bulkheads that would make building a buttress fairly uncomplicated.
A buttress really only uses two more inside corner sets and two more panels (or fillers) and then your bulkhead. The inside wall forms (opposite the buttress) should not form a seam right in the middle of the buttress because there is a good bit of pressure at that point.
They still use keyways here in Missouri. In Colorado, where engineers call for tons (literally) more rebar (because it is a different seismic region), I can only remember putting in one keyway, and that was in addition to the vertical rebar in the footing 2' OC.
I had a guy tell me once that a keyway was sometimes used to keep water from seeping between the cold joint of the footing and foundation pours. But I didn't really buy that...that's what water-stop is for.
The sills bolt on top of the foundation walls. The floor joists and floor fasten to the sills. This gives the lateral support to most foundations. The floor system is always installed before backfilling. I've seen bigger foundations with no pilasters or other supports. You need an engineer to do the calculations if you're concerned.
This seems surprising to me. Although the joists do add some support against inward collapse in the direction they run, they offer nothing to the walls in the other direction. I guess they help support the long wall, but on a 24x 36 foundation the "short" wall isn't all that short, is it? Clearly the foundation walls have to be strong enough to support the pressure of the back-fill WITHOUT relying on the joists! That said, it makes sense to wait until the joists are in place before you use heavy equipment to back-fill against a new, incompletely-cured concrete wall.
In the direction perpendicular to the joists, the decking provides lateral support. It doesn't bow up because of the weight and blocking. The foundation wall is constrained at the bottom by the footer and at the top by the floor system. In the middle, you rely on the concrete and its reinforcement to resist bending. Maximum lateral force is at the bottom of the foundation wall. Footers are the most important thing. The support given by the floor system is important, but not as important as the footers. At least, that's what I've been told. I've seen foundations collapse when backfilled without the floor, but also seen some that didn't. I don't know the intimate details. Would depend on wall thickness, height, how much steel, the kind of soil, etc. That's what engineers are for. I build what's on the print. If it falls over, not my seal on it.
Good explanation- thanks for clarifying that. It's the floor membrane which is doing the reinforcement in the direction perpendicular to the joists, which is generally the shorter wall and hence less of a job otherwise. The footing and even the floor slab contribute on the bottom side.
The support given by the floor system is important, but not as important as the footers.
The stress cracks we commonly see here occur in the sidewalls. The buried and walkout walls will lean together, connected by the floor system. That leaves the side walls in tension. Block foundations almost always have diagonal cracking, less common with concrete, but it happens there too. Problem is insufficient footers or the wall/footer connection.
These houses aren't anywhere close to falling down, but it's discomfiting to go into the basement and see daylight through the cracks, the same on both side walls. Often with previous patches along similar lines.
What my engineer stressed was the importance of building a wall that would withstand the earth load by itself. I'm probably overbuilding with my 9'x1' footers (for 15' of backfill), but we don't have any side wall cracks. My footers extend on the outside of the wall. As long as my wall/footer connection is intact (lots of steel), for the wall to lean, the footer must pick up that 15' of dirt sitting on it.
Houses with buried basements (not walk out) are a totally different matter, much simpler and can rely on the floor system for reinforcement, although not normally necessary.
The catastrophic failures you've seen, did the backfilled walls fail midpoint with the side walls intact? That would be a wall, not a footing, failure. No? PAHS Designer/Builder- Bury it!
I remember being in dark and hot basement after the whole rest of the house was built (but basement floor not poured) putting in lateral bracing for backfill. (I was a framing carpenter.) So, my point is, having a floor on, where I worked, was not considered sufficient to resist the pressure while it was being backfilled (with the heavy equipment and the falling dirt being a pretty intense "live load"). After the fill was in place, though, of course we took out the bracing. For a more permanent solution, you could put re-rod and grout in some (or all) cores of the block. The basement floor would help secure the bottom of the wall, and the first floor deck with its joists would help at the top, then the reinforcing rod in grout would help the wall in the middle resist "bending". But like someone else said, you'd be better off to seek the advice of an engineer.
I agree that in a typical foundation it is the floor system that locks the top of the concrete wall into place, but in a walk-out condition, the opposing wall has no lateral stability being framed out of wood. I am not concerned with backfilling so much as bowing over time or the fact that a truck may one day be parked against the house above this wall.
I agree with the 3' footing, offset wall and rebar. This is sort of where my gut was leading me.
Less concerned,
I agree that in a typical foundation it is the floor system that locks the top of the concrete wall into place, but in a walk-out condition, the opposing wall has no lateral stability being framed out of wood. I am not concerned with backfilling so much as bowing over time or the fact that a truck may one day be parked against the house above this wall.
Exactly.
Even with a concrete opposing wall, unless one of them has a retaining wall footing, they can lean together. That truck isn't much compared to the weight of wet dirt. Combine them and you could really have something. PAHS Designer/Builder- Bury it!