ACI 9.7.6.4.1 - Compression Bar Confinement

[JustUseSteel]

9.7.6.4.1 states that the requirements apply "where longitudinal compression reinforcement is required". What exactly does that mean? If a beam works without the top bars considered in the analysis, then.. confinement is not required? Even if those bars, in reality, are stressed substantially? What if I need two but use six (can expand on reasons if important); if the analysis works with just the corner bars, can I ignore confining the center bars?

Next, the way I interpret 9.7.6.4.4. is that every compression bar needs to be either:
A: wrapped by a hooked transverse bar
B: 6" max on either side between two bars wrapped by hooked transverse bar

However, I am told from superiors that this situation below meets the code as one side is within 6" of a confined bar.

Another potential concern with this design is the lack of cover between the top strands and top mild bars. I think this is a non-issue as in my mind the worst it would do is delay bar development, but they have development beyond way beyond the code mininmums.

 

[andrews2]

I would say you do not need to meet the requirements of this section. If you do not need and are not counting on the compression in these bars to provide extra strength, I don't see any reason to provide confinement for them. In my opinion, this provision is more applicable to a beam with axial load than a pure flexural member. My question is why are you providing the 2 extra top bars if you don't need them? What is the purpose of the second set of bars 4" from the corner bars?

The way 9.7.6.4.4 is worded, I agree with your superior. If you are within 6" on only one side then you are technically not violating the statement of "No bar shall be farther than 6" clear on EACH side." Such bars would only be farther on one side, not each.

For the cover issue, you still want to be able to get concrete all around your reinforcing. I would make sure the clear distance is not less than the diameter of your largest concrete aggregate.

 

[lexpatrie]

I feel like this is intended more for columns.

 

[JustUseSteel]

I would say you do not need to meet the requirements of this section. If you do not need and are not counting on the compression in these bars to provide extra strength, I don't see any reason to provide confinement for them. In my opinion, this provision is more applicable to a beam with axial load than a pure flexural member. My question is why are you providing the 2 extra top bars if you don't need them? What is the purpose of the second set of bars 4" from the corner bars?

The way 9.7.6.4.4 is worded, I agree with your superior. If you are within 6" on only one side then you are technically not violating the statement of "No bar shall be farther than 6" clear on EACH side." Such bars would only be farther on one side, not each.

For the cover issue, you still want to be able to get concrete all around your reinforcing. I would make sure the clear distance is not less than the diameter of your largest concrete aggregate.

Thanks for your input! In that particular beam, it does need all four top bars for flexural strength. But, being prestressed, sometimes the amount of top steel is controlled by transfer stresses. So, for standardization, that might mean adding two more top #9 bars. Then, it seems I can pretend the middle two bars don't participate flexurally and use just the outer four for analysis - but in reality those middle bars will be stressed.

With respect to the cover, it doesnt seem feasible to abide by those requirements when designing a precast beam for torsion. I'd probably need a different topic to go into detail.

I feel like this is intended more for columns.

That would make sense, but this is the beam section of ACI and a beam specific provision.

 

[lexpatrie]

Ok, we've kind of had a similar discussion going about cross ties, could you replace the (4) top bars with 2 larger bars just at the edges, then you've got the confinement satisfied whether it's a larger single bar or 2 bundled bars or is there some horizontal spacing you need to satisfy I've forgotten?

Sometimes these top bars are there so they can hang the shear stirrups and aren't there for strength, if the section works as pure concrete on the top face when it's in compression, the confinement for the compression bars, which is needed to prevent them from buckling, I suppose, seems unnecessary.

What is the span on this thing? Shear reinforcement suggests there's a very heavy load on it anyhow.

 

[m7rhman]

Beams could have a compression force (beam-column action) typically when it is part of diaphragm and acts as collector or chord for lateral loads (which typically will have the force as tension or compression depending on the direction of the applied lateral loads). In such case the beam capacity needs to be checked as a column for combined loads (axial and bending) using interaction diagram. This details is same as for column to prevent buckling of longitudinal reinforcement. If you do not have substantial axial load, this details not needed. However, in ACI 318-19, if shear exceed certain amount, the distance of stirrups cross the beams limited to d/2.

 

[LearningAlways]

If the beam needs these for deflection control or moment capacity, hence they are used as part of a transformed section, I'd argue they need to be confined properly.

But you did question the development of the bars when they are so close to the strand, I'd agree with that assessment. Yes, development happens slower but I'd just be sure that you have enough development for the required transfer stresses, with transfer stresses being at their highest at the beam ends you may want to check into that.

 

[Flotsam7018]

JustUseSteel - I agree with your interpretation of 9.7.6.4.4 and that the each is saying that neither side shall be greater than 6". I view the position that it's only in regard to one side as being analogous to having a column braced against buckling along one axis but not the other. Additionally, it doesn't satisfy the every other requirement, which is always a bit awkward with even numbers of bars.