Wood Diaphragm Supporting Basement Wall

[MCurry]

Hello all. I just searched this forum but didn't find an answer to my question. I'm designing a residential foundation, cut into an hillside, and I'm treating the uphill wall as a basement wall (supported at top and bottom - not a cantilever wall).
The floor diaphragm stresses are a bit high, and will require a blocked diaphragm.
My question is this - have any of you seen a basement wall failure because of an overstressed floor diaphragm or the connection of the diaphragm to the perpendicular walls?

Thanks - I'm looking forward to your response.

 

[scottiesei]

This is done up North quite a bit so maybe one of our more northern friends will chime in. Triple check your calcs., maybe have an associate check them and trust your design! Thicker plywood and double ledgers at the side walls may help you sleep better.

 

[JAE]

Keep in mind that wood creeps under constant loading. We looked at a welcome center up in the Dakotas a few years ago which had this situation. The whole building had deformed due to the lateral earth load on the wood sheathed diaphragm. This was due to lack of engineering analysis, not just that it was wood. But wood framed structures have a higher degree of flexibility that you don't always find in concrete or steel structures.

 

[MCurry]

Thanks scottiesei and jae. I have not seen (or haven't recognized) a failure from earth pressure supported by a wood diaphragm. I guess I'm surprised - there are quite few large houses with basements around here (central virginia), and I know many are not engineered. I'm sure if I checked I'd find inadequate anchor bolts and diaphragm nailing. scottiesei said it right - "trust your design" - I just find that I can "sell" a more expensive structural detail if I can picture the failure in my head.

With that in mind -jae, how did the Dakota building deform? How bad was it?

Thanks to both of you for responding - its my first post on this site, seems like a great resource.

 

[kkoloj]

I have been involved in a couple of custom homes where we resisted soil pressure with the diaphragms. However diaphragms are only part of the issue here. Once the load is in the diaphragm where does it go? The next big step for us was designing the lateral system for both seismic (I'm in CA) and soil pressure combined. These projects were steel/concrete floors with steel moment frames or concrete shear walls for the lateral system. Just something else to think about.

 

[JAE]

MCurry,

The deformation of the building we looked at was pretty severe in that the interior of the building was wide open....cathedral ceiling supported by long glue lam beams, and the building was quite long if I remember right.

The basement was of course under ground on one side and the fill actually came up to within about 4 fee of the eave of the rear upper wall, which had an extended rear concrete block wall to interface with the earth. I seem to recall our conclusion that the original engineer (It was engineered) didn't really follow the load path through (so I second what kkoloj said about following it through).

We ended up temporarily bracing the basement walls and eventually re-excavating and installing some additional structure to better transfer the lateral earth loads away from the diaphragm.

There are lots of wood framed homes that successfully resist earth on one side without any problem. Just do the math and watch not only strength, but the stiffness as well.

 

[MCurry]

Thanks to all for sharing your expertise!

 

[sundale]

I never load a wood diaphragm with unbalanced dirt forces. This is done all the time and works well 95% of the time. The remaining 5% that has big problems will incurr the wrath of lawyers and the licensing board.

The shear numbers on the side walls are usually quite large and the allowable diaphragm shear values given in the code should probably be ratioed by 0.9/1.6=0.56... If you should some how demonstrate that this works numerically, then as the other posters said, this leaves no strength in the diaphragm for seismic/wind and creep deflections should be considered: flexible diaphragm.

A cantilever retaining wall is typically OK for residential construction. If possible, use the basement slab and downhill stemwall for sliding resistance and then your heel only need work for overturning. Also keep in mind that the contractor is going to want to backfill the wall ASAP so his framers can get to the main structural floor easier; having a retaining structure that does not need the help of the flexible diaphragm is a good idea from a sequence perspective.

 

[HouseBoy]

What type of foundation is going to be used? Spread footing or drilled and socketed concrete piers?
We design alot of hillside foundations. Soil type and below the footing soil profiles are a big factor in determining what foundation system to use.
Perhaps you can utilize buttresses in the front wall to provide stability without requiring the diaphram and the wall spans horizontally.

 

[MCurry]

Here's the whole story. A new client (architect) came to me with a house under construction, with retaining wall already built. Spread footings, braced by floor diaphragm. I've been trying to get the wall to be supported by the diaphragm, but the forces are pretty high (end with a 550 plf diaphragm shear, plus or minus). Delivering the earth-pressure-generated diaphragm shear to the side walls was even more problematic. So after farting around with it for too long, I convinced the architect to reduce the height of the retained earth so that everything works.

Thanks everybody for your input. Invaluable!