Unsupported Residential Foundation Design Methodology 1

[bigmig]

So how do so many engineers design a basement retaining wall with a footing at the base that is barely wider than the wall, and claim "the loads balance out"?

The footing below a 9 foot retaining wall is 20 inches wide, 12 inches thick. It has no overturning resistance and is clearly designed to act as a pin-pin model.

A few questions (asking for a friend):

1. how does a 1/2" diameter anchor bolt at 4 to 6 (!!) ft in a 2x plate on center support a top of wall reaction of nearly 450 plf. The bolt has 1/4 to 3/8 of gap because they drill it with a 1 inch bit so they can slide the sill plate over the bolts easier. Do not say friction.
2. when you have a walkout basement, and one of your walls clearly does not 'balance' against an opposite opposing side of the house, then what? Do not say "we pick it up in internal framing" or "the J bolts pick up everything".
3. if you pour your slab perimeter with a 1/2 inch expansion gap (ACI says don't pour them tight) how exactly does that slab at the base form a reaction against your wall? It is air. Does air transfer load?
4. how do you transfer 450 plf into a floor in a condition where the floor joists are parallel to the wall. I have seen the plans. There is no blocking between joists.

I see really smart people doing this (I see your plans come across my desk once in a great while), and am just wondering, what am I missing? Were you born before the age of physics, or perhaps in a world where gravity and fluid pressure do not actually exist? Are your contractors magically buying sill plates with the anchor bolts literally 'grown' with the tree the board was cut from? If so, please teach me your magical ways.

 

[jerseyshore]

The reality is that the forces are almost always less than the design loads and there are a lot of additional "supplemental" load paths that help to balance out.

Residential design is based around redundancy so it's never as simple as force at top of wall goes directly into that one bolt.

But anyone that does a lot of residential will tell you they see walls with issues every week. Issues because of the lack of sill anchorage. Or parallel framing.

I'd like to think that most of the engineers on here detail anchorage and specify blocking for the parallel conditions.

 

[SWComposites]

1. this has been discussed many times on this forum. it doesn't calc out but its been done forever so in the prescriptive code
2. no idea, other than magic.
3. presumably there is a expansion filler material. otherwise, just something that has "worked"
4. thru the floor sheathing, lol. probably doesn't calc out. but it has been long done and is in prescriptive code

 

[JoshPlumSE]

How are you analyzing the wall?
a) You could model it as a cantilever retaining wall that is free at the top and fixed as a base. That base footing would need to resist significant moment.
b) You could analyze it as a pinned-fixed wall where the top diaphragm or slab restrains the top and there is, therefore, less moment at the base.
c) You could analyze it as a pinned-pinned wall, I suppose. But, there would have to be some major sliding resistance at the base... though if there is a basement slab, that sliding resistance could be easily handled.... especially since there would be resistance from the opposite wall as well.
d) You could analyze it as a wall that is pinned on both sides, the top and fixed (or pinned) at the base. If there is reinforcement is both directions, this is probably more realistic.... My guess is that this would be closer to the reality of how the wall will actually behave.
e) Lastly, does the footing have a heel that extends out into the other direction under the retained soil. I've seen a lot of retaining walls whose have a small 'toe' part of the footing, but a larger heal.... Meaning the gravity weight of the soil helps to stabilize the footing.

If you go with d), then I suspect you won't get much overturning at the base and the footing will be more about gravity resistance.... Though I do prefer to have the footing and basement slab connected and poured together.

 

[TRAK.Structural]

These are good questions, and also ones that I'm interested to see responses from those that have done residential for a lot longer than me.

1. If I were designing this connection I would tighten up the spacing of the anchors. Another option could be a ledger that floor joists are face mounted to; this would provide direct load transfer through bearing. Curious is anyone does this??

2. I think the floor diaphragm would re-distribute this unbalanced load to the perpendicular walls that would act as shear walls. Footing sliding resistance on these perpendicular walls has to be adequate.

3. Don't know if I've ever seen expansion filler in a residential basement slab. If it were there, I would suspect that the horizontal reaction at the base slab level would be resisted by the entirety of the sliding resistance of the wall footings (all 3 walls). And don't forget about passive resistance on the vertical face of the footings.

4. No idea on this one, I would spec blocking for a few joist spaces.