Tie-in BTW Concrete Basement Wall (N) and Wood Basement Wall (E)

[KootK]

Basement walls made of wood studs are a thing that happen here. Yeah, I know...

Anyhow, I've got a condition where an addition with concrete basement walls is to tie-into one of these wood basement wall things.

I struggle to figure out what to do here with respect to differential settlement. Obviously, neither the existing footing nor the existing stud basement wall is going to be stiff enough prevent differential settlement.

Any ideas?

Here are some of mine:

1) Build the first bit of the new basement as more wood basement wall. This doesn't prevent differential settlement much but, maybe, promotes a gentler transition.

2) Somehow arrange the floor -- and to a lesser extent the roof -- framing to be more favorable should differential settlement occur. I'll probably do this in combination with whatever else I do.

3) Put some larger pads in under the new walls near the tie in to keep new building settlement low there and not induce much new soil stress in the zone of influence of the existing foundations.

4) Put the new stuff on helical piles. This means that the new building wont settle and, also, will not add soil stress to the zone of influence of the existing foundations. On the other hand, if the existing building settles of its own accord (moisture change etc), differential settlement may wind up worse because whatever caused the existing foundation settle probably won't take the new foundation along for the ride.

5) Include a big disclaimer that differential settlement should be expected.

6) Demo some of the existing basement walls and rebuild them on stiff grade beams. Two problems with this:

a) the price tag.

b) where do you stop?

 

[XR250]

I would do the useless dowels and provide the disclaimer. There probably won't be any diff. settlement and you are likely meeting standard of care.

 

[phamENG]

How old is the existing structure? Is it old enough that any future settlement would likely come from some outside source and not a condition that exists currently (somebody digs out a basement up to the property line next door, a pipe rupture erodes a 6cy hole under the building, etc.)?

Is there a geotech involved? If so, what are their thoughts on the likelihood of differential settlement?

I've done additions on piles where they are structurally isolated. But the original structures were on sand and the geotech was pretty confident there was no reason to expect future settlement.

For most houses, I'd just do as XR suggests. Dowel the footing to keep any significant movement from happening at the junction (assuming there's any concrete at all?) and then hope for the best.

 

[KootK]

How old is the existing structure?

20+ yrs.

Is there a geotech involved?

You amuse me so.

 

[KootK]

Dowel the footing to keep any significant movement from happening at the junction (assuming there's any concrete at all?)

Probably 18"W x 6"H or 8"H. Maybe a couple of longitudinal bars.

I'll do the dowelling but, as you can imagine, if that sees any real force, the existing footing probably just punches locally. And, even if it doesn't, the surrounding footing and wall will be so flexible that holding that one location together won't likely do anything meaningful as far as preventing differential movement up atr the floor level and away from the tie-in.

One could almost argue that the doweled tie-in would create a hard spot discontinuity in the existing and tend to exacerbate problems there. See the sketch below. Imagine that you're in the new basement looking back at the existing basement wall.

 

[phamENG]

You amuse me so.

Always worth asking. Besides, it's easier to feign righteous indignation when it's somebody else's project. So here goes: "What!? I can't believe you'd agree to design a structure with no geotech report!" Alright, with that out of the way, I'll go back to designing these 4 houses without geotech reports and hope the contractor actually hires one to check it...

20+ years is less than I like, but a pretty good amount of time. I think it's unlikely that you'll see any serious settlement out of the existing unless a) the new addition overloads and triggers consolidation in some deep layer that hasn't been influenced by the existing structure or b) one of the too-out-there-to-design-for events I mentioned above happens.

Apart from the fact that it just seems like a terrible idea, why not just give them a wood basement in the addition? The one they have has lasted 20+years...so it seems unlikely that there's a swarm of voracious termites in the neighborhood.

 

[KootK]

Besides, it's easier to feign righteous indignation when it's somebody else's project.

Just so. I appreciate both the honesty and the advice.

a) the new addition overloads and triggers consolidation in some deep layer that hasn't been influenced by the existing structure or

Yeah. That was my impetus for the screw pile and "pad at the end" options. Which creates its own problems depending on how things unfold.

Here, our movement problems are:

a) Frost. We'll be below frost depth though. And, somehow, actually having frost susceptible soil here seems to happy rarely.

b) Expansive clays. So the moisture problem thing. Yes, it would be nice to have a Geo on this.

c) Flood plain stuff. I'll confirm that this is not flood plain. If it is, I may have to just abstain.

why not just give them a wood basement in the addition?

I'm glad you mentioned that as it's something I've considered. Durability aside, can you think of any structural pros or cons to doing that? I see it as pretty agnostic so far.

Plebian homes here never get geo. Not even in whole neighborhood projects somehow. I was out at a house a few weeks back where an abnormally large bathtub seems to have appreciably settled the last 1/4 of the floor plan somehow. Just crap luck being the one house where the geotech report probably would have made the difference.

 

[phamENG]

Durability aside, can you think of any structural pros or cons to doing that?

Durability is the only concern I have, really. As my former boss used to say, equilibrium is equilibrium. If the forces are balanced, you're good.

The Southern Pine folks but some real effort into promoting them about 20 years ago: Permanent Wood Foundation Manual

 

[Enable]

Plebian homes here never get geo. Not even in whole neighborhood projects somehow. I was out at a house a few weeks back where an abnormally large bathtub seems to have appreciably settled the last 1/4 of the floor plan somehow. Just crap luck being the one house where the geotech report probably would have made the difference.

One consideration might be that it should get you closer to replacing the weight in-kind, and thus be less likely to induce new settlement.

What I mean is that if you replace the 8' of dirt (or whatever it is for the basement) with a structure that is similar in terms of weight (really bearing stress at the new footing locations), you'd not expect increased settlement beyond what has occurred naturally at your founding level. I'd wager you would have a greater chance of the weights being a wash if you build a wood foundation. Of course, if your structure even with wood construction, is significantly heavier than the area of soil being replaced, that really is a moot point. But may be worth it to do the math to see how close the new weight is to what you're removing.

 

[Eng16080]

To be honest, I've never been overly concerned with differential settlement between existing foundations and new additions, provided the contractor doesn't over-excavate for the new footing. (Perhaps I've been overlooking something important though.) I also have not dealt with wood foundation walls before.

Ideas:

1. If the basement design allows it, place the new footing on top of the existing, rather than relying on what we all seem to agree are "useless dowels."
2. Is it possible to not have concrete walls next to the existing building? Maybe start the walls a few feet away and support the floor above with beams. This would give a smoother transition between the potential new settlement and the existing.