Is this a laterally unsupported fnd. wall?

[TORCHMAN]

Gents,

See below image of ground and basement plans. Looks to me like the fnd. wall on the north side is unsupported. The one on the southside, because there is an elevation jog in the fnd. wall, I want to say that it is laterally supported. What would you say?

 

[KootK]

Sounds about right. Even at the south condition, you're not pushing against much diaphragm. Just the landings. Fine if it's concrete. Less so if it's wood.

 

[TORCHMAN]

Thanks KootK. Yes, the landings are wood.

When I design the laterally unsupported fnd. wall, I typically consider the connection at each side of the fnd. wall to be fixed. I typically spec one layer of reinforcement that works for both moments if possible.

Do you typically spec 2 matts of bar?

 

[milkshakelake]

I generally specify rebar on each face. But that's more because I don't want to run a calculation and use different rebar in one area of the foundation wall; might get screwed up on job site. It's also generally accepted where I practice. In some places, one layer of rebar might be the norm.

If you're specifying one layer of rebar for negative and positive moment, it'll have to be in the center of the wall, which isn't very efficient. Low d for both directions. That being said, if it works, it works.

 

[jayrod12]

I probably wouldn't consider it fixed if it was up to me, but that would be situationally dependent. On a single family residence with an 8" wall, I would design the wall as pinned where it gets diaphragm support again and only put bars on the inside face.

On a multi tenant or commercial building with 10" concrete walls, fixed all day with 2 mats of bar, but those walls have 2 mats all the time anyway.

 

[TORCHMAN]

If you're specifying one layer of rebar for negative and positive moment, it'll have to be in the center of the wall, which isn't very efficient. Low d for both directions. That being said, if it works, it works.

That's what I am going with. The d is low but does the trick. I figure it is better to put one matt of 15M than 2 mats of 10M from both a labour and material perspective.

I probably wouldn't consider it fixed if it was up to me, but that would be situationally dependent. On a single family residence with an 8" wall, I would design the wall as pinned where it gets diaphragm support again and only put bars on the inside face.

On a multi tenant or commercial building with 10" concrete walls, fixed all day with 2 mats of bar, but those walls have 2 mats all the time anyway.

Interesting. I analyzed as fixed on both sides, which creates a negative moment that is higher than the positive moment, but 2/3 lower than the positive moment if it were simply supported. (wl^2)/8 vs (wl^2)/12. I used to think of it the same, where I put the bar on the inside face, but then if I consider it fixed, I get a negative moment that would be completely unaddressed if the bar is on the inside. These are very different results when it comes to analysis.

Another question. Do you assume that part of the lateral force is resisted by the footing rather than the fnd wall? I find the design quite conservative if I assume the fnd. wall to take the whole load. Below is a sketch. Is there any guidance document on this?

 

[jayrod12]

I know of no direct document guidance on it. Tends to be a "what you're comfortable with" type design.

Although I understand the point about negative bending on the outside face of the wall if considered fixed ends, however I then would ask, what's the outcome if it's not reinforced there? In my mind it's some flexural cracking on the outside face of the wall. To me, those would usually be acceptable as there is a waterproofing/dampproofing installed on the outside, and the cracks would not project through the entire wall so no leakage potential.

But again, situationally dependent. It also would depend on how far it needs to span.