Concrete Foundation Wall with Cantilevered Masonry Above Detail 2

If you are tying the slab into the concrete stem wall, be sure to get your proper hook development length of your bar between slab and wall. With an 8" thick, inset ledge, wall, you may not get there.

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Good point JAE. Since I am providing much more steel than needed I am able to reduce my development length for my hook down to 6" so I should be ok on that front. I have attached a sketch of the third option for reference. Thanks!

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1551216435/tips/Option_3_f7vikk.pdf[/url]

Correction on my last post. The second option (8" wall with the 1.5" ledge) would not be ok.

OP said:

The second option (8" wall with the 1.5" ledge) would not be ok.

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Why not? In my opinion, a #5 hooked around a transverse bar is developed past the bar, per beam stirrup provisions. We do a very similar detail in my area with 8" wide grade beams (which have their own issues).

KootK
Your thoughts please...
For hooks of stirrups developed around a continuous bar... my recollection is that it works in beams because one end of the stirrup is hooked around a bar with significant tension while the other end is hooked at a high compression (pinched) zone.

With a bar hooked only 4 or 5 inches around a non- stressed wall top bar I’m not sure that works the same way.

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I have seen option 3 most (pour the slab on top of the wall, then lay the block). One draw back to option 3 is that it doesn't allow flexibility of construction sequencing; the slab has to be poured before the block can be laid. Of the other 2 options, the 10" wall with ledge for the slab allows for flexibility in the construction sequence. The 8" wall with ledge does not. I would not use the 8" wall with ledge. It's an extra step of forming and complication for no benefit other than saving a bit of concrete.

Regarding developing the standard hook in the wall, I would not use a standard hook; I would use a bent bar with full development length on both sides of the bend, so for a #4, go ahead and put a 24" tail on the hooked part in the wall.

That 24” Isn’t going to help you any when the top corner of the wall tears out at the hook elbow.

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Not a fan of tying a slab on grade to a gradewall. Slabs on grade prefer to float (i.e. not be restrained).

I would simply make the strip footing wider to resist overturning and/or consider friction and/or passive pressure to resist overturning.

JAE, interesting observation about the rebar development. You are making me rethink how I think about it.

Huh, unless you intend to hit that wainscot with a truck, I would not have thought twice about using your detail.

JAE said:

KootK Your thoughts please...For hooks of stirrups developed around a continuous bar... my recollection is that it works in beams because one end of the stirrup is hooked around a bar with significant tension while the other end is hooked at a high compression (pinched) zone.

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- If this is part of the story with the stirrup provision, I'm not aware of it (just checked ACI). If the voracity of your claim can be verified by digging deeper somehow, I'm certainly open to changing my tune.

- If your claim is accurate, this would seem to be a problem for the high shear locations of simple span beams. Near the supports, where there would be minimal flexural compression, transverse bar anchored stirrups would no longer be viable?

- The ACI commentary describes the phenomenon by which bar anchorage is improved by the presence of the transverse reinforcing which keeps splitting cracks over the hook closed up. This is similar to the concept that you've described but would not lead one to conclude that both a transverse bar and transverse flexural compression would be required to make a go of the anchorage.

gte said:

Regarding developing the standard hook in the wall, I would not use a standard hook; I would use a bent bar with full development length on both sides of the bend, so for a #4, go ahead and put a 24" tail on the hooked part in the wall.

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This is somewhat regional I think. In Eurocode, you are allowed to extend development around the corner of rebar bends so long as some attention is paid to the diameter of the bend. To my knowledge, however, we're not allowed to do that in north america without resorting to STM models etc. Regardless of the extension beyond the hook, I believe that our codes assume that crushing of the concrete inside the bend of the bar creates slip of a magnitude qualifying as "failure". All this said, I see this as more of an anchorage issue than a development issue, similar to JAE's comment about the corner of the wall coming off.

Motorcity said:

Not a fan of tying a slab on grade to a gradewall. Slabs on grade prefer to float (i.e. not be restrained).

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This is also regional I think. Certainly, in locations like mine with swelling clays, we stay away from the rigid connection between frost wall and SOG. In other areas, however, I believe that the connection is made and buildings constructed that way perform well. One advantage of the slab in notch business is simply architectural: the desire to hide the joint. I've done plenty of retail buildings where long lengths of curtain wall were underlain by SOG poured over frost wall. So far, so good.

gte said:

...allows for flexibility in the construction sequence. The 8" wall with ledge does not

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Is it at all possible to construct the frost wall with the bearing notch, construct the masonry, and pour the SOG into the wall notch as the last operation? Or is pouring the SOG into the notch with the block in place simply too messy? I'm often amazed at what concrete contractors can manage in the field. There are some interestingly cast bridge bents across from my house that will be the subject of another thread...

KootK said:

Is it at all possible to construct the frost wall with the bearing notch, construct the masonry, and pour the SOG into the wall notch as the last operation? Or is pouring the SOG into the notch with the block in place simply too messy?

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I suppose it depends on the width of the bearing notch. The OP mentions 1.5", but I would prefer a wider bearing seat, in which case I might worry about the stability of the block wall before the slab was poured.

It's usually 1.5" for me for obvious reasons. That would be ample bearing capacity to mobilize the shear strength of your typical SOG.

KootK - makes sense.

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I have used a 1.5" ledge many times through the years without issue. When I can go wider I might use 2" or so.

In this situation I am leaning more and more towards the third detail option. I just discovered this afternoon that the client is going to have some pretty sizable vehicular loading on the slab so it will now be 9" thick. Thank you for all the good input and discussion. Look forward to hearing more.

I would consider making the footing a skosh wider, using CMU down to the footing, and letting the slab float.

JAE said:

KootK - makes sense.

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I did notice something interesting in reviewing the ACI verbiage on this however. For stirrups, it's always referred to as "anchorage" and never "development". By definition, a shear stirrup always has a compression strut to anchor and be anchored too. Not so in a purely tension application which I'd expect to be more like an Appendix D thing.