Punching shear reinforcement

[handex]

Hi guys,

I have a job where I cannot increase the slab thickness enough to satisfy punching shear requirements. I have never yet had to deal with punching shear reinforcement as I usually prefer to increase the slab depth or provide a drop panel, but in this particular job it cant be achieved. As far as I can see the only methods referenced in AS3600 to increase shear capacity is the use of a shear head (with no guidance given), or provide closed ties in the torsion strips. For now I am going along the lines of the closed ties in the torsion strips, but I have read that in some cases they arent completely effective due to anchorage slip (my slab is 600 thick so anchorage shouldnt be a problem). Are there any other pitfalls that I should look out for?

Also I have a set of drawings produced by another australian engineering firm that has used "shear cages" at each column location, consisting of standard open ligs the width of the column face in each direction. I have been reading up and it seems this form of reinforcement has been proven effective in improving punching shear capacity and is referenced in several international design codes, but since it is not referenced in AS3600 what are the rules in applying it here? Does it fall under the "testing" clause in the punching shear section? I guess I also have the same question regarding proprietary stud rails (ancon etc). They have been proven to be effective in multiple research papers, but what is the legality in applying them to structures that fall under AS3600? I realise they have been used in Australia so I guess I am just interested in seeing how they are justified.

Cheers

 

[rowingengineer]

Is this a PT slab or a normal slab?

I normally reference the special study requirements of AS1170 for any deviations of the Australian standards.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[handex]

As of now, the slab is conventional RC because this is what the client wants. We are however going to give a PT option, so it could end up that way. Are you asking that in regarding to utilising the precompression into the Vuo calc?

Thanks for the reference to AS1170, that was what I was looking for. I have used other codes in the past to check or upgrade my members designed to Australian standards but have never designed a member (or structure) by a method that is not recognised by the Australian standards, so it is good to know that the option is there if it is required.

 

[Trenno]

Doesn't the Euro / British codes take into account the amount of tension reinforcement you have over the column in their punching shear calculation?

I guess it would also be wise to put in some integrity bars (bottom) to avoid catastrophic punching shear failure.

 

[asixth]

AS3600 is deemed to satisfy the bca. I forgot what the bca says about designing outside the standards but it does allow it provided it's based on sound research. Which studrails are.

I recommend Reid or Ancon. Reids provides literature based on research at Curtin Uni.

I would never go past a shear stress of 0.1*f'c or 4N/mm^2 for a 40MPa concrete. If your up that high then the slab is too thin.

 

[bigglesinaus]

What do people classify as a shear head? Speaking to a senior person in the office he was under the impression that Shear Heads consist of steel I-beams or channels welded in a cross shape and placed over the column. This approach is uncommon and used when other solutions have been exhausted.

Regarding use of stud rails. The design is based largely on research and development from the manufacture based on a research paper(s) in conjunction with AS3600 Clause 9.2. But they advise that the design be verified against BS 8110 or BS EN 1992, Eurocode 2. [From Ancon website]. The program on their website will automatically determine design and detail of shear reinforcement.

 

[asixth]

I think so. I have never detailed one. I have looked through my collection of drawings from 1980's buildings onwards and haven't seen one detailed.

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[rowingengineer]

the senior engineer is correct

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."