Reinforcement Detailing of Simple Beams 5

[StructuralAddict]

Hello Everyone,

In the design I am working on now, I have secondary beams supported by main beams. The main beams are not designed to resist torsion. This means that the secondary beams should be simply supported on the main beams in order to ensure that moment does not transfer from the secondary beams to the main beams.

So, how can I design a simply supported connection? What is the difference in reinforcement detailing between a simple connection and a fixed connection?

I appreciate if you could provide me with example drawings (if possible).

Thanks in advance!!

 

[jayrod12]

I think you need to design the primary beams for compatibility torsion if they will see it. It's likely that minimum stirrups would provide enough torsional strength

 

[KootK]

X2 for Jayrod's comment. Additionally:

So, how can I design a simply supported connection?

You don't. The strategy that I'm familiar with is this:

1) Design the secondary beams as simply supported.

2) Design the girders for compatibility torsion.

3) Provide the secondary beams with top steel = 1/3 bottom steel.

From a detailing perspective, these faux-simple span beams will look like any other concrete beam.

In my opinion, it's important to maintain some negative flexural capacity in monotonically cast beams, even when they are designed to be simply supported. The beams will attract negative moment no matter what you do and, without some negative flexural capacity, your shear capacity may be adversely affected.

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.

 

[StructuralAddict]

Thank you jayrod12 for your prompt response. I am also confused between the reinforcement detailing of simple connections and fixed connections. What would be the difference between them? I read something related to the influence of development length on controlling the connection behavior, but I am not sure how that works. Any thoughts about this issue?

 

[StructuralAddict]

Thank you KootK for your great explanation as always
It is very clear now!

 

[StructuralAddict]

Just a quick follow-up. In Etabs, should I release the moment at the ends of the secondary beams? Or is it better if I keep the joints as fixed?

Thank you!

 

[jayrod12]

That depends on what you're designing. For the secondary beam bottom steel, moments released would be conservative, for secondary beam top steel fixed would be the conservative route. For primary beam design then fixed would be the conservative route.

In this case where you want this to act as a simply supported beam then you would want to model it with end moments released.

However as KootK pointed out, design your primary beams for the compatibility torsion, and then provide about 1/3 of the secondary beam bottom steel as top steel that extends into your primary beam. It is always beneficial to maintain a bit of negative flexural capacity.

 

[KootK]

I vote for pinning the joints in ETABS. I think that strategy will provide you with the most useful output for design purposes.

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.

 

[StructuralAddict]

Thank you very much again jayrod12 and KootK. I will model the secondary beams assuming pinned ends in Etabs. This will give me the maximum bottom reinforcement required. Then, I will add top reinforcement = 1/3 of the bottom reinforcement near the supporting girders. Meanwhile, I will make sure that the girder is designed to be torsion compatible by checking if the stirrups are sufficient.

 

[hokie66]

For compatibility torsion in an edge beam, you don't really design, you just follow the rules for minimum continuous reinforcement top and bottom, and minimum stirrups. With secondary beams framing into the edge beam, the twisting is resisted by flexure of the secondary beams.

 

[JedClampett]

Monir87, you ought to invest in the CRSI Detailing Manual or ACI Document SP-66. They have a lot of real life or close to real life examples of reinforcing detailing.
Like hokie66, I prescribe to "Field of Dreams" detailing theory. If you assume it, it will behave that way. Just make sure you reinforce according to your assumptions.

 

[rapt]

"Field of Dreams" is ok as long as it provides a load path to supports. If your dreams have been twisted by inappropriate inputs into or outputs from the brain, then it may be useless!

In terms of crack control, it does not matter what you dream, cracks will happen where elastic stresses are too high, before redistribution to whatever field of dreams you have chosen. So if you want crack control (and this includes compatibility torsion) you need reinforcement where the elastic stresses say you need it.

 

[SteynvW]

"the twisting is resisted by flexure of the secondary beams."

So in other words, if you design your secondary beam so that it complies with deflection limits as simply supported it will automatically create minimal torsion forces in your primary beam.

 

[hokie66]

Yes, the rotation will be minimal, thus the torsional shear stress will be low. You still need to provide continuous top and bottom bars in an edge beam, as well as code required closed stirrups.

 

[chekre]

Hi
So if i understand well detailing doesnt have to do much since whether we assume pinned or fixed end the detail will always be the same for the connection between secondary and primary (top bars anchored in the primary beam with a 12db hook). At the end the structute will behave as you design it in condition to do all the neceasary checking. Please correct me.

 

[KootK]

if you design your secondary beam so that it complies with deflection limits as simply supported it will automatically create minimal torsion forces in your primary beam

This will be true most of the time but designers still need to keep their wits about them. Whether or not the girder is shielded from torsion stresses is a function of the relative stiffness between the flexural load path in the secondary beams and the torsional load path in the girder. A common trouble spot is where you get a secondary beam tying into a girder close to but not at a column. The torsional load path at such a location is very stiff and some serious girder torsional cracking can take place en route to redistribution. These issues can, of course, be ameliorated by making thoughtful layout choices to begin with.

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.

 

[JedClampett]

One place I have heard of problems is where two beams frame into opposite sides of a supporting beam, but they're slightly offset (like two feet). The span between the beams is subject to torsion, no matter what you do. If this is happening, it needs to be laid out differently.

 

[hokie66]

chekre,

No, the structure will perform in the way it wants to. We just try to predict and detail accordingly.

The situations described by KootK and JedClampett are essentially the same. As they said, layout choices can prevent this problem.

 

[chekre]

Hookie66, i meant by detailing the connection details between the secondary and primary since as far as i know there is no hinge detail between elements built integrally.

 

[hokie66]

Cast in place concrete structures don't often have intentional hinges. We may model joints as hinges, but that is just to make analysis simpler.