ACI 318 21.9.5.1 vertical rebar considered "effective" or can they be non-continuous betwe

[OUe]

When I read ACI 318-11 section 21.9.5.1, I read that all vertical reinforcement must be considered effective in the sense that the vertical bars shall be relied upon for shear in two directions. I also understand this to mean that all bars shall be considered effective for the purposes of yielding and to not over-estimate the moment capacity by putting all coupling bars at the wall edges. In effect, it's supposed to be detailed and designed consistently.

I'm responsible for the review of a retrofitted special reinforced concrete wall in an existing concrete building. It is spanning between T-beams of concrete whereby the wall effective thickness (12") matches the bottom tapered thickness of the t-beam web on each level. My problem is this: the shear reinforcement needs to be at least 0.0025 of the gross concrete area based on it being a special reinforced wall. Since all the bars are required to be considered effective, what would you detail for your tension between floors? The original engineer has rebar checked only for shear transfer (#5 at 12"oc) for the diaphragms at each level, but the code isn't explicitly clear to detail the tension bars (#6 at 8"oc e.w. both faces in this case) between floors, except in the above instance.

Thanks in advance for any helpful insites.

OUe

 

[OUe]

Due to time constraints, I'm moving forward with the review requiring that the engineer provide continuous vertical reinforcement at the boundary compression zones. I'll continue to monitor this thread if anyone can provide me an answer. It seems to me that the wall is considered not continuous and is not allowed because all bars have to be considered effective...

 

[KootK]

I reviewed that ACI clause and I disagree with you interpretation. I believe that 21.9.5.1 is telling designers what they can utilize for reinforcing, not what they must utilize for reinforcing.

Based on what you've shared, the retrofit sounds like a pretty common design approach. That said, it is necessary that the boundary tension reinforcing be continuous vertically. Is that not the case? Are you able to post some sketches/details?

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.

 

[Deker]

ACI 318-11 21.9.5.1 does require not require that bars be continuous, it requires that bars that are developed be included in the analysis. If bars in the flanges, boundaries, or web are developed, they must be utilized in the analysis since they have implications for the code sections inspired by capacity-design, such as neutral axis depth for special boundary element triggers or strength reduction factor for shear vs. flexurally controlled walls. If they are relying on the vertical web reinforcing for flexure then obviously those bars need to be developed (continuous), but if they are relying only on the boundaries for flexural resistance then I see no particular need to make the vertical web reinforcing continuous (although you would still need to provide shear transfer). I view this as somewhat similar to a conventionally reinforced coupling beam with skin reinforcing that terminates 6" past the ends of the beam.

 

[OUe]

The SEAOC Volume 3 has an expert opinion on this matter. The way I read it is that if you assume the bar to be effective, which is a more ductile system and fits the intent of the code, then you must design the bar to behave in ductile behavior. Kootk, while you may not agree, there are some in the SEAOC community that take a fairly conservative approach to this. The engineer did not even take the time to define whether boundary reinforcement was required. Therefore it's a bit of a if/then review response at this point.

Thanks for your comments!

 

[Deker]

The way I read it is that if you assume the bar to be effective, which is a more ductile system and fits the intent of the code, then you must design the bar to behave in ductile behavior.

Assuming the bars are effective does not by itself increase ductility. In fact, adding reinforcing beyond what is required by analysis will generally decrease ductility, especially if it is not accounted for when trying to ensure a flexural failure by capacity based design. This is what the SEAOC Seismic Design Manual is warning against in the quote below. What is important is making sure that your detailing matches your design assumptions so you don't underestimate the capacity of the wall. If the reinforcing is there and it's developed, it needs to be included in the analysis. This is different from requiring that all reinforcing be effective.

The 1995 and earlier editions of ACI 318 and the 1991 and earlier editions of the UBC required wall boundaries to carry all moment and gravity forces. This practice results in higher flexural strengths in walls, which can lead to poor earthquake performance because it makes shear failure more likely to occur. By ACI 318 Section 21.9.5.1, this design practice is no longer permitted.

 

[OUe]

Deker - I don't disagree with anything you've said. I think you probably said it better than I did.

Cheers!