Basement Wall Custom Home 3

[AZPE]

I do alot of basements for custom homes and the ceilings that they want are usually 10' with 24" deep trusses. The restraint force per foot of wall is easily in the 1,200-1,600plf range.

I don't see how the assumption that the floor diaphragm will take the load works. I run numbers and those are huge forces. The other issue is stair openings. No floor framing to restrain it, even if it is in the middle of the floor, there is not a continuous compression member/diaphragm to resist the force and restrain the wall.

Typically when I get loads that are just too high for my comfort level I design the walls as cantilevered and require the contractor to backfill before placing the floor framing and then having a 1/4" gap at the ends of joists/trusses so that the wall can rotate.

Of course this is more costly because the footings are much larger and the wall can be as well so contractors don't like it. I always get the "I've been doing this for 30 years and I never have had to do this!" Of course 30 years ago the basements were 8' ceilings with 2x12 floor framing that was all above grade making it 5-6 feet of retaining, now we have 11-12' retaining! What are your thoughts

 

[IceNine]

I find myself in the same exact situation quite often. I too have been designing them as cantilevered retaining walls, and arguing with the contractor about it.

Glad to hear I'm not alone.

 

[COEngineeer]

AZPE, when I design basement wall here I use PCA concrete tank design table and assume the wall is fixed at the bottom, fixed on both sides and use the table to design my wall. Then I will make sure the footing is big enough so my assumption that the bottom is fixed is met.

OR, I can also assume the wall is pinned on 3 sides and put "standard footing" but then I will use a lot of reinforcement in the wall (almost like 1 away slab).

What do you guys think? Do you think my approach is correct? Typically the basement wall I design are 10" thick with 12" thick footing that is about 2-3' wide. Especially with the 55-65 psf lateral load that the geotech has been giving me. I dont get a lot of complaints about the footing width though.

 

[AZPE]

I purchased the tanks manuals and the only problem that I see using them is that there will be quite a bit of reinforcement in both faces of the wall if you consider the edges fixed. At the fixed edges you would need bars on the retaining face while at the middle of the wall you would need bars on the inside face. I believe that to properly reinforce a wall designed like that would take a very detailed set of plans and a very adept contractor, both of which are hard to find in the residential world... I do like those manuals though for checking assumptions on what is really happening. The problem with any approach is that if it is an approximation (which the tank manual and typical retaining wall design are) you have to be able to live with some loose ends when it comes to your "numbers". Thanks for the great reference!

 

[csd72]

AZPE,

Everything to do with concrete is an approximation, but it works because the stiffness of the reinforcement tends to make it behave the way you have designed it.

regards

csd

 

[COEngineeer]

AZPE, I always put my rebars in the middle of the wall. I usually go as thick as 10" and design it using the tank book. With the rebar in the middle I can take care moth negative and positive moments. I also use more dowels than the vertical to take care the larger moments at the bottom of the wall. Other engineer in the office use the largest moment (positve or negative) and design the whole wall for that moment. I take the moment a few feet from the bottom and adjust the dowel spacing and length at the bottom to take care of the larger moments. Good luck!

 

[LGARG]

Hi i agree with CoEngineer, i always put my rebars on center to take positive and negative moments. we always calculated as a Restrained retaining wall, But we do quick calcs as cantilever to provide a temporary backfill height. i think that the floor diaphragm could take this force, maybe the top of the retaining wall deflects a little so the true behavior will be a mixture between cantilever and retaining wall

LG

 

[MSEMan]

We've been designing a fair number of wire faced MSE pressure relief walls for commercial structures where basements are more on the order of 20-25' deep. In these applications the MSE wall usually runs full height 6 inches of so of the building wall. The concept could probably be applied to residential construction, maybe even employing the MSE pressure relief wall only in a top section of wall.