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Basement Wall Bracing
2

Basement Wall Bracing

Basement Wall Bracing

(OP)

I have a 54'x34' residential basement with 9 ft. backfill on the front wall. The contractor will be backfilling with washed stone. I compute a load of 472 plf into the floor diaphragm. I don't normally like to go 54 ft. between shearwalls on a rectangular plan, but really do not want to get into interior shearwalls as the sustained loads end up being pretty high (near 500 plf). The stairweil is also interrupting my tension chord of the floor diaphragm. The foundation walls are precast concrete (Superior Walls). i never used to worry about things like this but am a little more risk averse in my old age. What would you guys do?

RE: Basement Wall Bracing

If I read this right, the wall should span vertically in which case you don't need shearwalls...you have some large openings and a door (is this the basement? or soil on 3 sides?) which may require some special attention.

Dik

RE: Basement Wall Bracing

I recommend looking at the floor diaphragm as two, separate, three sided diaphragms, since the stairwell interrupts one of the chords.

DaveAtkins

RE: Basement Wall Bracing

(OP)

Quote (dik)

If I read this right, the wall should span vertically in which case you don't need shearwalls...you have some large openings and a door (is this the basement? or soil on 3 sides?) which may require some special attention.

The wall does span vertically, but the floor diaphragm needs shearwalls to resolve the load - either using the end walls only or with some interior shearwalls as well.

@DaveAtkins - i'll check that out.

RE: Basement Wall Bracing

Why can't your porch floor brace the top of your wall against the rest of the building first floor? Looks like floor joists are oriented to help that also.

RE: Basement Wall Bracing

You should be able to tie it into the stair core and the endwalls... 54'x34' is a decent footprint.

Dik

RE: Basement Wall Bracing

(OP)

Quote (oldestguy)

Why can't your porch floor brace the top of your wall against the rest of the building first floor? Looks like floor joists are oriented to help that also.

They can for sure. I am concerned about the rest of the floor diaphragm being able to take the load.

RE: Basement Wall Bracing

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Basement Wall Bracing

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Basement Wall Bracing

Of course the second one will try to act like the first until those baby shear walls rock out of the way.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Basement Wall Bracing

(OP)
Thanks Kootk. Without the extensions, looks like I have about 6.6k going into to each wall over 5.5 ft. or about 1,100 plf. I'll see if I can get enough Simpson clips on the LVL!
If I go with the extensions, I may still have an uplift problem which would have to be resolved. I may have that problem without the extensions as well.

Might be better to use the wall to the left of the powder room for a single drag strut. Would not have an uplift problem and will be easier to connect to. That still does not solve the interruption of my diaphragm tension chord by the stairwell but i have never seen that be an issue in practice anyway.

RE: Basement Wall Bracing

(OP)
Sorry, I was writing my post after you replied.

RE: Basement Wall Bracing

Ok you structural guys looking only at the foundation. The whole structure includes the floor. That's why computations only including the walls would maybe say failure, but they work when the floor is there, properly tied in. You don't need all those extra walls.

RE: Basement Wall Bracing

(OP)

Quote (Oldestguy)

but they work when the floor is there, properly tied in. You don't need all those extra walls.
My concern is failure of the floor diaphragm as its tension chord is interrupted by the stairs. I have 9 ft. of backfill along the entire front of this house and pre-cast foundation walls that do not provide any propped cantilever effect. Have I ever seen this be a problem in practice - no, but we all should worry about it maybe a little. As DaveAtkins stated I could try to count on two three sided diaphragms but i am concerned about the shear deflection under the sustained loads. I realize, in reality, the lateral stability of most building can be attributed to a series of three sided floor or roof diaphragms as it is not always possible or practical to have continuous diaphragm chords - but the sustained load cases are the ones that keep me up at night.

RE: Basement Wall Bracing

Why would you consider two three sided diaphragms? You have one diaphragm with an opening in it. You would design for the shear and moment based on the opening. If you need examples a book with a good number is "The analysis of irrgeular shaped structures" by Malone and Rice.

RE: Basement Wall Bracing

(OP)
Hmm, there is an opening in it, but what about the disruption of the tension chord? What would take the moment?

