Overlapping hoops

[Tstruct]

I am a little confused about overlapping hoops, which we call double/triple rings in my area. I am in an argument with another engineer about detailing of overlapping hoops. The other engineer's practice is to place 2 regular hoops immediately next to each other to make it a overlapped hoop, whereas my practice is as ACI 318-19 states in figure FIG.R.18.6.4. See attachment for both types.
Is the detail used by the other engineer correct?

 

[BridgeSmith]

I don't understand what the discrepancy is. The attachment shows 2 ways to do overlapping shear reinforcement, one with closed ties and the other with open stirrups. Either seems acceptable. the one on the left is simpler and uses fewer bars, but could make it more difficult to place interior longitudinal bars.

 

[KootK]

Is the detail used by the other engineer correct?

I wouldn't say the either is "wrong" but I prefer your method as it seams easier to construct the reinforcing cage.

Possible reasons to prefer the overlapping setup:

1) The people assembling it prefer it for some reason.

2) Maybe you're trying to use both hoops for torsion. Even at this, most engineers don't count interior legs for torsion.

3) Maybe you're trying to improve the anchorage of the tie hooks. Not that this will be necessary in most situations.

The constructability of the overlapping hoops is probably quite dependent on whether or not the cage will be pre-fabricated or assembled in place.

 

[Tstruct]

I think I have to make the question more clear. There are total 3 types of overlapping hoops in the attachment.
The one in the top where I mentioned "2 consecutive rings" is the detail used by the other engineer. The problem here is this type of overlapping doesn't have any intermediate leg to resist shear when the crack travels inside, all the legs are at the edges. Whereas the remaining 2 details in the bottom (From ACI) are the ones I use for the opposite of the above stated reason.

 

[KootK]

Ah... you're right, I did misunderstand.

The problem here is this type of overlapping doesn't have any intermediate leg to resist shear when the crack travels inside, all the legs are at the edges.

Whether or not this is an issue will depend on the width of your beam and what code you're using. Some codes have limits on the transverse spacing of stirrup legs. IIRC, ACI added this fairly recently.

Bundling stirrups side by side like that doesn't sit well with me. We take some liberties with the anchorage of beam stirrups to begin with. Doubling up hooks etc only makes that worse.

 

[KootK]

Related thread: Link

 

[KootK]

 

[Tstruct]

Thank your very much Kootk, your valuable response is really very helpful for me. Keep helping.

 

[KootK]

You're most welcome Tstruct. Thanks for the kind words.

 

[rapt]

I agree I do not like the "2 consecutive rings".

My preferred option is actually an outer ring full width of the beam and an inner ring. I think this gives a much more stable reinforcing cage.

 

[BridgeSmith]

Our typical configuration, when double ties are needed for bent (AKA pier) caps, is the second one (bottom left). It gives us closed ties with internal legs, without needing to have 2 different ties.

 

[Tstruct]

Thank you Rapt and Bridgesmith

 

[Tomfh]

I prefer a complete outer ring too. Continuous bars beat lapped bars.

 

[BridgeSmith]

I wouldn't consider any any of the configurations the OP showed to have lapped bars. They all have hooks. All 3 have fully developed shear reinforcement. What the first (top) one doesn't have is any internal legs to reinforce the core. I haven't seen the research to show why, but per the table KootK posted, it appears not having a large cross section of unreinforced concrete is important.

 

[Tomfh]

Yes they have hooks, but the hoops are overlapping, which isn't as robust as continuous bars. The tension has to jump from hoop to hoop.

 

[BridgeSmith]

I'm not sure I understand what you're saying, Tomfh. Each hoop is a closed tie, at least in the configuration we use. I don't see how it matters whether the tie is the full width of the concrete section or not.

 

[Tomfh]

In some rare circumstances the horizontal parts of the hoop go can into tension (e.g. torsion), in which case an unbroken horizontal bar is preferable to spliced hoops. I doubt it's an issue in 99.99% of cases. I wouldn't say the overlapping method is wrong. Just that I'd prefer not to break a bar if I can avoid it.

 

[Tstruct]

Sorry I didn't understand why you wouldn't consider these configurations?

 

[Tomfh]

BridgeSmith isn’t saying he wouldn't consider using them. He's saying he doesn't consider them lapped.

 

[BridgeSmith]

Thanks, Tomfh. You've got it. I think any of the configurations would meet the requirements for full tension development of the bars, either by continuity around the bends, or by hooks.

There is slightly less steel running horizontally across parts of the top and bottom in the second configuration and depending on the size of the bar shown in green in the illustration, potentially a much smaller amount across the top in the 3rd configuration. However, as Tomfh noted, the situations where this would matter (high torsion) would be very rare. Mostly, it's the vertical legs of the bars that matter. In respect to that, the configurations at the bottom of the illustration are apparently better, since some of the vertical legs run through the core of the concrete.