Integrity Reinforcement in Transfer Slabs 9

[KutEng]

Just want to bring back an old post by KootK that got no real answer:

https://www.eng-tips.com/viewthread.cfm?qid=388712

Integrity reo has just been introduced into our Australian concrete standards so I don't think many of us will have this figured out yet.

This is a tough one because transfer slabs really benefit from integrity reo since a failure in a transfer slab can be catastrophic, however trying to get adequate integrity reo over your columns seems like a huge ask.

It's almost as if this clause is pushing us away from designing flat plate transfer slabs by making it so unfeasible that no one is willing to use them anymore. Generally, on some of our transfer decks we transfer columns at ground floor that continue up to 15 stories (around 7000kn of load). It would be almost impossible to fit enough reo over your column in these cases.

Would love to hear from some people who have managed to satisfy this clause in a transfer slab, or if its generally left out of transfers.

 

[rapt]

The robustness case is a supporting member failing (possibly due to an accidental action) meaning that the supported member has to redistribute its loads elsewhere. The supported member is assumed to have only its combination load combination applied (D + psic, (I was wrong, it is not technically the permanent case, but for office/retail/parking in AS codes psil and psic are the same value)).

The column punching case, the slab is failing in punching shear presumably because its load is higher than the design capacity of the connection. So the slab has full ultimate load on it (or someone under-designed the punching shear).

 

[Toby43]

I've not designed any major/large flat slab structures nor high rise, so just a few comments from someone on the outside looking in...

Steveh49 - Punching failure was observed at the Newcastle Workers club in the 1989 Earthquake. Looking at the photos, there was no bottom "integrity" steel, just ripped out top bars left hanging in the breeze. See "investigation of the failure of the Newcastle Workers Club" by Melchers.

In general, real punching failures seem to have the following features (technical only - procedural etc.. always compounds the problem) -
1) During construction due to
a) Concrete Strength gain issues
b) back-propping insufficient
c) Poor bar placement (top bars too low)
Such examples would be "Harbour Cay Condiminium", Baileys Crossroads "Skyline Plaza", "2000 Commonwealth avenue" - All cases where integrity reinforcement would have reduced the likelihood of progressive collapse as they "pancaked".
2) Degradation over time - "Pipers Row". Integrity reo would again have helped catch the slab - interestingly enough, as the slab was not caught effectively, but some continuity existed, the progressive collapse spread horizontally
3) Seismic

I feel the integrity reinforcement, in addition to catching the slab, also gives some restraint to the column to prevent 2-storey buckling, which would progress the collapse, in a disproportionate manner. That said, I would conclude that edge and corner columns would need to be designed for this 2-storey condition along with an additional horizontal load from the failed slab. Is this done in common practice?

Again just a few comments from someone on the outside looking in...
Regards
Toby

 

[Toby43]

I should have added that my reason for commenting on those failure case studies, was that punching shear capacity predictions, can be quite different, depending on the code/reference used. So in my view, even a slab well designed for punching shear, still has an inherent/residual risk associated with it (indeed all aspects of shear in reinforced concrete are vastly more uncertain than other areas of behaviour). As such, since Engineering is a profession that predominantly deals with uncertainty, it is my view that punching integrity reinforcement is an economical way to mitigate life safety risks associated with this uncertainty.
Now as to the query about how mush integrity steel in each face - In my opinion as it starts to punch on one side, the propagation of the shear crack would unzip right around the column, thus considering more steel depending on predicted force transfer into each face would not be relevant (stiffness of integrity less than "about to be" punched slab)
I feel that if integrity steel is in one direction, that post punching the slab would span "one-way" to the strip that has the integrity steel and thus hang. This however contradicts my thoughts on column stability post punching.

 

[rapt]

Toby43,

I would tend to place the reinforcement relative to the load coming into the column on each side, not relative to the side lengths as was suggested previously. Side lengths are misleading as for a very rectangular column, on the long sides most of the load comes onto the ends and very little along the remainder of the length, so for a square grid the sides are about equal no matter the column shape.

If it was a transfer slab with a transferring column on one side of the column, then most of the load will come in on that side, so it should have more of the reinforcement.

Yes, if you put it all in one direction it will possibly redistribute to that, by why wait for the redistribution when there is no benefit to reinforcing one way compared to 2way. And the whole idea of this reinforcement is to try to avoid it unzipping in the first place!

