ACI Deep beam + torsion

[PMR06]

Does anyone know of a design guide, ACI code reference, or other information regarding concrete deep beams with torsion?

In particular, I am looking for guidance on combining the minimum areas of vertical/horizontal steel from deep beam with the longitudinal/stirrup steel from a torsion analysis. I could combine the steel areas, but the reinforcing gets out of hand pretty quickly. Per commentary, the deep beam vertical/horizontal steel is "intended to control the width and propogation of inclined cracks", not necessarily a direct strength calculation, which might suggest they are not additive?

Thanks in advance

 

[KootK]

Tough question. I'm going to tepidly vote for additive based on my presumption that a bar that is yielded in tension via torsion cannot simultaneously be doing much to limit crack widths.

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.

 

[jayrod12]

And I'd be inclined to think differently kootk.

I would feel that the torsion reinforcement could pull double duty. It's likely at service level torsion loads there would be capacity left in the bars to provide crack limiting. And at ultimate loading no one gives a crap about serviceability they just want it to stay standing.

 

[KootK]

I've done me some book lernin' in an attempt to sort this out.

When we employ deep beam theory, we are generally making an intelligent, but not quite correct, assumption about the flow of stresses in the member. One of the primary purposes for the H&V reinforcing is to allow a ductile redistribution of forces to occur when a member transitions from the stress distribution that it "wants" to the stress distribution that we've reinforced for.

All this means that, at the ultimate condition, the supplied H&V reinforcing can be expected to be yielded, and therefore unavailable to resist torsion, at some locations within the member that would be rather difficult to predict.

Some questions for the OP:

1) Are we talking about equilibrium torsion or compatibility torsion?

2) What are the proportions of your member? A 300Wx2000H beam will resist torsion quite differently than, say, a 900Wx1200H beam will.

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.

 

[PMR06]

@KootK,

1) This would be equilibrium torsion. I have a transfer girder picking up a multi-story column above. For "architectural" reasons, the CL of column above and CL of girder/columns below are offset about 6".
2) The transfer girder is about 2'-0" wide x 4'-6" deep.

I am trying to get rid of the eccentricity/torsion all together. The more I look at the load path, the less I like it! For pricing purposes, I combined the steel areas, and that's probably what I will proceed with unless I can find documentation otherwise. The steel is intense, but constructible. Hopefully the price will help motivate the Architect to line things up though!

 

[KootK]

Bad news: at those proportions, and with that loading, your torsion design is rightfully an STM design as well IMO. And that makes it a 3D STM problem not fit for man or beast.

Good news: I think that you can forgo the 3D and, instead, just multiply your vertical load by .75/.5 and ignore explicit torsion considerations. The stirrup spacing should be much more agreeable. Use closed stirrups of course.

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]

I suppose that the ratio would be a little higher to account for the fact that the CL of your ties are inboard of the perimeter of the beam. You'd also need to ensure that you can deal with the torsion at the beam ends.

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]

I'm going to change my original answer too. Like Jayrod, I'm fairly confident that the stirrups and side reinforcing are not additive. Of course, I also do not feel that it's appropriate to combine strut and tie design for flexure and shear, if that is what you're using, with sectional method torsion design. Both mechanisms would load the same compressive struts without accounting for that explicitly.

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.

 

[PMR06]

Thanks KootK. I agree, I'm not sure combining the two methods is valid either. Preliminarily I think I had enough steel in there to cover the cost in final design. However an appropriate, rigorous analysis would need to be done. The good news is, it looks like the Architect is lining things up and this problem will go away!

It is an interesting one to keep pondering though, especially the 3D strut & tie modelling. When I have some spare time I might look into that. Check back in about 30 years though!

 

[KootK]

I tried it once for some pile caps that were loading piles laterally. Ended up giving up in frustration and going back the sectional methods. Congratulations on your persuasion win!

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.