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.