AS 3600 Commentary on Punching Shear

[QSIIN]

Hi All,

Looking at punching shear, CL 9.2, concrete shear stress fcv may be taken as 0.5*sqrt(f'c) only if a "shear head" is provided - no further guidance or description on this is given. My understanding is that "shear head" in this context are open steel sections cast into the slab itself. I have always ignored this item, as I have never detailed steel sections to combat shear.

However, reading the AS 3600 Commentary on this section, it states that using closed ties, studs, or shear heads allows the use of 0.5*sqrt(f'c).

Depending on your beta-h ratio of the column,fcv can be significantly less than 0.5*sqrt(f'c) (limited to max 0.34).

Is using 0.5 when providing closed ties/other shear reinforcement unconservative? Or have I over designed my punching shear in the past having never seen this statement before/misinterpreting the definition of a shear head?

Any thoughts?

Thank you

 

[Trenno]

I'd be happy I over-designed punching shear situations.

Ancon seem to take a limit of 0.34*sqrt(f'c).

 

[rapt]

AS3600 does not define a method of calculation of a Shear Head. It is not simply some ties added.

You would have to use the method in ACI318 for shear head design in my opinion as that is where AS3600 says the limit comes from.

 

[QSIIN]

Thanks for your reponses.

Of course I'd be glad to have over designed for punching, but I'd be happier to understand by how much.

I guess what I'm really wondering is, is the AS 3600 commentary wrong where it states "Where shear reinforcement, in the form of closed fitments, shear studs, shear heads and the like, is provided, the value of fcv is taken as 0.5sqrt(f'c)." ?

 

[rapt]

It has to be designed as a Shear Head. Not just ties added using the normal shear formulae. AS3600 doers not define a calculation method for Shear Heads.

 

[M-H]

I am wondering to find a source for designing double-head shear studs in Australia. I have designed shear studs based on ACI 421.1R-99 and Euro code but the only thing about shear studs that I could find in AS3600 was in the AS 3600 commentary C9.2.3 "Where shear reinforcement, in the form of closed fitments, shear studs, shear heads and the like, is provided, the value of fcv is taken as 0.5sqrt(f'c)." Having said that, I checked the Ancon calculations, it's based on Lim & Rangan Paper which was published in 1998. Is anybody here to help me to find a local code or material or designing shear studs?

 

[rapt]

There isn't a local code for this currently!

The AS3600 shear design method assumes Torsion reinforcement, and shear studs cannot provide this, so the AS3600 formulae cannot be used.

AS3600 does allow the design of shear heads but gives no rules on it. So you would have to revert to another code for this, probably Canadian or ACI318.

 

[M-H]

Thanks for response, I relocated to Sydney recently and I'm not sure the designing by ACI will be acceptable here? Generally,which code do you always refer in these cases?

 

[Retrograde]

Ancon have software available (Shearfix) that can do the calculations.

https://www.ancon.com.au/products/punching-shear-reinforcement/design-information

 

[M-H]

I have checked the Ancon calculation method and it is based on Lim & Rangan Paper which was published in 1998. It seems outdated! the results of Ancon method is weaker than ACI method for the same case.

 

[Retrograde]

It seems outdated!

If it ain't broke don't fix it. The rectangular stress block approximation is older than that but still sees a lot of use.

 

[M-H]

You're right, I used the worst possible word. I've checked the Ancon source and as authors "Lim & Rangan" mentioned, the results of their tests and the formulas which are developed by them need to confirm by more tests, but I couldn't find any other test in Australia which confirm their test. According to ACI 421.1R-99 we can consider shear studs as normal stirrups and continue the designing. Why we can't do the same?

 

[rapt]

Not using AS3600 shear formulae as they assume any shear reinforcement is Torsion reinforcement.

AS3600 does not give a methodology for increasing shear strength using reinforcement like shear studs. The reinforcement referred to for increasing shear capacity is provided in a torsion strip and is defined as "closed ties". The logic is defined very clearly in the Commentary.

 

[QSIIN]

The commentary also very clearly states that shear reinforcement in the form of closed fitments, shear studs, shear heads and the like, is provided, the value of fcv is taken as 0.5SQRT(f'c).

I guess in this case, we are all in agreement that the commentary is outright wrong with this statement?

AS3600 does not even define what a shear head is, however this clause of the commentary implies that a almost any type of shear reinforcement (closed ties/studs) can be considered a shear head, that can all take advantage of the increase shear stress capacity.

 

[rapt]

No, AS3600 does not say those can be considered a shear head. As you suggested AS3600 does not define shear heads and does not tell you how to design one.

So Yes, the comment section is incorrect. Bus so is any assumption about AS3600 defining how to design a shear head.

It simply gives a method of defining torsion reinforcement to improve the shear capacity of the connection through increased capacity to transfer moment to the column.

Read the code. 9.2.2 - NOTE!