Concrete Beam Stirrup Question

[vandede427]

Attached is a sketch of a beam. It's 28" wide by 36" deep. There's a 12" x 12" clipped out of one corner providing a bearing surface for a hollowcore slab.

My question is this. The max stirrup spacing is 16" based on d/2. Do I have to place a pair of each stirrup @ 16", or can I stagger them at 16" (one at 0-32-64 and the other at 16-48-80).

If they were staggered, then the left face would have a bar at 16" o.c. but the depth of the stirrup from the beam face would vary. Essentially I'd only have a stirrup at the face every 32". But does that really matter? Is there anything that says that stirrups have to be as close to the side faces as possible. It is crossing the shear crack no matter what.

Perhaps I limit the width (b) in the shear calcs to the narrower dimension.

Assume that I'm ignoring any torsional loads and only considering shear.

 

[csd72]

If you use the whole section in the shear calculation then you should include both stirrups in pairs at 16" spacing (i.e. not staggered).

 

[vandede427]

but the concrete contribution is the only thing dependent on the beam width.

the steel contribution is only based on area of steel crossing the shear crack.

 

[csd72]

But it is the interaction between the two that is the issue. If you are relying on the full with of the lower portion then I would recommend that you tie it in with the steel otherwise it could potentially crack and separate.

Also have you considered how the corbel action works under the cross beam?

 

[vandede427]

For this discussion, ignore any corbel or torsion design of the stirrups.

I'm asking that if the steel crosses the shear crack, does it matter if it does it as close to the face as possible or closer to the inside of the beam?

 

[csd72]

An interesting question,

It is really the taller of the two stirrups which is doing most of the work so personally I couldnt see how you would justify having it at more than every 16" spacing. The other stirrup you may be able to justify every second one but I would not see the point as it would just confuse the guys on site.

 

[hokie66]

Agree with csd72. Space and size the full height stirrup for the shear requirement, use the other one for the corbel. Place them together at the same spacing.

 

[ishvaaag]

Agree. On ly if I had problem with the shear capacity I would start to think in using the small stirrup for shear in the beam.

 

[ishvaaag]

Yet of course the main stirrup will have to acknowledge the extant torsion.

 

[weab]

When you said to ignore the "corbel" effect, I think you hit the nail on the head. I guess that I agree with the consensus as long as the load has the ability to reach the tall stirrups.

This means, of course, that you can't ignore the "corbel" effect. Depending on your load conditions, you may end up just as much stirrup-like steel when you are done.

 

[hokie66]

vandede,
The outside leg of the tall stirrup, as well as acting as beam shear reinforcement, has to take the vertical reaction of the floor slabs as hanging reinforcement. Add these two together. Do a strut and tie model if you don't get my drift.

 

[vandede427]

This is my fault. I've distracted everybody with torsion, corbel, hanging slabs, etc.

let me re-ask my questions. see attached sketch.

Is the shear strength of Beam B any less than Beam A?

I would argue that they are equal. The concrete contribution to shear is based on bw which is purely concrete dimensions, not concrete inside a stirrup area. The steel contribution is not defined by any width, but only depth.

Now, all that aside I'm not going to detail my stirrups like this on purpose if I've got a simple rectangular beam. But I've got an odd shaped beam with mutliple type stirrups and a corbel on each side of it. I'd like to design the tall stirrups for the shear/torsion and then just use the secondary ties for corbel. That way my main stirrups can be at 16" max and be governed by the max spacing for shear/torsion, and then the secondary ties can be spaced at 32". I was just trying to justify not having to place all 3 types of ties by blindly following a max spacing, if the corbel stirrup is only needed at 32".

 

[hokie66]

The shear strength of your Beam B is less than that of Beam A because of lack of confinement of the portion of the concrete outside the stirrups. After initial diagonal tension cracking, the shear force is redistributed so that part is carried by the reinforcement and part by the compressive concrete above the crack. If this compressive concrete is unrestrained (think column concrete), its capacity is reduced.

 

[vandede427]

if that's the case, then why isn't something about confinement mentioned in the shear portion of ACI chapter 11 like it is for torsion and compression? ACI specifically talks about confinement for those two items but says nothing for shear. it confinement were part of the steel shear strength contribution, then wouldn't some form of bw be in the equation along with d, or some form of Acp like in torsion?

The only thing that I can find, in the code, that alludes to this is in the commentary that says

"research has shown that shear behavior of wide beams with substantial flexural reinforcement is improved if the transverse spacing of stirrup legs across the section is reduced."


and to all, please don't ever take me as argumentative. i'm just in the pursuit of knowledge, sometimes to an extent that may be to far.

 

[tai99]

You'll find answer to your question by thinking in terms of propagation of cracks on deformed/deflected beams. Note that without confinement, the concrete performs poorly under tension. Also do not forget the relationship of shear and diagonal tension.

 

[shaneelliss]

There was an article in ACI structural journal in the March/April 2009 issue that presented some test results of wide shear reinforcement versus narrow shear reinforcement (and other arrangements) in beams that could help in this discussion. Basically, the test results showed that the narrow shear stirrups inside a wider beam did not provide the shear resistance that following the code prescribed calculations indicated it should.

 

[vandede427]

ah, now there we go. that's what i've been looking for. a man with some concrete (lots of pun intended) answers.

I've probably got that issue laying around here somewhere.

 

[miecz]

Getting back to your original sketch, the use of the shorter stirrups for shear reinforcement is outlawed by ACI Article 12.13.1 and R12.13.1.

 

[ash060]

Vandede427

Check the PCI Handbook 6th edition chapter 4.5

They have exactly what you are looking for.