Concrete Beam Stirrup Question
Concrete Beam Stirrup Question
(OP)
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.
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.






RE: Concrete Beam Stirrup Question
RE: Concrete Beam Stirrup Question
the steel contribution is only based on area of steel crossing the shear crack.
RE: Concrete Beam Stirrup Question
Also have you considered how the corbel action works under the cross beam?
RE: Concrete Beam Stirrup Question
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?
RE: Concrete Beam Stirrup 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.
RE: Concrete Beam Stirrup Question
RE: Concrete Beam Stirrup Question
RE: Concrete Beam Stirrup Question
RE: Concrete Beam Stirrup Question
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.
RE: Concrete Beam Stirrup Question
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.
RE: Concrete Beam Stirrup Question
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".
RE: Concrete Beam Stirrup Question
RE: Concrete Beam Stirrup Question
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.
RE: Concrete Beam Stirrup Question
RE: Concrete Beam Stirrup Question
RE: Concrete Beam Stirrup Question
I've probably got that issue laying around here somewhere.
RE: Concrete Beam Stirrup Question
RE: Concrete Beam Stirrup Question
Check the PCI Handbook 6th edition chapter 4.5
They have exactly what you are looking for.