Unsupported Residential Foundation Design Methodology 3

[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.

 

[Eng16080]

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.

The detail that most engineers use in this situation generally doesn't calc. out. This has been discussed on here many times. The IRC prescriptive requirements seem to allow this large anchor bolt spacing and that's what most engineers use. By the numbers you would probably need double or triple that number of anchor bolts. I suspect in most cases these design loads are never realized, especially if the backfill is well drained material and there's an acceptable drainage system in place (gutters, foundation drains, etc.). I know I'm not supposed to say the bad word, but friction is also helping. Most engineers are generally hesitant to require a 12" o.c. anchor bolt spacing (by the numbers) whereas 4 to 6 feet is (somehow) allowed by the code and is common practice. I know this isn't a great answer.

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".

Are you asking how lateral pressure on the foundation wall is being resisted if there is no foundation wall on the opposite side of the basement? If so, I would consider the wall force being transferred into the floor diaphragm and from there into perpendicular walls (similar to your typically diaphragm/shear wall design). At this point, I'm assuming the anchor bolts ("J bolts") have already transferred the load into the floor. (As an aside, I generally don't use J-bolts, but headed anchor bolts instead).

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?

If you're asking how the horizontal force at the base of the foundation wall is being resisted, I generally consider this occurring at the connection between the wall and footing, not at the slab. I'm sure the slab is contributing some, but I agree this should not be relied upon with there being a gap/expandable filler.

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.

This is a bad detail, IMO. I always either reverse the joist framing direction to be perp. to the wall or use blocking. In some cases I may also design the wall to span horizontally, like at an opening for stairs. Otherwise, what resists the load, just the floor sheathing? Most likely the sheathing would buckle under any significant load.

 

[DoubleStud]

I do not rely on the floor framing. I look at the geometry of the basement wall and make sure it has enough jogs so you have perpendicular wall. I look at how long the basement wall span from corner to corner and design the wall as Fixed at the bottom and pin pin on the side. The top is free. I put enough horizontal reinforcement in the middle of concrete wall to make sure wall can span. If the horizontal span gets too big, I add counterfort. My footing is bigger to resist some moment.

I do share the same thoughts when I look at other people's drawing. I see all of the time where they put the vertical rebars on the interior face with minimal horizontal rebar. It was for sure designed to span vertically from footing to floor framing. I don't get it.

 

[jerseyshore]

I do not rely on the floor framing. I look at the geometry of the basement wall and make sure it has enough jogs so you have perpendicular wall. I look at how long the basement wall span from corner to corner and design the wall as Fixed at the bottom and pin pin on the side. The top is free. I put enough horizontal reinforcement in the middle of concrete wall to make sure wall can span. If the horizontal span gets too big, I add counterfort. My footing is bigger to resist some moment.

I do share the same thoughts when I look at other people's drawing. I see all of the time where they put the vertical rebars on the interior face with minimal horizontal rebar. It was for sure designed to span vertically from footing to floor framing. I don't get it.

Houses in the US have had the same general construction design for what, ~200 years? and basements aren't falling down everyday. For me, when it comes to basement wall issues/ failures it goes like this:

1. Horizontal cracks from lack of reinforcement in 8" hollow block.
2. Out of plumb walls on the parallel sides from lack of blocking.
3. Excessive settlement from poor soil/ water issues.
4. Sliding failures when they cut slab out for french drains and don't brace it.

And #1 accounts for like 90%+ from what I've seen in the thousand basements I've been in around NJ.

Designing a basement wall with literally any vertical reinforcement and adding a note to specify blocking on the parallel walls covers probably 99% of basement wall issues IMO.

The floor framing exists in a house and has incredible strength, even if some of it is unquantifiable. Not using it seems bizarre when it has worked successfully for centuries.

 

[bigmig]

I found a niche in forensic engineering because of a boss years ago. When the chrome wears off and the new home market plunges, houses still need fixin'.
What I learned along side many insurance inspectors and lawyers, is that "they aren't falling down everyday" is a myth that exists in the heads of people who haven't had to call their insurance company and file a claim because they received a supeona from an angry clients lawyer.