 

[Toby43]

Rapt,
I would contend that the pre-punching load distribution into the column is no longer valid once the integrity reinforcement is utilised. That is why I'm not perturbed by one-way integrity. Of course you would put it both ways if possible. For the long column (wallum), with punching initiating at the short face, the integrity reinforcement on this short face would not be effectively utilised until the entire perimeter of the slab had punched through some distance (indeed some dowel action may be at play though). Thus once slab has totally punched, all integrity reinforced would be in play.
Adjacent penetrations, as mentioned above been one case where it is not possible to provide two-way integrity, to me are the same as punching initiated on one face. Punching shear design provisions (to prevent punching) account for penetrations etc.., yet i feel the post-punching behaviour is a whole different animal.
To me the role of integrity reinforcement is to minimise the chance of progressive collapse in the vertical direction, if for whatever reason (under-designed, construction quality etc...) punching failure does occur and is thus independent of the punching shear design that aims to prevent punching (which would rationally take into account load distribution into the column).

Cheers
Toby

 

[aaronPTeng]

All,

Been a while since my last post, But with the new AS 3600 I have seen the above interpreted in so many different ways.

Firstly RAPT I agree codes cant substitute logic and engineering judgement. When it comes to this clause I have a few questions/ comments I would like to see what people think. We cover off full design and PT D & C work. so we have already started this debate with several engineers and everyone seems a bit scared off making a call any direction with regards to this clause (and others!).

I believe I understand the intent of this clause. Consider the following situation

In the attached image, Looking at the Canadian code it allows for the use of tendons (13.10.6.3). If the tendons in the X-X (directly over the column ill get to this bit next) have sufficient As.m to carry the load then how will the failure occur? If you get significant load reversal or something major happening and we get a full connection failure the tendons over the column will pick up the tendons close to the columns in the Y- Y direction (on the bases they have the correct As min)

Secondly with regards to this "zone" where the reinforcement needs to be I have never seen a straight failure of any structure directly vertical at the column face. Is there any examples of this, why is it not column dimension + D (slab depth) each side?

I have been trying to re locate a photo and paper I read years ago which reviewed structures after a major earthquake in Mexico and it shows the failures happened as per above (with respect to the PT holding it together. (after the concrete cracked all the way around the supports. Not at the column face!).

I am contributing to this not to "cut corners" or "make the code do what I want it to" just me thinking logically about the structure.

Thoughts?



"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[KutEng]

Think I can help with a few of your points

Secondly with regards to this "zone" where the reinforcement needs to be I have never seen a straight failure of any structure directly vertical at the column face. Is there any examples of this, why is it not column dimension + D (slab depth) each side?

The reasoning for this is that the bottom integrity bars will tear out from the failure cone if they have nothing to bear on (like the horizontal face of the column)

I have been trying to re locate a photo and paper I read years ago which reviewed structures after a major earthquake in Mexico

This paper might be of some use to you

 

[aaronPTeng]

AR, with regards to the zone, I agree in the RC solution of bottom bars, but in PT if you had a tendon with a high point right next to the column then it wont tear out.

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[Ingenuity]

I have been trying to re locate a photo and paper I read years ago which reviewed structures after a major earthquake in Mexico and it shows the failures happened as per above (with respect to the PT holding it together. (after the concrete cracked all the way around the supports. Not at the column face!).

The following photo is not from a Mexico earthquake (1985 nor 2017) but it is a slab soffit photo post-Northridge (California) earthquake of 1994.

Unbonded PT flat plate - typical USA practice of banded/uniform tendon placement.

Yellow circled tendons are those placed directly over the column core.

Not a pretty picture - but it worked. Not much concrete with significant capacity around the column face.

 

[rapt]

aaronPTeng

You obviously do not understand the intent of the clause in AS3600 as you persist in considering the PT tendons in it.

I explained the intent much earlier and it does not involved the PT tendons. IT is purely a requirement for "bottom" reinforcement at the columns to provide ductility in the case of a punching shear failure. It has nothing to do with Robustness involving the failure of the supporting member which the tendons will assist in. If it did, we would not allow the reinforcement to be terminated as we do. It would have to be continuous full length of the frame.

So it is no use quoting the logic in the Canadian or ACI codes regarding PT tendons.

