Connection at top of foundation wall to slab-on-deck for soil loads

[bnickeson]

I have a commercial basement foundation wall resisting about 14' of soil load, so the reaction at the top of the wall is significant - about 4.7 kips per foot ultimate. Typically when we have this condition, our top of foundation wall goes all the way up to the finished floor level so that the top of the wall can laterally bear directly against the side of our slab-on-metal deck. That makes this condition easy to design as there is no connection required to resist this lateral soil load, and typically there is an exterior stud wall over this location so that it can't be seen. However, on my current project, the basement only occurs at about 50% of the building footprint, so we have several walls that are directly below finished spaces and the architect would prefer the top of wall to be below the slab-on-metal deck.

So now we have the question on how to connect the deck to the top of the wall to resist this large out-of-plane load. Headed anchors can't resist the load as the breakout is about 75% overstressed. We don't have beams framing perpendicularly into the wall at all locations so we can't use the beams either. And since we're only using a 4" total slab thickness with the metal deck, developing rebar into the slab for shear friction doesn't appear to be an option either.

I'm wondering if anyone else has come up with a great idea in the past that could be considered? Or are we kind of SOL and I'll have to tell the architect we need to bring the top of wall up to top of slab? Another idea I'm considering is to hold the top of wall down 1/4" to 1/2" and pour an overlayment on top of it to get the finish and elevation perfect. Though I don't know if that will stand up to power-troweling.

 

[TRAK.Structural]

At locations without perpendicular beams framing in, can you thicken a strip of the slab to develop bars? Or even just thicken the slab with a downward haunch to provide the physical bearing along the top of the wall like the typical scenario you described where the slab braces the wall.

What about a strut (steel angle) that goes perp to the wall and fastens to the underside of the deck intermittently until you can transfer the load?

 

[jhnblgr]

What it look like in plan view?
From a construction standpoint it will likely be poured after backfilling, it would be wise to build it with pilasters or retaining wall design methods. Is there live loads on the slab above where wheel loads will apply pressure to the wall?

 

[jerseyshore]

From a construction standpoint it will likely be poured after backfilling

I agree. Whenever I have these walls, I design it twice as a cantilever and restrained. They won't be able to build the slab on deck without backfilling first so best to just design it accordingly.

 

[bnickeson]

At locations without perpendicular beams framing in, can you thicken a strip of the slab to develop bars? Or even just thicken the slab with a downward haunch to provide the physical bearing along the top of the wall like the typical scenario you described where the slab braces the wall.

What about a strut (steel angle) that goes perp to the wall and fastens to the underside of the deck intermittently until you can transfer the load?

That might solve one side of the basement, but not the sides with the beams framing into them. I'd like to use one relatively consistent detail all around if possible. And a steel strut is a consideration, but given the amount of load and how many you'd need, it probably isn't all that practical.

I agree. Whenever I have these walls, I design it twice as a cantilever and restrained. They won't be able to build the slab on deck without backfilling first so best to just design it accordingly.

I would disagree with this. We only go this route if the contractor requests it to be backfilled prior, and they are forced to pay the difference between the two designs which is often significant (this cost usually makes them rethink needing to backfill prior as well). We've done a couple dozen of these types of buildings in the last few years and have only had one issue with them backfilling prior to the deck being poured. We will sometimes allow them to build a small ramp up to the deck off a corner of the basement, otherwise they have to use a lift for any power trowels or other equipment they need.

 

[jerseyshore]

That might solve one side of the basement, but not the sides with the beams framing into them. I'd like to use one relatively consistent detail all around if possible. And a steel strut is a consideration, but given the amount of load and how many you'd need, it probably isn't all that practical.


I would disagree with this. We only go this route if the contractor requests it to be backfilled prior, and they are forced to pay the difference between the two designs which is often significant (this cost usually makes them rethink needing to backfill prior as well). We've done a couple dozen of these types of buildings in the last few years and have only had one issue with them backfilling prior to the deck being poured. We will sometimes allow them to build a small ramp up to the deck off a corner of the basement, otherwise they have to use a lift for any power trowels or other equipment they need.

I can see it making a bigger difference for tall walls. But for shorter basements we'll design it cantilever just in case.

Most times when we have a metal deck on wall detail we'll use a shelf for the deck side. So basically reverse your wall detail. SOG isn't doing anything so that can just sit right on top. Then you have a nice concrete stem.

 

[HTURKAK]

So now we have the question on how to connect the deck to the top of the wall to resist this large out-of-plane load. Headed anchors can't resist the load as the breakout is about 75% overstressed. We don't have beams framing perpendicularly into the wall at all locations so we can't use the beams either.

You have other options. One of them , Design the basement walls having top support with the available upper limit of headed anchors . That is, if you can not increase the resistance of the deck , then reduce the earth pressure say , choose the backfill ( say crushed stone, provide drainage etc ) .