Townhouse with walk-out basement. Need retaining wall? 2

[Ben29]

This is my first time designing townhomes. The overall length of the row of townhomes is 126ft. The width is 33ft. The rear wall retains about 9ft of soil. The front wall retains nothing. I think I need to provide a cantilever retaining wall at the rear since the soil forces are not counter-balanced. Are the owners going to think I am crazy?

And since the floor trusses run parallel to the rear wall, would you add full depth blocking or a 2x6 strong back between the floor truss for the first 2 bays?

 

[JLNJ]

It's not so much about whether or not you need a retaining wall.

The question is, "Where does the lateral soil pressure from the soil on the high side go?" You definitely need a load path for it. Your job as the engineer is to figure out where you want these forces to go and how to get it there.

 

[XR250]

Dump the load into the floor diaphragm, and take it out in the perpendicular party walls. They will need to be sheathed in plywood and designed as shearwalls with a modified LDF due to the sustained loading.

 

[jayrod12]

Dump the load into the floor diaphragm, and take it out in the perpendicular party walls. They will need to be sheathed in plywood and designed as shearwalls with a modified LDF due to the sustained loading.

Definitely do this. However be extremely mindful of the connection of the blocking to the wall on the top side, you may need additional bays of blocking, i.e. 2 or 3 spaces. Also the connection of the diaphragm to the sidewalls are going to see more than typical load.

 

[XR250]

The devil is def. in the details! It would be best to run the party walls up to the bottom of the sheathing. Unfortunately, the builder will likely complain.

 

[Ben29]

ok - I will see if I can get that to work. This will transfer 17000lbs at the top of each party wall. There are (2) 2x4 walls at each party wall, so really that is 8500lbs at the top of each 2x4 wall. Seems reasonable.

I am reading between the lines and I am hearing you say that cantilever retaining walls aren't typically used in townhouse construction.

 

[JoelTXCive]

I don't think it's a cantilevered wall because the top of wall is not free. You have a pinned-pinned wall. Or fixed-pinned.

 

[jayrod12]

Also, generally speaking the builders of these townhouses are often just glorified single family home builders. So keeping the reinforcing the way they're used to seeing it is a boon to the project. Once you start flipping reinforcing from inside face to outside face to satisfy your design assumptions, that's when it gets built wrong.

 

[Ben29]

Also, generally speaking the builders of these townhouses are often just glorified single family home builders. So keeping the reinforcing the way they're used to seeing it is a boon to the project. Once you start flipping reinforcing from inside face to outside face to satisfy your design assumptions, that's when it gets built wrong.

Jayrod, if that is the case, then won't the builder also screws up when I tell them to block the floor diaphragm and nail floor sheathing at 2 1/2"o/c?

 

[XR250]

8,500 lbs seems excessive. Based on my estimate of the unit width, that is about 1k/ft reaction at the top of the wall. I would expect about 1/2 of that.

 

[JStructsteel]

What is the slope of the hill upwards from the top of the wall? I would be worried about that surcharge if there.

 

[Ben29]

XR250: I calculated 810 plf at the top of the wall.

 

[jayrod12]

You're right, they likely will. But you can retrofit in blocking and sheathing nailing well after they've screwed that one up. You can't add reinforcing after the concrete is already there. And although you are supposed to be able to review the reinforcing before the concrete hits the forms, there's been more than one occasion where something gets poured before it gets reviewed.

 

[Ben29]

Thank you all for the advice. I designed a beefy floor diaphragm and I convinced the architect to go with CMU block demising walls at the first floor. Now I can sleep better at night.

 

[Aesur]

I would check if the diaphragm and shear walls can work to restrain the wall, however it is also worth noting that in my experience I have almost always went back to a cantilever wall or soil nail/shotcrete wall for the following reasons:
1. You still need to deal with sliding, does the structure have enough weight to resist sliding using friction?
2. Party walls are sometimes used as plumbing walls as well, can you realistically use the full wall as shear walls? Are there chances that the walls will be chopped to pieces being 2x4's after the plumber is finished with them?
3. Using the structure to retain soil imparts extremely high shear forces into the diaphragm and walls, resulting in many times blocked diaphragms and close nail spacing, contractors hate this, especially smaller ones who specialize in homes/townhomes as they just aren't familiar with this type of construction.
4. Staging of construction - what supports the wall until the structure is built?

 

[XR250]

XR250: I calculated 810 plf at the top of the wall.

That is like 60 psf/ft. Might want to spec washed stone backfill a good ways out to get that down to something more manageable.
Trying to get 810 plf reliably into the diaphragm ain't easy.

 

[Aesur]

To add to XR250 another option may be to use GoeFoam or similar material to get the forces down as well.

 

[Ben29]

I found that the weight of my structure isn't enough to combat the sliding force. Is there anything else I can do, aside from specifying a massive retaining wall?

 

[XR250]

I found that the weight of my structure isn't enough to combat the sliding force. Is there anything else I can do, aside from specifying a massive retaining wall?

Again, try to get your lateral pressure to something more manageable. I can't imagine your structure will slide with CMU party walls.

 

[Aesur]

As XR250 said, the best option is to reduce the lateral earth pressure though soil modification if you are using the structure to support the wall. From a global sliding perspective, if you don't have enough weight, you will need to look into using passive pressure though the use of turndowns/keys on your slab or perimeter. Maybe you could put a key on the bottom of the retaining wall footing and tie into the building slab/footing for sliding.