Post-tensioned beam reinforcement detailing AS3600

[BentEng]

Hi

CL 8.3.1.3 of AS3600 (2018, used to be CL 8.1.10.4), gives minimums of the quantity of positive moment tensile reinforcement required at midspan to be extended beyond the face of the support. 50% at a simple support, 25% at a continuous.

Do PT strands count towards this, even as they drape up to the negative moment zone at the support? Is that still considered as "tensile reinforcement extending past the face of the support"?

For example, if 75% of the moment at midspan is taken by the PT, can I provide just 25% of conventional reinforcement at the peak moment zone and terminate the bars when no longer necessary, meaning there would be no conventional bottom reinforcement at the face of the support.
I'm guessing this is acceptable, as there are plenty of PT slabs out there with no conventional bottom reinforcement (not considering new integrity reinforcement requirements).

 

[Agent666]

I believe a lot of people have interpreted it that way to stay competitive (counting PT as part of the conventional reinforcement to count towards minimum requirements). But I'm dubious as to whether that was the code authors intent. Everyone's doing it, so everyone does it.

I'm sure Rapt will be along to school us shortly.

I recall having this conversation a year ago with a Australian PT designer, who was doing some preliminary work for us on a NZ project that obviously had to comply with NZS3101. I questioned them on the fact they were not complying with our minimums, they were adament it was allowed. But from memory at the time reading through NZS3101 it seemed to be a bit vague but we tended to think best to take a conservative approach and not count it. Well the PT wasn't having any of that, everything was boiled down to the minimum to give best price for their piece of the pie (flat slabs and transfer beams). Not withstanding that we as building designers end up putting it all back and some in slabs to deal with seismic diaphragm forces.... So I guess it was a moot point in the end.

 

[BentEng]

It's definitely vague, especially when there's a conventional reinforcement diagram in section 9, but nothing for PT. I've seen dozens and dozens of buildings with PT slabs and no btm Reo, so I would think that it's almost a universal understanding that the tendons are applicable to the clause.

But for deeper members where the tendon is very far from the soffit towards the column end of the span, it just doesn't feel correct.

Maybe the new integrity reinforcement clause in Section9 is intended to cover this?

 

[rapt]

Are you talking about beams or slabs. Your discussion appears to be related to beams as it is talking about section 8, but then moves to slabs which are covered by section 9.

The only 3 reasons I can see for a percentage of the bottom face tension reinforcement to reach the end support are

- robustness

- truss analogy for shear

-seismic load reversal

If you can think of others, let me know.

The truss analogy for shear requirements are now covered by clauses 8.2.7 and 8.2.8. This applies to slabs and beams. In the earlier codes this was also covered in the above mentioned 8.1.10.4 as a minimum tension force requirement at simply supported ends (should have been at any point of contraflexure!!). That was actually a double up with the tension force for shear requirement so was removed from the 2018 version which is now much more complete on this. Prestress tendons can contribute to this, even if not near the tension face, as long as they are in the tension half depth of the section. This would only apply to bottom reinforcement if there is no rotational restraint at the end, otherwise it would be top at the support and bottom at the point of contraflexure.

AS3600-2018 requires checks on Robustness, but does not provide minimum requirements to satisfy this for beams (I lost that one, but will hopefully get it in the next release) except for some minor rules for edge beams. So the designer has to work that one out themselves for beams. It is in section 9.2.1 for slabs. Bonded tendons near the supports will contribute to robustness, so they are included in this requirement as well.

So based on this, I would suggest that the % rules apply to both bonded reinforcement and prestressing tendons.

AS3600 does not require that there be bottom reinforcement if the prestressing tendons are adequate for strength and crack control. I would always provide at least nominal bottom reinforcement in PT beams, even wide band beams, but some PT companies disagree with me on that. Slabs do not require bottom reinforcement if the PT is adequate and restraint conditions are satisfied.

I have discussed the integrity reinforcement requirement for the bottom of slabs, drop panels and band beams at columns (section 9.2.2) in other threads. It is purely to improve punching shear performance and prestressing tendons do not contribute unless they are in the bottom of the section at the support (highly unlikely so not even mentioned in the code).

If seismic design requirements need to be met, the moment reversal requirements at the support face will normally require that bottom reinforcement is supplied for slabs and beams. Also, the ductility requirements for beams in 14.4.6(a) will require compression face reinforcement at the supports equivalent to 30% of the tensile capacity of the tension face reinforcement and tendons (similar to the NZS rule for moderately ductile beams). Prestress tendons would not contribute to this.

 

[BentEng]

Hi rapt,

I don't mean for this to be a complex theoretical discussion, merely an interpretation of the detailing rules for beams and slabs, specifically the % of positive moment tensile reinforcement required to extend beyond the face of the support. I've said beams because this is the example I'm looking at, though I think the question is applicable to both beams and slabs.

I don't think 3600 explicitly says why or what this requirement is for, it's just a detailing rule, by which I am happy to oblige.

Though I would always place bonded reinforcement to the bottom of PT beams, technically the code allows no conventional reinforcement if the PT provided satisfies 100% of the demand (beam or slab).

So then, if the % rules apply to both conventional reinforcement and PT, that is to say that, although the PT may be in the negative moment zone at the face of the support, because 100% of it was in the positive moment zone at midspan, this detailing rule is automatically satisfied. Or if 75% of the positive moment reinforcement is provided by PT at midspan, then 25% can be provided by bonded bars that terminate when the moment has reduce enough, and again, no btm reinforcement at the face of the support.

Then we move onto integrity, robustness, reversal etc.

 

[rapt]

BentEng

I was not making it a complex theoretical discussion. But a designer needs to be able to sort through this themselves as the code cannot give a specific rule for every situation you might encounter. The code clause does not cover this well for PT and never has. When AS1480 and 1481 were merged, some areas simply copied the 1480 RC rules without considering how they are imp[-acted by PT, and this is one of them. So you have to think through what controls it.

AS3600 clauses 8.2.7 and 8.2.8 tell you how much tension force you have to develop at any point in the frame, as well as into the support.

Robustness then also requires a certain amount extending into the support.