Basement Wall Bracing 2

[XR250]

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?

 

[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.

Dik

 

[DaveAtkins]

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

DaveAtkins

 

[XR250]

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.

 

[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.

 

[dik]

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

Dik

 

[XR250]

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.

 

[KootK]

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.

 

[KootK]

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.

 

[KootK]

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.

 

[XR250]

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.

 

[XR250]

Sorry, I was writing my post after you replied.

 

[oldestguy]

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.

 

[XR250]

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.

 

[sandman21]

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.

 

[XR250]

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

 

[oldestguy]

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.

 

[XR250]

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

 

[sandman21]

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.

 

[XR250]

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