The only interpretation possibly required is how much reinforcement is required on each face. This has been discussed above. Otherwise, the clause requiremments are pretty obvious. If designers are having problems "interpreting" them, I suggest they find another profession.

 

[Gishin1]

RAPT, I think you're out of touch with general practice in the industry... haven't you heard, punching shear isn't real? And AS3600 isn't the cook book, you're far too flattering of practicing engineers...RAM concept is the cook book!

Engineering logic....what's that?

In all seriousness, there are PT transfer out being built now that don't have any shear reinforcement, and trust me, they aren't very deep either. We don't see collapse because how often do we see ultimate events? I dread the day we get a real earthquake..

I honestly don't think the torsion strip philosophy in AS3600 is appropriate for the types of slabs being built today, a stress based approach as per ACI/NZS is more appropriate. Additionally, there are further reductions in capacity for deeper slabs. 3600 has acknowledged this in the "beam shear" section but no mention in the slab shear section.

We really should move away from this "beam shear" / "slab shear" terminology. It's just one way or two way shear.

 

[rapt]

I doubt that I am out of touch with general practice in industry, with regard to codes, software and the level of competence of some engineers. Fortunately not all are the same. And I doubt that too many engineers would ever say I have ever flattered them. Wishing that they understand engineering principles is not flattering them. But writing codes, that is what we have to assume.

RE the serious bit above,

We are seeing failures. There are several buildings in Darwin that are currently propped very heavily to stop collapse until the punching shear problems in their RC transfer slabs can be rectified. I understand these were found when one of the slabs nearly collapsed during construction (with only about half of the building above in place), but the builder noticed some abnormal deformations around some column heads and propped it in time. I understand there are similar problems in Canberra and possibly one in Newcastle (by the same Canberra engineer) and over the last several years a couple of buildings have been demolished due to punching shear problems in Canberra. I do not think either of these consultants used popular commercial FEM design software, but I could be wrong.

I agree about earthquakes. Hopefully new buildings based on this new rule will be safer in earthquakes. But there are more problems than punching shear with our current buildings in a real earthquake. Walls with very high concrete strengths and single reinforcing layers are going to be a major problem. As are many connection details and ductility problems.

I agree about the AS3600 punching shear rules in general. But we can only change so many things at once. There was not sufficient time to change the Punching shear rules this time to fit in with the publishing schedule which is controlled by the BCA publication cycle. Interestingly AS3600 used to be similar to ACI stress based approach but was changed to the current approach! I have never liked the new approach but have never been able to get it changed. It will be changed in the next code cycle.

ACI code would disagree with your logic of 1way/2way shear. It treats them both very differently! BS code and in some ways, Eurocode are really the only ones that treat them the same way.

 

[Gishin1]

Of course, I was being very sarcastic..no disrespect intended, RAPT. Just quoting some of the rediculous things I've been told.

Regarding treating beam/slab shear different, I suppose I mean in the sense of how engineers understand the mechanism of shear. I think analysis/design can have different rules and approaches, sure, but some engineers think a wide shallow band beam can't possibly fail in punching because it's a "beam" - even if it's 3000w and is only 50mm deeper than the slab! It's a lack of understanding how the load travels through the slab to the support, either from one direction or many.

 

[aaronPTeng]

RAPT and All

I am not "persist in considering the PT tendons" You are famous for quoting people dont apply logic etc, yet here with a simple question on mode of failure based on logic (in which the photo above by ingenuity shows a load path) causes such a negative response. Lets move past the professional arrogance.

Forget the "clause" The statement was based on the failure mode in this situation. Not trying to get the clause to do what I want it to.

The new standard has a clause (which is this thread) use of integrity reinforcement. I am saying in a PT situation is there an additional load path provided to the structure. I am also asking my peers for an open opinion about this additional load path.

Also with regards to the points above. I am currently involved in several of the mentioned projects (there are many) in which I see "runs" provided to me which would make your blood boil. Along with real life examples of structural failures of several elements. I do not believe codes or software should ever be fool proof as it should only be used to validate a design not drive it.

Blindly following statements is as bad as ignoring them.



regards,

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[Trenno]

If designers are having problems "interpreting" them, I suggest they find another profession.

Releasing the code commentary a few years after the code publication doesn't help matters.

 

[Settingsun]

Side by side commentary and code like ACI and CEB. If code writers are having trouble writing understandable codes and timely commentaries...