RE: Basement Wall Bracing

Change that porch to have a concrete slab floor. A little reinforcing in it you have a deep horizontal beam. That's just one of several items that keep that wall upright. A little effort at securing the ends and this may well do the job without other assists. Even a plywood floor there will do plenty. Even look at the concrete section over those interior basement openings in front wall (doors) as a tension chord with plenty of re bars there.

RE: Basement Wall Bracing

(OP)
They are using and aluminum decking product. Maybe I can change his mind...

RE: Basement Wall Bracing

The chord shown in KootK would be designed for the loading, and then collected back into the full diaphragm. You can try looking up diaphragms with intermediate offset.

RE: Basement Wall Bracing

(OP)
Looks like I may need to spend $100 on a book. Still seems a little sketchy

RE: Basement Wall Bracing

The situation changes in a significant way when your shear forces are delivered directly to shear walls rather than to "floating" chords: the chords no longer need to be continuous in order to do their jobs. And that's a very handy thing for your particular situation.

You can replace your real diaphragm with the hypothetical one below and proceed as usual, treating the diaphragm as simple spanning. If you're a stickler for precision, you can also give the sub-diaphragm in the middle some attention easily enough. I'd be inclined not to bother.

I also note that in most respects, this approach would produce the same results as DaveAtkins' double cantilever proposal. As such, I consider this approach to serve as validation for Dave's. Not that Dave needs me to validate his recommendations of course.

Screwed up my sketch a little. Should be A & C in the second one, not A & B.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Basement Wall Bracing

(OP)
ok, makes sense. thanks

RE: Basement Wall Bracing

Notice there is no floor shown. Assuming this guide as valid for your case, the manager of this WEB site has a step you might as well take. Red flag it and explain you want to delete it and it will be gone.

RE: Basement Wall Bracing

(OP)

Quote (oldestguy)

Notice there is no floor shown
I think it is not shown for clarity. These panels are not meant to span horizontally as a unit. An individual panel can be designed that way - such as at a stair opening, but not a group of panels.
I want to thank everyone for all of their input!

RE: Basement Wall Bracing

Agreed on the vertically spanning. We've had these guys come in and talk to us. The walls are essentially a concrete joist and slab system turned vertical. I'd be surprised if the panel as a whole has any capacity to span horizontally. As far as I'm aware, only the slab spans horizontally between ribs.

RE: Basement Wall Bracing

(OP)
Jayrod,

They can design a concrete girt at the top of the wall - typically to span stairwell openings.

RE: Basement Wall Bracing

Fair enough.

Around here these walls, at the time they came and told us about them, were not a delegated design item. They required the EOR to take responsibility for them. Granted they are just standard concrete and rebar type designs, but we weren't willing to go down that route. Similar to the girder-slab system compared to Deltabeam.

RE: Basement Wall Bracing

(OP)
Around here they use a dedicated engineering firm that designs each job. I would not want to take responsibility for them either.
My neighbor used them on his house. His garage was on a hill so he just set the walls and piled some dirt on the outside to try to balance the interior gravel fill and poured the slab without attaching the slab to the top of the wall. Needless to say, it is leaning outwards and the slab has dropped about 5". I'm glad I was not involved in that one.

RE: Basement Wall Bracing

I gave KootK a star for agreeing with my approachthumbsup2

Seriously though, my thinking is that if a chord is interrupted, the conventional wisdom is that the diaphragm will fail. However, it truly CANNOT fail, because in order for the diaphragm to tear apart on the tension edge, it would have to overturn the basement walls which are parallel to the tension chord. Which will not happen.

DaveAtkins

RE: Basement Wall Bracing

Quote (DaveAtkins)

I gave KootK a star for agreeing with my approach

Sweet. Diplomacy has earned me a good deal more stars than technical expertise I reckon'. I agree with your latest 100%. Sadly though, you can only star me once per thread...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Basement Wall Bracing

I would think $100 is worth it if it would have solved your issue and future issues faster.

Link has the basic procedure and might help in your situation. The Analysis of Irregular Shaped Diaphragms

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