Concrete nib and minimum reinforcement 2

[derim01]

Continuous RC nib 150mm wide is supporting hollow core slabs. The design is passing for nib height 200mm and given rebar and ultimate loads.
In order to make work easier the contractor increased the nib height to 400mm, to make the bottom level of the nib equal to bottom level of the beam. Even though reinforcement and loads are the same, the design fails for the minimum reinforcement which shall be doubled.

Clause 11.8.5 — Asc /bd shall not be less than 0.04(fc′ /fy).(ACI318M)

I get the idea that sudden failure in concrete shall be prevented and therefore this clause is in place, but isn't it a bit harsh to penalize the increase in the height in this case? The beams are already casted. Do you have any comments or suggestions?

 

[JedClampett]

Well, I'm not sure what a "nib" is, and I suspect that no one else is either. Is it corbel, a ledge or something else? Or give a sketch.

 

[bugbus]

As I understand, a nib is to a corbel what a slab is to a beam, aka a ledge.

The issue seems to be that you now have 200mm height of unreinforced concrete? If it works for the 200mm high ledge, the same strut and tie model should work for the 400mm high ledge. But there might be some issue with crack control.

 

[hokie66]

The OP needs to provide a sketch due to the confusion here. To me, a 'nib' us a short, stubby wall. But that doesn't seem to fit the description of his problem.

 

[SWComposites]

Was the reinforcement moved down 200mm, or is there 200mm of unreinforced concrete? if the latter, tell the contractor to cut it off.

 

[derim01]

It is a continuous corbel that supports hollow core slabs. Reinforcement is stretched as height is increased.
Sketch is on above link.

 

[hokie66]

In that case, bugbus is on the right track. Your concern should not be about 'minimum' flexural reinforcement, but about ensuring that a strut and tie model works.

 

[OldDawgNewTricks]

Regional vernacular for construction terms is an interesting subject. A nib is defined as a pointed projection such as the tip of an ink pen or the beak of a bird. So a nib in construction should rightly be a discrete corbel rather than a continuous ledge. But I digress...

I concur that if ACI 318 is your governing design code, then the minimum primary tensile reinforcement in brackets and corbels is based on the dimensions of the concrete, so doubling the depth could be a problem. A few thoughts:
1. You cited ACI 318-11. You might want to check more recent editions if they are applicable because corbel requirements have been updated in since 2011.
2. I believe the ACI 318 strut-and-tie provisions contain the same minimum reinforcement requirements for corbels, so that may not help.
3. You might want to also look at the PCI Design Manual. They have examples of ledge design that might be of some use.

 

[bugbus]

hokie66, I’ve normally heard a short stubby wall being referred to as a hob.

 

[Smoulder]

Nib is the British name. I guess you're the engineer who's been asked to solve this? Tough one but black and white. If you reject, make sure to acknowledge that it's undoubtedly stronger what they've done but you have no room to move.

Thanks for posting. Good data point on material cost vs convenience. Wonder if the extra steel would've swayed them to stick with the design. Maybe ask that off to the side.

'If you reject ' - maybe someone else sees a loophole I don't.

 

[Smoulder]

Ah, forgot to say that the British min steel was lower. Maybe it would've passed (0.13%).

 

[rapt]

BS8110 minimum steel is very outdated, as is BS8110.

There is no way to justify that minimum in slabs, even though it has been used for many years!

 

[KootK]

PCI has some excellent, free, recent documents on the design of ledge beams that you may find useful: Link

In my mind, even the PCI documents are conservative when it comes to the kind of ledge that typically supports precast plank. The ledge beams shown in the PCI documents tend to have proportions typical of the kind of ledge beam that supports double tees at the perimeter of parking structures. In contrast, a ledge supporting plank will have these features:

1) Loads that tend more towards the distributed than the concentrated. I feel that this make longitudinal reinforcement requirements that are intended for load distribution less critical.

2) Ledge widths so narrow that, from a flexural perspective, the ledges probably work without tension reinforcement. And, what tension reinforcement there is will rarely be successfully anchored with any manner of detailing considered "reasonable" for this situation.

...but isn't it a bit harsh to penalize the increase in the height in this case?

I do feel that it is a bit hash and would, personally, let the existing beams pass without replacement or reinforcing.

I feel that a plank supporting nib that develops any meaningful cracking is probably headed for trouble. In this sense, I feel that doubling your nib depth is about the best damn thing to ever happen to a nib.

If I had to chose between your original nib and your modified one that doesn't meet the min reinforcing requirement, I'd take the modified one all day, every day.

 

[Smoulder]

Yep agree on the technical side. If you're going to let it go think about whether you'll be okay to write a letter saying all to code. If not then think about how likely it is you'll be asked. If you think it's hard to back out now, wait until it's all built.

 

[hokie66]

I think the minimum steel limits you are talking about are for flexural steel. This is a non-flexural element. If it complies with your strut and tie analogy provisions, then you would be good to go.

 

[Smoulder]

@hokie The strut and tie part refers back to the empirical part for min steel. Thinking more and a loophole might be that this is a nib and not a corbel. Its got. Some validity because the consequences of failure is lower because localised but a corbel would drop a whole bay.