 

[Trenno]

I didn't even touch on the fact that when the recent AS3600:2018 was first published, there were a multitude of blatant errors.

We're not all squeaky clean.

 

[rapt]

Jishin1

I know a lot of the weird and grossly incorrect interpretations of the code that are used to get around code clauses. Unfortunately we cannot program the code to lobotomize the idiots who often deliberately do this. e.g. the idiot in Victoria doing contract PT design who gets around the flat slab fire rating rules by using banded/distributed tendon layouts in flat plates and calls them one way slabs.

Also your band beam punching shear point. I have been pointing out that band beams need to be designed for punching shear for 40 years. I also think there should be a band of reinforcement over the column in the slab direction. Used to be considered good practice 30-40 years ago. Ignored now by most.

ArronPTeng

"I believe I understand the intent of this clause."

As I explained, you do NOT know the intent of the clause. I explained the intent a long way back in this thread and again just above. PT tendons do not come into it. Look at the ACI318-2011 version of the clause (I think it was that one). They have changed their mind since then, we have not.

Trenno and Steveh49, many things are out of our control including everything mentioned in your posts. Australian Standards will NOT do side by side code and commentary or same day release. We have requested it many times. I am not at liberty to go into the cause of the errors, and we are just as frustrated as everyone else.

 

[StructEng23]

This post is becoming laughable now and quite personal and quickly diverging away from the main topic.

We as "PROFESSIONAL ENGINEERS" are required to (and encouraged) to maintain our professional development, and one of the ways this is done is through forums like this to discuss topics where there are short falls in literature and codes.

This integrity reinforcement is a hot debate at the moment and as an experienced PT engineer myself, is something I have been following closely...collaborating with many of my colleagues and industry contacts.


A few questions/comments for RAPT:

- Why are you taking many of the comments so personally? As above, this is a thread for discussion on people's views and interpretations. We should be encouraging this conversation, not shutting people down with comments such as "you do NOT know the intent of the clause" and "If designers are having problems "interpreting" them, I suggest they find another profession". I agree with many on this thread that a detailed commentary for the Code would have assisted peoples interpretations.

Is there an update on when the Commentary will be made available???

- You stated ""There was not sufficient time to change the Punching shear rules this time to fit in with the publishing schedule..."
Was 9 years between Codes not sufficient time? Seems to me that it was a very rushed job when it was finally released with all the errors in the text.


One for both RAPT/aaronPTeng and back to our actual topic:

- I think RAPT you are missing what aaronPTeng is trying to say here. He is not trying to exclude or ignore the integrity reinforcement, but more exploring the reason for the integrity reinforcement. It's to introduce more robustness into the structure so the system has a progressive collapse, he is not debating this. What he is asking about is more about utilizing the PT to assist in achieving the structural integrity clause more in line with the Canadian clause:

Clause 13.10.6.3 The bottom reinforcement required by Clause 13.10.6.1 shall be provided by one or more of the following:
(d) continuous tendons draped over column capitals, with a minimum total area of prestressing steel calculated using Equation 13.26, but with fy replaced by fpy

The photo provided above by Ingenuity is actual proof behind this concept that it works and is achieving the intent of the clause.



Another question, the integrity reinforcement required in AS3600 is based on N*...

Where is the background theory that N* is based on 1.2G + 1.5Q?

Should this not be for the governing fire load combination or earthquake load combinations??? Is this not why it was introduced, for disaster situations?

 

[aaronPTeng]

StructEng 23

Yes You understand what I am asking,

Rapt I may not of written it correctly but when I was talking about intent, its not the numbers or formula etc. Its the idea of this clause giving structure robustness.
As another idea of giving it robustness (forget the clause above) what impact does PT have to achieve the same out come - Robustness in the structure. For years I (and my engineers) have been detailing PT in the fashion I have described for robustness. (for many reasons with lateral loads etc however I dont want to go off topic)

StructEng, with the N* I a also trying to track down the research/ real life examples to when this robustness needs to happen. All I can find at this stage is the "overloaded event" example in the photo above at mexico.

Ingenuity

I have spoken to a university about this one and am discussing possible research and testing on the different methods for robustness. Before adding 100t of reo to a job it would be good to see some of this theory actually tested.

Now we are having a engineering discussion.

Regards,



"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976