Upturned Concrete Beam Design Checks

[bookowski]

For simplicity assume I have a single span one way slab supported by upturned beams on each end. In my case this is a 14" very heavily loaded slab and the upturned beams are full story walls. In practice I know that this will be two pours, the 14" slab and then the rest of the wall/upturn beam. I'm ok with this part.

What special consideration/check should I be doing at the slab to beam interface? The slab needs to deliver shear to the beam, the shear strut needs to develop sufficiently into the vertical bars to effectively 'hang' from the rest of the beam. This implies that I should have development of my vertical bars past some point in the joint correct? Measured from where? It can't be from the bottom of beam or upturned beams wouldn't work.

I've done upturned beams in that past but this one has > 1200psf on the slab so I want to make sure I'm covering my bases.

 

[KootK]

1) Analytically, I'd apply "hanger" beam steel provisions similar to what we have in the Canadian code. They're intended for beams but the theory applies equally to slab/wall connections. This will be less conservative than what you'd wind up with STM. What it will come down to is that your inside face wall rebar will need to be able to deal with 100% of the tension load.

2) Practically, you'll want to use small diameter vertical wall reinforcing with lapped u-bars at the bottom. There will be longitudinal bars in the corners of the U-bars and you'll want to run your slab bottom steel up over top of those longitudinal bars.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bookowski]

Thanks Kootk. I planned on u-bars that are left projecting form the first pour to tension lap with the wall verts. with long bars over these and turned up as well. This is my typical detail. The loading on this one has me a bit more concerned though whether or not that's sufficient.

What is the hanger beam concept, is there any quick summary/example that you can point to? I don't believe we have anything like this in aci (or I've missed it).

My concern is the development into the vertical bars from the slab shear. Do you agree that this is a design check and if so how do I show that I am ok. With 14" I should have Ldh if I'm measuring past the compression block of slab - so I'm likely fine if that's sufficient. Just want to make sure - I can imagine a failure mode where I'm left looking at some nice u-bars projecting from a freestanding wall.

Part 2: Back to the old favorite of punching. This upturned beam/wall is actually two columns below. There is no way to get punching to work on these columns if the punching perimeter is the column. I'm using the wall above as a continuous support, the same as if it was below, and ignoring punching around the column perimeter. kosher?

 

[hokie66]

Agree with KootK. In addition to the U-bars at the bottom of the wall, I would want U-bars to the slab. Maybe even some diagonal bars, from bottom of slab to outside of wall, across the opening corner. Suggest reading the work by Neville about efficiency of opening corners.

 

[bookowski]

Just to clarify - the slab needs shear reinforcing because of very heavy loading relative to span (~1250psf unfactored) so my concern is that shear getting into 14" section of beam, developing into the 'hanging' bars.

 

[KootK]

@Book: I'm away from the office otherwise I would send you a killer sketch. Basically, you assume that the compression in the slab is concentrated over the lower 25% of the slab cross section. Then you run a 45 degree line from the top of that compression zone down into your wall (extension of the wall into the slab I supposed). Any rebar that is anchored below that line is fair game for "hanging" the shear load. There's also a stress level below which no serious shear cracking is assumed to take place and one can forgo these requirements. In this case, I might feel good about being below that stress level but I wouldn't take advantage of the "out".

The detailing that I've been describing basically gets you a hanging support incapable of serious moment transfer. If you want moment transfer, you'll need to do something like what hokie has suggested. And that's basically what the stem/footing wall connection looks like for retaining walls in the CRSI manual. Serious stuff.

In my heart of hearts, I would love to run your wall a foot or so below the underside of the slab. I've never been able to sell that for one of my projects, however, so I won't engage in the hypocrisy of pretending that it's a truly viable option for you.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

These provisions imply vertical bars that, like normal beam stirrups, could be considered developed simply by virtue of the fact that they wrap around longitudinal bars. 15M tops for me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bookowski]

I found some images of what I think the are the csa provisions - looks like I'd have the 'hb' term as zero. I don't see how this ensures that you can actually develop the force into that hanger reinf quickly enough - but having only looked at it for a minute I may be missing something.

I agree with the bit of a downturn, I was thinking the same thing. I may try and sell this. Feels a lot better.

 

[bookowski]

Simul-posted. Thanks, those were the provisions I was looking at. Imagining that diagram with hb = 0 seems a bit iffy. I guess the idea is that you are truly 'hanging' by virtue of dowel action/development of your bottom bars which are anchored via the longitudinal bar and ties, rather than trying to develop that strut into the vertical reinf/ties. I definitely will feel more comfortable if I can get a small downturn as well.

 

[KootK]

Another option that I used in one extreme case was to install a continuous steel plate at the bottom of the wall with welded rebar couplers on it to anchor the starter bars into the wall. Then you've got to contend with fireproofing issues however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.