RC beam to beam connection - hanging reinforcement? 2

[mats12]

English is not my 1st language, so I think thats an expression to use.
I have RC beam that is supported on another RC beam. At connection I should provide 'hanging' reinforcement right?
I was hoping that someone can help me out and post some details/sketch about how this reinforcement looks like / proper way to do it.

Tnx.

 

[KootK]

This is the gist of it. Your code may have applicable requirements in addition. Mine will only allow me to use the stirrup legs closest to the beam for many geometries rather than all stirrup legs. And it allows me to spread the hanger bars out beyond the sides of the supported beam a bit.

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]

My code also allows the hangers to be omitted if the computed shear stress is below a threshold value.

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.

 

[JAE]

I think it is interesting that ACI has never codified hanging steel for conditions like this...unless it is in 318-14 now.

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[KootK]

Same. Anybody know if it exists in any of the non-318 ACI docs? Some stuff does...

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.

 

[rapt]

In ACXI14 there is a discussion on the effects of loads applied not at the top on critical shear location. But nothing on hanging shear.

But the whole logic of shear design is that the load starts at the top. If it is not applied there, it must be raised to the top before the logic of shear design can be applied. So hanging shear reinforcement is required.

Remember, the code is not a text book on design. it is essentially a set of limits. The designer must understand design logic and this is one that is important. It is assumed in the code that the person using it understands the logic behind concrete design and the way concrete members work.

 

[slickdeals]

The only place I have found hanger stirrups discussed in any detail is in ACI 314. I have attached excerpts from the document and highlighted it as well. I don't think this is codified in ACI 318-14.

 

[KootK]

Thanks for that Slick.

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.

 

[mats12]

Kootk!

Thanks for this.

Just to make things clear - in equation you posted is:

R* - reaction of secondary beam
fsy - characteristic yield stress of the reinforcing bars
phi - diameter of suspenso reinforcement/hanger bars

?

 

[mats12]

Also - I have another issue. Primary and secondary beam has to be the same dimensions. They are both heavily reinforced.

I dont know what to do at connection - reinforcement congestion is a problem. Can I bent rebarsa of secondary beams as shown bellow? it might be an issue since rebars are pretty large in diameter...

In other option, I can put rebars of secondary beam in the middle of primary beam rebars, but in this case the distance from concrete edge to middle of tension rebars is approx. 75 mm. Is that too much? Im worried about cracking...

 

[JAE]

In most cases the secondary bottom bars simply slope up to rest on top of the bottom bars without any direct shop-bending.
This requires them to slip up the sides of the secondary beam stirrups, which is not a big issue.

In your case, with the double mat of bars, your second option would work - with the secondary bottom bars extending between the two layers.

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[jayrod12]

If you are concerned about the increased cover for the primary bars in the secondary beam, run the checks. The CSA code has a way to check cracking, I can only assume other codes do as well.

Make sure the increased cover doesn't affect the moment capacity significantly either. Typically though, I just see it done as JAE has indicated. Slight sloping of the bars near the ends as opposed to shop bent.

 

[KootK]

Kootk! Thanks for this.

You're welcome. Always a pleasure working with you mats.

R* - reaction of secondary beam
fsy - characteristic yield stress of the reinforcing bars
phi - diameter of suspenso reinforcement/hanger bars

Almost. Phi is a safety factor. The stuff I posted was Australian.

With regard to your congestion issue:

- Draping the bars is common but can be problematic if your bars are large and you're trying to step over two layers of girder steel. One thing you've got going for your here is that your secondary beams are pretty long so you've got lots of length over which to drape the bars.

- I wouldn't drape the bars AND feed them between the girder layers. The secondary beam bars would be entering the girder at a slope and that would cause more congestion problems than it would solve.

- There's a third way per the sketch below. Basically a non-contact lap splice with some extra ties to make that work by the numbers. I don't see this often (unless I spec it) so I can only assume that the other alternatives are more cost effective. Darwinism at work as it were.

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.

 

[mats12]

After some thinking I agree with KootK about not putting secondary beam rebars between the girder. I think I'll go with option 1 or 3.
I appreciate help guys!
thank you all