The current underpinning contractor in my town of business has a 3 year waiting list on underpinning installation. 3 years.
I think that what has happened is that people are afraid they will lose work if they follow physics, and that they haven't seen house foundation splitting, bowing and deflecting the $8k tile floor owned by the lawyer from New York. And in the back of their mind, that validates looking a little the other way.

Maybe they will go their entire career, skirting the risk, while their clients hold the hot end of that stick, like friction holding the top of wall up (because it sure aint the anchor bolts), or the drain behind the wall working so the real fluid pressure never really occurs. Just food for thought. I appreciate the honest and candid responses here. That is why I like engineers. Most have trouble talking like politicians. It is a reality. The older I get the more I realize that people, and generations change. The way I did it in my early career....that is not the way we do it now. Some of it is for the bettter, some of it (like this example) is not. The question of "how in the heck is that standing" after looking at my calcs is a good test of our humility. We don't know everything, and we have to be humble and admit that.

 

[DoubleStud]

Houses in the US have had the same general construction design for what, ~200 years? and basements aren't falling down everyday. For me, when it comes to basement wall issues/ failures it goes like this:

1. Horizontal cracks from lack of reinforcement in 8" hollow block.
2. Out of plumb walls on the parallel sides from lack of blocking.
3. Excessive settlement from poor soil/ water issues.
4. Sliding failures when they cut slab out for french drains and don't brace it.

And #1 accounts for like 90%+ from what I've seen in the thousand basements I've been in around NJ.

Designing a basement wall with literally any vertical reinforcement and adding a note to specify blocking on the parallel walls covers probably 99% of basement wall issues IMO.

The floor framing exists in a house and has incredible strength, even if some of it is unquantifiable. Not using it seems bizarre when it has worked successfully for centuries.

I am curious, what reaction do you typically use on the top of the wall? Do you do blocking in 1 joist space? What spacing? What nail pattern? I do use blocking for joists parallel to the wall @ 24" o.c. with 5 nails. It is just for extra bracing. 5 nails every 24" is not a lot of load.

The thing is most of the basement walls I design are quite long and tall. Most of the time it is on a hill side so the basement is a walk out basement. The one I fear the most is the basement wall being backfilled before the floor is being framed, even though I say it on the general notes not to do that. A lot of time they have to backfill before framing the floor. So I do my best to make my walls strong without any bracing. But it is the contractor's risk if they decide to backfill and the top of the wall deflects too much.

 

[jerseyshore]

I am curious, what reaction do you typically use on the top of the wall? Do you do blocking in 1 joist space? What spacing? What nail pattern? I do use blocking for joists parallel to the wall @ 24" o.c. with 5 nails. It is just for extra bracing. 5 nails every 24" is not a lot of load.

The thing is most of the basement walls I design are quite long and tall. Most of the time it is on a hill side so the basement is a walk out basement. The one I fear the most is the basement wall being backfilled before the floor is being framed, even though I say it on the general notes not to do that. A lot of time they have to backfill before framing the floor. So I do my best to make my walls strong without any bracing. But it is the contractor's risk if they decide to backfill and the top of the wall deflects too much.

Unless my wall is really tall, I usually just specify 3 bays of perpendicular full depth blocking at 4'-0" o/c.

I've seen some plans where people just show blocking for 1 bay, but for new walls I always go 3.

If I have a unique wall or really tall wall or something I'll use whatever reaction is actually on the wall and check each element.

Walls backfilled before the floor is constructed is definitely a concern, but anecdotally, it feels less common than it used to be. It's on of the few things that inspectors and builders seem to look for than years ago. I think any modern CMU wall with dur-o-wal every other course plus vert. reinforcing in the neighborhood of #4's @ 24" o/c can withstand quite of bit of lateral load even without the floor.

 

[DoubleStud]

How about the connection of the plate to the concrete like what OP was saying specifically when the joists sit on top of the concrete wall (instead of hanging from it).

