Basement Wall Top Restraint - Wood Framed Building

[skired]

I am designing a wood framed building with a 12 ft basement, retaining about 11.5 ft. The first floor framing sits on an interior shelf in the concrete basement wall.

What load path do you typically use to restrain the top of basement wall and transfer the lateral earth pressure into the first floor diaphragm? Are you sizing sill anchors to transfer the load? Wall bearing on the first floor framing?

 

[jayrod12]

Generally speaking it's anchorage for the sill, but on taller walls they're tough to get to work out, as you're likely finding. If there's a pocket, I don't see an issue with counting on the bearing, but you need to make sure that gets installed that way.

 

[XR250]

At that amount of backfill, global stability and overall sliding of the floor diaphragm should be checked. Also diaphragm bending and shear.
I have seen the first two be a problem.

 

[DoubleStud]

I generally do not rely on restraining the top. The restrained top is just extra factor of safety for me. If the wall is long, I would add counterfort. I rely on the basement wall to span horizontally and cantilever vertical.

 

[jerseyshore]

I've always seen and used the wood floor as restraining at the top unless it was going to be built in multiple stages and backfilled early. Never seen anyone check the connection at the joist bearing end side, but the parallel side should definitely be detailed to brace top of the wall appropriately.

 

[skired]

At that amount of backfill, global stability and overall sliding of the floor diaphragm should be checked. Also diaphragm bending and shear.
I have seen the first two be a problem.

Assuming the retained heights are similar around the building why would you have a global stability or overall sliding problem?

And wouldn't the load on the diaphragm be more akin to axial load than shear?

 

[skired]

but the parallel side should definitely be detailed to brace top of the wall appropriately.

I usually add blocking for the first few bays as necessary

 

[XR250]

Assuming the retained heights are similar around the building why would you have a global stability or overall sliding problem?

And wouldn't the load on the diaphragm be more akin to axial load than shear?

Could be a walk-out basement and that answers your other question also.

 

[XR250]

I usually add blocking for the first few bays as necessary

A few bays of blocking ain't gonna cut it with any significant amount of backfill. Do a free-body diagram on your blocks and see.

 

[ntattose]

I agree w/ jayrod regarding bearing. The messy thing for me is when the joists are parallel to the wall. In that case I use blocking for a couple of spaces between the joists to transfer the reaction from the top of the wall into the diaphragm.

 

[JAE]

The IRC provide some guidance on the requirements at the top of basement walls re blocking, anchors, etc.

However, after sitting on my city's building code committee a few times we always found the IRC tables and provisions to be a little "iffy".
We generally had difficulty getting their recommended anchors, etc. to work with our structural calculations.

As an engineer, a 12 ft. wall dumps the design into your lap (the IRC doesn't go to 12 feet I believe) so you need to run the numbers and determine the correct wall reinforcement, anchor rods to the sill plate, and the connection (nails or Simpson connectors) into the floor joists - whether they are perpendicular to the wall or parallel with blocking.

 

[jerseyshore]

I typically add a few rows of blocking at 4' o/c too, but that's for 7 or 8 ft walls. At 12 ft you have to look at it a bit more than that though. It's a lot of force and outside the typical IRC walls as JAE mentioned so you can't just slap up the same details you normally would.

 

[bones206]

I generally do not rely on restraining the top. The restrained top is just extra factor of safety for me. If the wall is long, I would add counterfort. I rely on the basement wall to span horizontally and cantilever vertical.

I rarely do residential work, but I share this philosophy. Around here basements are almost always backfilled before floor framing or even basement slabs are installed. We had a thread on this topic a few years ago and I was living in a neighborhood with a lot of new construction going up at the time. Using string lines and a level, I was able to confirm the two way action of backfilled basement walls based on the deflected shape.

 

[XR250]

Here is 8 ft. of backfill, 8 ft wall @ 40 PSF/FT, 2 bays of blocking using 2x10's
122 PLF uplift at joist where blocking ends. Most of y'all use I-joists IIRC, so the uplift will be worse due to the increased depth of the framing.
Anyone design for this load? Probably not. I have seen this be an issue once in house. The floor was pretty humped up.

 

[skired]

JAE, Jerseyshore I know, and that's why I'd like to rely on bearing on the first floor framing at least where framing is perpendicular to the wall. Getting the sill anchors to calc out is getting heavy. JAE, which Simpson connector have you used for something like this?

XR250 do you typically use 40 PSF/FT as at rest pressures for basement walls or is that just for the example?

 

[milkshakelake]

Taking a cue from cold-formed steel joist construction, I think pocketing with sammy anchors or something similar could work for a heavily loaded foundation wall. Waterproofing would be a hassle, so it would have to be a lot of PT joists or wrap them in a waterproofing membrane (which no contractor will actually do).

 

[XR250]

do you typically use 40 PSF/FT as at rest pressures for basement walls or is that just for the example?

Depends. I usually use 30 as I call out washed stone backfill, but probably should be using 45 or 60 as they usually cheap out and just use site clay.
IRC has tables for 30,45 and 60 depending on the soil type.
That being said, in my experience, basement walls always perform better than they should.

 

[JAE]

I don't have a specific Simpson clip I use - perhaps an A34 or A35 to fasten the joist to the plate - but again - depends on the force.

Most all houses around my neck of the woods use the raw excavated dirt (clays) to backfill over a perforated drain pipe taken to daylight or to a sump.
Even so - 50-60 pcf for at rest clay pressure would be the minimum I'd typically use....the drain pipe does clog up over the years and most residential projects don't include clean-out pipes to grade.

Spanning basement walls horizontally doesn't work often as the horizontal span can be large compared to what we see around here with 8" or maybe sometimes 10" thick basement walls.
Backfilling the walls prior to the floor diaphragm construction can cause significant inward bowing. Usually you see temporary braces propped against the inside faces at regular intervals.

 

[skired]

JAEmy real challenge is the getting the force out of the concrete into the 2x plate - sill anchors aren't cutting it.

 

[XR250]

Try a Simpson FWANZ