Post Tension Slab Design for New Zealand

[Ophirian]

I've been doing PT slab/beam design in Australia for more than 5 years and I would say AS3600 has a straight forward requirement and easy to follow guidance.

I'm recently involved in PT slab design in New Zealand and I was surprised to find that the code is not as straight forward as that of AS3600 when it comes to design requirements. Though there is a clause in chapter 19 that says somethings about PT design, I find it difficult to follow especially that I am looking for similar requirements in the Flexure and Service clause of AS3600.

To make things worst, Ram Concept does not support New Zealand code at all.

I guess my question is, has anyone design PT slab/beam in New Zealand who can give me some guidance?

Cheers!

 

[Agent666]

I've seen a few people just use Rapt with plain old vanilla AS3600 as the design code. PT isn't really that popular here, generally contractors put a price premium on it due in part to lack of experience in the use of PT.

The fundamental principles are similar in terms of first principles analysis of sections, etc. However some of the detailing requirements are a little different though so I've seen a few people come up with answers that don't necessarily comply with NZS3101 if they don't rely on a few hand checks or design conservatively with regard to some of the limits.

Things like max stirrup spacing and the like are different. You might need to manually compare limits instead of reliance on the in-built calculations and the like, so still need to do some manual checking.

I'm not a guru or anything with respect to PT, but I have done some peer reviews which involve a PT component.

 

[Ophirian]

Thank you @Agent666.

When you do your peer review do you intentionally check it against AS3600?

What do you do when some clause in AS3600 is not met? Do you consider it as strict compliance?

 

[Agent666]

When you do your peer review do you intentionally check it against AS3600?

Definitely not, the design needs to comply with NZS3101, usually I send them away to prove a design works to NZS3101 with a few suggestions on what might not work. I'm not even that familiar with AS3600, used it about 15 years ago for a few things I designed in Australia

The point is doing a design to AS3600 using some design software almost gets you there if that's what you are familiar with, a good means of checking the design or forming a good starting point. I believe NZS3101 is roughly based on ACI318, unsure if its the same with the PT section (definately some of the equations are the same). But that might open up a lot of literature if it is any help with understanding.

 

[rapt]

I would agree with that comment about NZS concrete being roughly based on ACI. It is a bit of a mixture. There is some As3600 in there as well.

But remember, a lot of the ACI rules are un-bonded PT specific. And NZ uses bonded PT!

You could check against both codes with some added checks on earhtquake as NZS has its own rules on that, especially on ductility, maximum bar diameters and shear detailing.

 

[Ophirian]

A follow up on this thread (though not entirely PT related), I just found that NZ 12.5.6.9 Requires for additional Bottom reinforcement at support location called as integrity reo. This is something I have not come across in AS3600 and the ACI.

It basically protects the slab for falling upon punching failure but for a transfer slab, it would require a lot of rebar to satisfy it. I'm quite skeptical about it. What are your thoughts on the code?

 

[rapt]

AS3600-2018 requires this as does ACI318 and the Canadian code CSA23.

 

[hokie66]

The integrity reinforcement protects against abnormal loading, and I don't know why anyone would argue against it. As to the Australian code, I think integrity reinforcement was introduced as good practice when seismic design was first considered.

https://structurepoint.org/publication/pdf/PCA-Concrete-Structures-Integrity-Detailing.pdf

 

[Agent666]

I'm just curious as the integrity reinforcement requirement got me thinking if anyone considers the reinforcement as being required for footings for punching shear or if its primarily intended for only elevated slabs supported from columns as the wording strictly suggests to add robustness. Footing is simply the same arrangement upside down after all, albeit with a thicker slab (I hope!).

 

[hokie66]

I think it is only for elevated slabs. The abnormal loading conditions envisioned would not normally affect the footings.