 

[jerseyshore]

How about the connection of the plate to the concrete like what OP was saying specifically when the joists sit on top of the concrete wall (instead of hanging from it).

I always specify 4' o/c max. I think IRC allows people to go up to 6' o/c, but I always specify 4' spacing and if it's a unique situation I'll check it a little closer.

A modern basement wall failing at the top plate seems to be the least likely place for an issue to occur.

 

[DoubleStud]

I just feel like the anchor capacity should at least match the nailing of the 3 bays 4ft o.c. blocking? OP made some good points about the oversize hole on the sill. I guess people really do rely on the static friction?

 

[jerseyshore]

I just feel like the anchor capacity should at least match the nailing of the 3 bays 4ft o.c. blocking? OP made some good points about the oversize hole on the sill. I guess people really do rely on the static friction?

That's why we always specify 4 ft for both the bolts and the blocking. At worst case, if the blocking and anchor bolts align, the wall is only spanning 4 ft horizontally between top supports. Otherwise if they don't happen to align (which I'm sure is more typical) the spacing is even less.

 

[XR250]

Unless my wall is really tall, I usually just specify 3 bays of perpendicular full depth blocking at 4'-0" o/c.

I usually install a double 4 ft. back and then ladder frame the joists.
Counting on blocking being installed correctly with I-joists or trusses is wishful thinking.
I don't believe i have seen a connection failure where anchor bolts were actually installed -regardless of the spacing.
That being said, for walls over 8 ft I generally go to 24" O,.C. for the bolts and add a Simpson FWANZ. 10-11 ft. is about my limit before I go with buttresses or cant. walls.
My favorite is when the contractor tries to save money by constructing the side walls as partial height masonry or concrete (walkout basement) and then wood framing on top of that without letting me know. Makes getting the front wall pressure out really fun.

 

[Eng16080]

I do not rely on the floor framing. I look at the geometry of the basement wall and make sure it has enough jogs so you have perpendicular wall. I look at how long the basement wall span from corner to corner and design the wall as Fixed at the bottom and pin pin on the side. The top is free. I put enough horizontal reinforcement in the middle of concrete wall to make sure wall can span. If the horizontal span gets too big, I add counterfort. My footing is bigger to resist some moment.

I do share the same thoughts when I look at other people's drawing. I see all of the time where they put the vertical rebars on the interior face with minimal horizontal rebar. It was for sure designed to span vertically from footing to floor framing. I don't get it.

This is an entirely reasonable way to design a foundation, and based on some plans I've seen, it's more common in the west and midwest than it is here where I'm located in the northeast. Overall, this method seems to give a better load path in avoiding the debatable concrete wall to wood framing connection. I occasionally use counterforts/buttresses for really tall walls and other oddball conditions where I can't justify having the wall span vertically.

 

[GaStruct]

I put enough horizontal reinforcement in the middle of concrete wall to make sure wall can span. If the horizontal span gets too big, I add counterfort. My footing is bigger to resist some moment.

Are you able to get the counterfort to calc out for overturning? What spacing do you generally use and how long is the counterfort? When I have looked into using them I just analyze the wall as a simple span horizontal beam (so my loads are higher than yours this way) and the counterfort footing becomes way too big. Should probably make the effort to work up the real loads

 

[GaStruct]

My favorite is when the contractor tries to save money by constructing the side walls as partial height masonry or concrete (walkout basement) and then wood framing on top of that without letting me know. Makes getting the front wall pressure out really fun.

Contractors do this in every single walk out basement in my area. It's for cost but also aesthetic so you don't have a big tall ugly concrete wall with no siding as the grade slopes down.

Besides the front wall load, these side walls are now cantilevered retaining walls that aren't designed as such. I have this fight with them too often.

 

[XR250]

I think I posted this previously. We are working on a walkout basement repair now that has 12 ft. of backfill on the front, full height masonry on the sides but they used a ladder floor truss on top of side the wall sheathed in foam board. Well, that racked and the whole floor and front wall have moved backwards. We are adding some helical tie-backs to stabilize it.