Basement Wall Behind Staircase Opening. 1

[palk7 EIT]

Hi,

I received this detail regarding a staircase opening in basement where the floor joist/ framing does not provide any top support for the basement wall. Under this case I see it more like a cantilever wall, in that case shouldn't the reinforcement be on the other side? as opposed to this dwg. why is it provided on the inside face, is it because the garage slab is considered as lateral support? but there is no positive connection between the slab and the wall and also the slab is on the fill side. Really appreciate your inputs on this.

Thank you

 

[KootK]

I'd love for these to be cantilevered retaining walls but, to save money, I usually see this treated as the wall spanning horizontally out to locations where the top of wall lateral restraint is back in play. In extreme cases, in commercial work, I've added a horizontal girt beam to the outside of the wall as close to the top as is practical.

Another issue that I often see ignored is that the diaphragm top of wall restraints can have rather significant thrusts in them under this model.

 

[palk7 EIT]

Thank you KootK, in that case then at the ends of that horizontal span (where the top of wall lateral restraints back in play) those vertical bars will be like the end supports or columns for this horizontal span?

On the other thought would it be ok to epoxy re-bar pins from the garage slab into the foundation wall @ every certain distance (let's assume 10 or 16" O.C) and assume that the wall is laterally supported at the top?. in that case it feels more satisfying to see the reinforcement on the inner side.

 

[XR250]

The reaction into the floor system at the end of the horizontal spanning member can be pretty large. Can take a fair bit of detailing to get the load into the diaphragm.

 

[Celt83]

On the other thought would it be ok to epoxy re-bar pins from the garage slab into the foundation wall @ every certain distance (let's assume 10 or 16" O.C) and assume that the wall is laterally supported at the top?. in that case it feels more satisfying to see the reinforcement on the inner side.

Where does the load go once it is in the garage slab? the wall is only laterally supported if the top reaction is resisted by something.

As an example lets say that is a 10ft tall wall with 60H at-rest pressure:
Top restraint reaction is then 1,000 lbs/ft

Your garage slab is 6" thick so weighs about 72.5 lbs/ft^2

Assuming your vapor barrier doesn't kill the friction coefficient you net about 21.75 lbs of resistance for 1 ft^2 of slab area, 0.3 friction coeff.

So to laterally restrain our example wall we need a 1 ftx46 ft strip of slab, really probably need 5-8ft more to account for poor compaction of the fill behind the wall.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[KootK]

Thank you KootK, in that case then at the ends of that horizontal span (where the top of wall lateral restraints back in play) those vertical bars will be like the end supports or columns for this horizontal span?

Yeah, that's one way to look at it.

On the other thought would it be ok to epoxy re-bar pins from the garage slab into the foundation wall @ every certain distance (let's assume 10 or 16" O.C) and assume that the wall is laterally supported at the top?

Sure, that's just another version of my exterior girt concept. Just be sure that you're comfortable with that from a durability & permanence standpoint. Your average homeowner doesn't know that they're garage SOG is keeping their basement from collapsing on them.

 

[palk7 EIT]

Celt83, I did a rough calc. for a 9' height and with factored load ended up with 1064.25 lbs/ feet (roughly). The weight of that floor slab of that strip of 1 feet long and 17' wide garage comes up to 1275 lbs. Plus that garage floor slab basically acts as a deep beam horizontally between the foundation walls, similar to that of a basement floor slab on grade that we consider as a lateral restraint to the bottom of the wall. So why would this be different

 

[Celt83]

The weight of that floor slab of that strip of 1 feet long and 17' wide garage comes up to 1275 lbs.

now multiply by the friction coeff. will yield much less than the per ft reaction at the top of the wall.

Plus that garage floor slab basically acts as a deep beam horizontally between the foundation walls

only if you detail the force transfer to the side walls, and again need to follow the load path through to completion as once the load is in the side walls it still has yet to be resisted just aggregated into different elements.

...basement floor slab on grade that we consider as a lateral restraint to the bottom of the wall.

Assuming a fully below grade basement, the rational argument is that the slab on grade acts as a compression strut from the base of the near wall over to the wall on the opposite side of the basement. Compression buckling of the slab is rationalized away by the slab self weight preventing the lift off buckling mode and the subgrade providing full bracing for buckling mode towards the ground.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[palk7 EIT]

Got you!, thanks