Anchorage of Positive Moment Reinforcement

[IreStruEng]

Clause 8.3.1.3 of AS 3600 states that "Not less than one half of the tensile reinforcement required at midspan shall extend past the
face of the support for a length of 12db, plus a cog or an equivalent anchorage;"

For 16mm diameter reinforcement, 12db = 192mm. Adding in cover and the thickness of the supporting wall/column is going to need be >=230mm.

Say two 16mm dia. bars are required at midspan of a simply supported beam for the flexural reinforcement. If both bars are taken 6db (96mm) past the face of the support is that clause deemed satisfied in your opinion?

 

[JAE]

Not technically by the language of the clause.
Doubling the bars required and halving the embedment conditions doesn't usually work out.

 

[Greenalleycat]

No I don't agree with that. The point is to ensure that some steel is fully developed...doubling the steel and putting in shitty development length on both doesn't meet this requirement

Also, I suspect that the development length required is more than 230mm?
You have 12db PLUS a cog (which I'm interpreting as a 90 degree hook?) - I assume there is an additional straight length of Ldh + the radius on the bend that you need to fit in as well?

 

[IreStruEng]

No I don't agree with that. The point is to ensure that some steel is fully developed...doubling the steel and putting in shitty development length on both doesn't meet this requirement

Also, I suspect that the development length required is more than 230mm?
You have 12db PLUS a cog (which I'm interpreting as a 90 degree hook?) - I assume there is an additional straight length of Ldh + the radius on the bend that you need to fit in as well?

Yes a cog is a 90 degree hook so it will be more than 230mm by the time you account for that.

It seems excessive to me that you'd have to have a really wide support for a simply supported beam that isn't doing much work. I don't have a specific scenario that I'm designing for right now but anchoring 16mm diameter bars from a simply supported beam perpendicular to a 200thk wall would require a local thickening of the wall at the bearing location.

 

[Lomarandil]

Yep... if your support is only 200mm thick, you need to not be using 16mm bars. Replace them with 12mm (or lap to 12mm for the anchorage?)

 

[KootK]

It seems excessive to me that you'd have to have a really wide support for a simply supported beam that isn't doing much work.

I agree. I don't know your code well enough to speak to it authoritatively so let's just assume that we're talking general principles.

Many codes will have some options with regard to how this is handled:

1) Fully develop the hooks. The Cadillac solution.

2) Assuming that the job of the anchorage is to restrain the last incoming strut as shown below, take the time to actually evaluate that as partial development.

3) In the case of ACI's treatment of beams on columns, just extend the hook past the support centerline and call that good enough.

Yes, we would all feel better about all bars being fully developed for yield at all locations. But that winds up being impractical in a lot of situations and I feel that the way most codes treat this stuff acknowledges that.

wall would require a local thickening of the wall at the bearing location.

Precisely. And that will cost $$$ if it means that the wall formwork has to be more complex. And that will cost you clients if your competitors aren't doing the same thing.

 

[KootK]