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?

 

[BridgeSmith]

You could design the rear wall as a cantilever retaining wall, i.e. extend the footing to the outside as the heel of the retaining wall, and reinforce it for the overturning moment.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[hokie66]

Use a cantilevered retaining wall and be done with it. Then you can just use stud walls for the interiors.

 

[EcoGen]

For sliding issues, do you have a slab on grade to depend on? If so, I would include all the friction resisting force between the slab on grade and soil, in addition to the total weight of your building too. Like others have mentioned you should also include the sliding resistance provided by party wall footings.

Regarding soil pressure value to use, without a geotech report, we have always conservatively used 60 psf/ft especially for walls restrained at top and bottom. For cantilevered walls with the top fres end, we would go as low as 45 psf/ft, again without any geotech reports. However, even if you do design as cantilevered, the top cannot be considered free end really. Also, this is all assuming proper drainage is provided to eliminate any water pressure behind the wall.

EcoGen Consultants LLC
Structural Engineers
ecogenconsultants.com

 

[jayrod12]

A cantilever retaining wall still wouldn't solve the sliding issues. Just because it's a cantilever instead of a pinned wall with a diaphragm, the total sliding force still needs to be accounted for.

And if you were to do it as cantilever, make sure you still have steel on the inside face of the wall. Because you'll end up connecting the top of the wall to the floor diaphragm, and therefore still will end up with it acting more like a propped cantilever. No steel on the inside face would result in some substantial cracking.

 

[BridgeSmith]

A cantilever retaining wall still wouldn't solve the sliding issues. Just because it's a cantilever instead of a pinned wall with a diaphragm, the total sliding force still needs to be accounted for.

We design freestanding cantilever retaining walls to resist sliding forces, why could not the rear wall of the townhouses not be designed as a semi-gravity retaining wall capable of resisting the sliding force? I assumed in my earlier post that with the additional weight of the structure above that will be applied to the top of the wall, the size and design of the heel would likely be governed by overturning, and the sliding resistance would likely be adequate between the weight of the soil on the heel and the superimposed load of the structure. The length of the toe would likely be governed by the bearing capacity of the foundation soil.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Ben29]

I'm just going to go with the cantilever retaining wall at the rear. I spent a good chunk of time running the numbers to try to get the other systems to work and I just couldn't justify it with the math.

I have a vapor barrier under the concrete slab, so I assume that the coefficient of friction is 0.1. Thus, this friction force doesn't really help.

Retaining walls used for basement walls are funny. Because you don't want it to move, hence you need to use the 60H pressure force. But if it acts as a true retaining wall then you are really looking at a 45H force and passive pressure is helping you.

I agree with Jayrod. I will use steel on both inside and outside face of the concrete wall.

I am sure someone will say I am conservative but at least I will sleep at night.

Thank you all for your insight. I always learn something here.

 

[jayrod12]

We design freestanding cantilever retaining walls to resist sliding forces, why could not the rear wall of the townhouses not be designed as a semi-gravity retaining wall capable of resisting the sliding force?

I'm not disagreeing with you. But the OP indicated that there wasn't enough sliding resistance for his design, and then other posters appeared to indicate that a cantilever retaining wall would solve the issues. But cantilever or propped, the sliding issue stands alone.

Personally I feel that the diaphragm method, i.e. using the party block walls and their additional dead load and direct contact with the soil below, would provide a substantial amount of frictional resistance.

But, not my design so I'm just indicating my concerns/comments. What the OP does with those aren't my concern.

I disagree with using a 45H force instead of 60H. The top of the wall won't deflect the same as a true cantilever because you're going to connect it to the floor diaphragm. You can't just wish away that support. Potentially you could detail it such that the wall isn't connected to the floor, but then the finishes won't work at the floor to wall junction on the outside of the house. Whether you like it or not, the diaphragm is taking some load. If you look at it like a propped cantilever, you potentially could reduce the actual load going into the diaphragm, but you can't eliminate it.

 

[BridgeSmith]

Personally I feel that the diaphragm method, i.e. using the party block walls and their additional dead load and direct contact with the soil below, would provide a substantial amount of frictional resistance.

For sure, there is substantial resistance by engaging the sliding resistance of the other footings, through the walls. It should be fairly straightforward to do that, with the floor diaphragm in place. It seems that completing the load path through the connections of the basement walls to the floor diaphragm (accounting for the moments generated between the bottom and top of the floor system, with proper blocking) would be the only significant work involved. There's in-plane shear in the party walls, but I can't imagine that would be an issue.

I disagree with using a 45H force instead of 60H. The top of the wall won't deflect the same as a true cantilever because you're going to connect it to the floor diaphragm. You can't just wish away that support. Potentially you could detail it such that the wall isn't connected to the floor, but then the finishes won't work at the floor to wall junction on the outside of the house. Whether you like it or not, the diaphragm is taking some load. If you look at it like a propped cantilever, you potentially could reduce the actual load going into the diaphragm, but you can't eliminate it.

I agree with you there.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[XR250]

Just left this jobsite. 9 ft. tall wall with 8' backfill, 32' long - nothing bracing the top. I can guarantee you it is not a cant. retaining wall as this is a tract house. As I have said before, basement walls always seem to perform better than they should. Sliding is the last thing I would worry about. Oh and the perp. stud wall had a small section sheathed with that stapled cardboard "energy brace" crap. Maybe that was doing all the work.

 

[jayrod12]

I've seen houses built like that. I've also been privvy to fixing houses with no blocking in that first joist space. The crack pattern almost ends up looking like shear cracks, start at the top corners of the wall going diagonally downward as you get to the middle of the wall length. What ends up giving away the failure mode is when I dragged a stringline at the top of the wall from one end to the other. Top of wall at the middle of the length had moved inwards a substantial amount, in one case it was over 6".

I'll find a picture of one instance and post it later.