Rebar Lap Question

[Lion06]

So, I was visiting a job today to take a look at rebar for a very large concrete beam carrying significant load (vertical, horizontal, and torsional). This is for an existing building, and the working space is minimal. The horizontal reinforcement is spliced at roughly the third points - there are hooked horizontal bars at the ends extending roughly 40% of the span at each end and then a straight bar in the middle lapped with each of the end bars.

The problem I noticed is that one of the corner longitudinal bars is not within the stirrups (only the straight bar over the middle 30% or so of the span). The two hooked end bars that are lapped with this particular straight bar are inside the cage, but the straight bar is outside the cage.

I raised this as a concern for a couple of reasons.

1) The bar has no confinement and is held in place only by ties to the stirrups (and bars that it's lapped with over the lap length.

2) Part of the reason that stirrups can develop Fy in such a short distance is because it's hooked over the top bar. If that isn't the case then that could have an impace on the effectiveness of the stirrups at this location.

The more senior engineer I was with didn't think it was a big deal because 1. the lap length was larger than it needed to be, and 2. There's a large amount of cover at this location > 6" so that the confinement issue really isn't a problem since that much concrete is not likely to spall off.

I'm still not convinced. Can I get some second opinions?

I see the point about so much cover being unlikely to spall off, but it still doesn't "feel" right when I look at it.

 

[bootlegend]

I would agree with you StructuralEIT, just because it seems logical to enclose it. ACI 318-05 section 7.13.2.3 does have a little note about splices being enclosed by U stirrups with 135 degree hooks. The section it refers to is for perimeter (torsion) beams, so I would think you could make a case to apply the requirement to your situation.

I have also seen something in the code about allowing extra concrete as lateral confinement, (not exact wording) but I can't find it now.

I don't know much about statistics, but I do know that if something has a 50-50 chance of going wrong, 9 times out of 10 it will.

 

[slickdeals]

Sketch please

 

[Lion06]

Here is a sketch. I have a section and a plan. tThe rebar is in red in the plan just to make it stand out from the beam lines. There is additional reinforcement that isn't shown in an effort not to clutter the sketch.

As you can see, one of the lapped bars falls outside of the stirrups.

 

[miecz]

According to ACI 318 Article R12.13.2.1, stirrup anchorage is provided by a standard stirrup hook, as defined in 7.1.3, hooked around a longitudinal bar.

 

[cntw1953]

If the end hook bars are in the tension zone, and the splice length is no less than required by ACI for non-contact splice, then I would go along with your senior on this. Especially if this is an isolate case.

 

[cntw1953]

Actually I shall say " if there is no significant axial force on the beam ....

 

[msquared48]

The 1.5" cover is the critical direction and you need the confinement here.

If the contractor says that it is too hard to relocate the outside bar, just provide a detail showing added extra cross-ties to confine the bar. One or the other has to be done. His mistake - His choice.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAE]

Is the longitudinal bar in tension or compression?

 

[Lion06]

It's in tension. Vertical load has it in compression, lateral load has it in tension, and torsion has it in tension - The net result is that the bar is in tension.

 

[BAretired]

I tend to agree that the bar in question should be enclosed by stirrups. It may be 8" away from the outside face, but it is only 2" away from the top.

I would have thought that the stirrups would be 8" wider so that they enclose a larger rectangle for torsional loading.

BA

 

[boffintech]

The bars should have been inside the cage. Reference ACI 318 7.1.1 A day without making a rodbuster re-work something he claimed was "to hard to do" is like a day without sunshine.

But seriously, StructuralEIT, don't keep us in suspense. How did it work out?

 

[vandede427]

the bar should be inside the cage to be used in your torsion calc. have you analyzed the beam with only 3 corner bars to see if it still works?

 

[Lion06]

I analyzed the beam with that corner bar gone. It still works from a strength standpoint. The beam is very large to keep deflections extremely small, so minimum steel was used (bending in both directions and torsional, even though it wasn't above threshold torsion).

Ultimately, the senior engineer said it's ok. When I'm put in that position and I'm making the call, I'm going to have him redo it. I don't like the way it looks and the "load path" (for lack of a better word here) looks incomplete to me.

Thanks for the input everyone.

 

[beton1]

Agree with MSquared approach. Give an alternate/remedial
detail. Especially at beginning of a job. Otherwise they will keep pushing and get sloppier (rodbusters).

 

[abusementpark]

If it is just minimum steel, I wouldn't worry about it.

 

[Lion06]

abusementpark-

It is just minimum steel, but the beam is a critical member. It's supporting the entire 16" stone facade for a 4-story building, and supporting a large bay. The stone has a fairly large eccentricity to the beam.

The beam is seeing torsion, strong axis bending, and weak axis bending. There is no redundancy, either, if this beam fails, that whole side of the building will topple over (I'm not exaggerating).

 

[miecz]

Well, I still think (see my earlier post) that the stirrups are not properly anchored as they aren't hooked around a longitudinal bar.

 

[cntw1953]

miecz:

Unless there is concentrate load or torque in mid-span, isn't shear & torsion critical near the supports? The end hooks are enclosed as mentioned in the OP.

I think there is nothing inconvenient to ask the contractor to correct it - a middle piece straight bar that can be inserted without much trouble. But, I wouldn't say it is necessary, as this bar was adequately confined by concrete mass on 3 sides, and there obviously has no cause for the bar to pop out/crack the top face (with std. covers only).
Again, this is my person opinion only. I will definitely try to talk the contractor into correction.

 

[miecz]

cntw1953

True, it would be worse if the unanchored stirrups were in a high shear region.