AS3600-2018 - Detailing for grouted ducts in precast walls 1

[li0ngalahad]

Hi
I am a bit puzzled about clause 17.7.5 on the AS3600-2019 code and I was wondering if anyone can shedsome light on it.

17.7.5 Grouted ducts to joints
Grouted ducts used to join prefabricated elements shall be fully confined using Ductility
Class N reinforcement anchored perpendicular to the duct in accordance with Section 11.

As usual, AS3600 is vaugue and unclear, and prone to be misinterpreted, especially without a commentary. It's quite ridiculous really, especially when you compare it to ACI or EC2. I was disappointed to see that the Amendment 2 Draft shows nothing about this clause.

Anyway, my questions are :
1) How do I design this "full confinement in accordance with Seciton 11"? Section 11 does not talk about full confinement as far as I know??
2) What does "perpendicular" to the duct mean? In a precast wall, horizontal reinforcement is anchored perpendicular to the duct, however I don't see that doing much in terms of confinement.
3) Do we really need to detail this even if my structure is designed with ductility 1? And if so what is the reasoning behind this? As long as my lap is in accrodance with AS3600, and I assume ductility 1, I would think such painful detail can be waived??

I have found a document from the NZ commission on the 2011 Canterbury earthquakes that talks about this, with a lo more info in terms of detailing and calculations. I wonder if anyone here is using the same formula and detail?

 

[tmac7285]

I think that's an interesting question Liongalahad, and I think you're on the right track looking at the NZ commission document. It'd be worth looking at the NZ concrete code too.

I was speaking to some of the Kiwi's in our office about this yesterday and they essentially said that after Christchurch and the failures identified with grout tubes as per your attachment, they began confining the grout tubes as per the sketches you attached i.e. ligatures around the grout tubes like one would detail for a column. That appears to be the intent of Clause 17.7.5, albeit that the reference to Section 11 is a bit clumsy as Section 11 doesn't specifically note any confinement requirements itself but instead references section in Section 10 and 14 in some clauses. Personally, I think Cl 17.7.5 would be more appropriate to be a sub-section of Cl 11.7.5, given that clause is about doweled connections in prefabricated concrete walls.

I'd be interested to hear other points of view on this clause.

 

[rscassar]

I like the detail on the right, not the detail on the left. I think single layer mesh should not be used, with the exception of facade panels that have been adequately detailed to no transfer vertical force.

 

[blihpandgeorge]

@rscassar I recently used the NZ standards and they do allow the use of single reinforced walls, including carrying seismic loads, but only in very specific cases including using low ductility values. We were using mu=1.25 so quote a bit lower than the mu typically used in aus.

 

[rapt]

blihpandgeorge
That's interesting about single reinforcement in NZ code. AS3600 also allows it for non-ductile walls! It is limited to doubly reinforced only for walls requiring ductility, with mu of 2 or above!

liongalahad
Had you pointed out to anyone that you thought that this clause needed reviewing or explaining?

 

[Agent666]

In NZS3101, it only allows the use of singly reinforced walls under certain conditions. There are quite restrictive axial load conditions, basically restricting their use to the panels self-weight or maybe a floor. They are most definitely not intended to be used in multi-storey construction by the code authors admissions. But they never expressly said so, so people use them. The provisions are written in a way that they say if the axial load is less than xxx, then you may use the following method...

What they don't say is when you are over this load, you are not supposed to use a single layer. Unfortunately, many engineers tend to think 'well I'm over this load, therefore cannot use that provision'. So they just design it normally like any other wall. They changed the strength reduction factor a few years ago to 0.7 (from 0.85), the intent was to make the use of singly reinforced walls less attractive (rather than explicitly banning them in certain applications) and also to effectively make the walls more or less elastic at the MCE level earthquake. But designers being designers just sucked up the change and carried on as they always had not going to double reinforcement.

There is a lot of unknowns, especially around biaxial seismic loading of singly reinforced which means they should be used with caution.

From discussions I've personally had with the code authors (NZS3101), their intent was that the provisions and hence use of singly reinforced walls should be limited to warehouse type industrial buildings or cladding elements only. They should not be used in multistorey construction, but we continue to see them being used. They were intending to have some seminars over here regarding the design of singly reinforced walls to help demystify when and where their use is appropriate.

Interestingly in ACI318-19 the use of a single layer of reinforcement is tied to shear, any shear higher than just the concrete component requires a double layer of reinforcement. This is a far more definitive cut-off, which would mean in any real wall you're going to require two layers.

Regarding the use of the ducts, etc. That advice has never been codified, it was interim advice put out by SESOC following the earthquakes in 2010/2011 to address what it thought were some pertinent observations they had made on building performance. Some people take this to mean the confinement specified is no longer required at all, given it was not included in our recent amendment to NZS3101. Some argue the code has always required the confinement of laps in certain applications, and this is where that equation comes from, the confinement of laps in areas of reversing stresses greater than 0.6f_y.

 

[Settingsun]

Interesting tale, Agent666. I wonder whether the code academics are hampered by construction industry representatives who have an eye on the dollars, or are simply too shy to say what they really think. You've mentioned peer review being required in NZ; maybe a sub-committee drawn from peer reviewers could advise on how the code is being misapplied/misunderstood to help improve the next version. Seminars in place of simple code revisions is crazy: reach a fraction of the audience at massive cost increase. Alongside code revisions - yes; substitute - no.

 

[Agent666]

Nothing moves fast over here....... same as anywhere else. It took 6-7 years to get some of the learnings from 2011 into our concrete code, albeit a lot of the holes exposed required further research.

