Resisting Overturning with 1st Floor Diaphragm

[JNEnginr]

Your thoughts please...

I have a rectangular building, whose length is twice the depth. Along the length on one side of the building, grade is high (10ft above slab) so I have a high foundation wall. Along the opposite side, grade is low, so foundation wall is low. Grade slopes down to the low section on the sides. So pretty much have tall foundations on 3 sides, low foundation on 1.

Instead of using a retaining wall with large footings on the high wall to resist overturning, i want to develop the force at the top of the wall into the floor diaphragm, and have the plywood take the load to the two tall sidewalls. The plywood would then be fastened as needed to the sill plates and anchored into the sidewalls.

Preliminary calcs show shear at the tops of the walls to be around 1 kip/ft. Seems high and would require a good amount of screws.

Is this how most would handle this situation? Pros/cons?

Thanks,

 

[dcarr82775]

We do not know the plan size of the building so that limits things. Generally it can be done in wood if the loads are low enough (small plan size). However, IMHO as soon as you calculate the required strengths/capacities including the duration factor for permanent loads it becomes prohibitively difficult to show that it is reasonable to actually build it that way. I would probably go with the retaining wall unless the building is very small

 

[JNEnginr]

The plan is about 90ft by 45ft. Concrete Foundation walls, wood construction.

 

[KootK]

X2 for Dcar's concerns and preference for retaining walls. Additional concern:

1) check diaphragm deflection to be sure that you're comfortable with it.
2) because grade steps, your diaphragm may too which can get ugly.
3) to keep load off of the diaphragm, the retaining wall should not deflect much. In this situation, I'll design for at rest pressure rather than active.

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.

 

[JAE]

Using wood diaphragms to resist permanent earth forces always concerns me.
I do use them but I try to be very very conservative with them.

We investigated a building once that had a long wood diaphragm with earth - very similar to what you describe.
It had all sorts of problems - mostly distortion of the diaphragm.

I think the main concern is long term deflection of the wood diaphragm. You can calculate initial deflection in your diaphragm based on various formulas but I think that with all the panel joints, nailing, misc. chords, and adjoining members there is a much higher predisposition to long term creep in the floor.

Therefore, we usually try to limit the shear and deflection of the diaphragm to much lower limits than you'd typically do for short term wind forces.



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[JNEnginr]

Would a combination of the two be appropriate? Say extend the footing slightly to reduce the amount of load being imposed upon the diaphragm?

 

[manstrom]

I think there is a provision in the code that does not allow wood to be used to retain lateral forces from concrete. I think.

Regardless, it is done all the time with out people even thinking about it in single family. Personally, I would put in a cantilevered retaining wall or counterforts to resist the lateral.

In calculating the lateral force to the wall, don't forget to include the dead load resistance. 1 klf sounds high.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[KootK]

Would a combination of the two be appropriate? Say extend the footing slightly to reduce the amount of load being imposed upon the diaphragm?

A combination of the two is probably a better reflection of reality. The trick is coming up with a reasonable split. In my opinion, you really can't do enough to stiffen the retaining wall if your strategy is to keep load off of the diaphragm. That's why I recommended designing for Ko. If you go with a 'half measures' retaining wall, it will be relative flexible rotationally and you should probably just design the diaphragm for the entire load.

If your diaphragm is simple enough, and you can just run axially connected floor joists from the high side to the low, you could possibly make the case that you're just transferring load from the the high (active) side to the low (passive) side without really involving the diaphragm. Obviously, the retaining walls on both sides would need to be designed accordingly.

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.

 

[Jerehmy]

Remember that wood diaphragms take a huge reduction in strength when using to resist dead load.

You would reduce the allowable seismic values by 0.9/1.6 = 0.5625 reduction in capacity for the dead load load case for ASD.