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?

 

[KootK]

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.

 

[XR250]

ok, makes sense. thanks

 

[XR250]

So this is a page from the Superior Walls Builder's Guide. For my situation, they are recommending no more than 34 ft. between shearwalls. Not sure where this comes from but I might as well follow it.

 

[oldestguy]

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.

 

[XR250]

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!

 

[jayrod12]

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.

 

[XR250]

Jayrod,

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

 

[jayrod12]

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.

 

[XR250]

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.

 

[DaveAtkins]

I gave KootK a star for agreeing with my approach

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

 

[KootK]

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.

 

[sandman21]

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