There is not going to be another amendment for some time we've been told. There are a lot of precedents here for accepting 'best practice' if it addresses some shortfall in the standards/codes.

Codes cannot tell you everything, it's a starting point I guess as far as the code writers are concerned. So the seminars simply build on the concepts and give further advice. But like you say behind a paywall....


I think sometimes they just don't want to be the one who says NO, some probably want to, but other forces pull them back from the edge. So they say some weasel words like 'should take account of...', or 'should be mindful of the fact that...', leaving the final decision to the designer. This implies some thought and knowledge on the subject from the designer, leaving the designer to make up their own mind on the matter, instead of outright simply stating 'do not do this!!!...'.

The problem seems to be designers only know what they know, and don't know what they don't know. So, when presented with some issue they don't understand, they either ignore it, misinterpret it, or make some assumption with no guarantee of achieving the same intent that the code writer vaguely implied.

You've mentioned peer review being required in NZ; maybe a sub-committee drawn from peer reviewers could advise on how the code is being misapplied/misunderstood to help improve the next version.

I certainly try to feedback any shortcomings or question what the real intent is if they are written poorly. The writers are generally receptive to this. I've seen a few things I've queried in the past get re-written which is pretty cool. But even then you're not involved in the process, they mearly take your idea and run with it (sometimes resulting in an equally confusing rendition of words).

 

[Agent666]

Back on the original subject, I noted this morning that SESOC republished their interim advice in late 2019 with some further updates. Their intent seems to be that unless the codes have been updated to address the specific points they had raised, that the prior advice is still considered relevant. Whereas I'd previously stated a number of people I've dealt with on reviews are of the view that because it didn't make it into the code that it is no longer considered best practice or is irrelevant.

The latest 2019 version is attached.

 

[rapt]

Agent666,

Thanks for the NZ clarification.

Many of the new rules added to AS3600-2018 for earthquake design, especially for walls and ductility, were based on the proposed NZ response to Christchurch. While the NZS code committee were trying to get it all included we did the same. Resulting in AS3600 provisions that appeared to some to be more stringent provisions than currently published NZS and ACI provisions.

As usual this resulted in comments that AS3600 was being too harsh and conservative, while in fact is was simply doing what others were planning. It seemed like a big step for AS3600 but the old rules were really a very basic version of the 1960's ACI rules so they were a long way behind. So there was a lot of catching up to do, and it was not logical to bring everything up to date with current NZS and ACI rules when we knew they were in the process of being updated in response to building performance in recent earthquakes in other countries.

 

[SRL V1.0]

Hi Rapt,
Firstly, thanks a lot for the time that you put into posting on this forum.

Secondly, is there any update that you are able to share about the 3600-2018 commentary? There was a posting from December last year that mentioned it may be coming out this year.

 

[rapt]

It is still being worked on. There has been an overall review of the initial draft by someone who is very good at that and very good technically and it has identified areas that need more work. Reviews of initial drafts by the whole committee and finishing off areas that were missed or need work are now being done.

I doubt it will now be this year due to delays for some people being able to put as much time into it as hoped due to Covid19 (Academics have been busy trying to save their universities from lack of students, needing to run courses remotely, etc). But definitely earlier rather than later in 2021.

For those who complain about commentary not being included with the main code, that is controlled by Standards Australia, not the AS3600 committee. The committee have requested it many times but are not allowed to even do it as part of the same development project. I assume they are not members of Eng-Tips (or I could be in trouble) so it is no use complaining about it here. Direct your complaints to the appropriate people directly.

I can confirm that the initial topic of this thread is being covered in a lot more detail. As is the earthquake section in general.

 

[QSIIN]

Can anyone recommend any articles, studies, product tech spec sheets for grout tubes?
I see a lot of offices simply specifying a 75mm dia metallic ribbed grout tube, but not a specific product.

Are lap splices (tension/compression) really effective through grouttubes? What length of a "ribbed grout tube" is required for anchorage of the tube itself?

I haven't seen much on this aspect of precast.

 

[Agent666]

QSIIN, this is the only manufacturers advice I'm aware of from NZ. It's basically regurgitating the SESOC advice posted earlier. Generally it's accepted here in NZ that you just (rightly or wrongly) provide Ld lap, I'm not aware of any justification for this except that's the way it's always been done. The bond stresses at the larger drossbach tube are quite a bit lower than around the perimeter of the bar its self, so if you're justifying the development of the drossbach tube like a deformed bar then it works.

 

[Agent666]

I also found this a few years ago, but never got around to working through the calculations to see if the Ld assumption held true.

 

[Retrograde]

https://www.psa.com.au/psa-category/grout-tubes-pipes

These are the tubes being used on one of my current projects.

 

[hokie66]

Retrograde,

I can see using plastic tubes just for passing grout through, but if they are structural, as I think is intended in this discussion, I would not want plastic.

 

[li0ngalahad]

Thanks everyone for their input

I still do not understand the meaning of this detail though, why would the grout tube require confinement? If I read the NZ document, they explain it by saying that when using grout tubes there is "reduction in concrete section area and a subsequent weakness develops" (see screenshot on the original post). If I fill grout tubes with grout having same or higher strenght than the panel, why would I get a reduction in concrete section? Especially if there is a methodolody in place to make sure no air-pockets are present within the grout tubes (with scanning or other methods), I see no reason why a grout tube should reduce the compressive capacity of a wall. Would be great to hear from someone here who understand this requirement better than me thank you

 

[Agent666]

The logic is it is a lap, Laps in reversing stesss regions require confinement according to NZ concrete code.

 

[hokie66]

Agent666,

What is your opinion of using plastic for these grout tubes? See Retrograde's link above.