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Integrity Reinforcement in Transfer Slabs
9

Integrity Reinforcement in Transfer Slabs

Integrity Reinforcement in Transfer Slabs

(OP)
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.

RE: Integrity Reinforcement in Transfer Slabs


1) Thanks for resurrecting this. I'd give you a star if I thought that such usage of the starring system was in any way appropriate.

2) I agree, the spirit of integrity steel is even more important for transfer slabs than regular slabs. That said, I don't feel that crazy amounts of reo over/under columns is the answer for this situation. More next.

3) Rationally, I like this, modified approach to integrity reinforcing:

a) Deal with punching shear with real, steel reinforcing, either stud rails, ties, or stirrups. Where the reinforcing exists in plan, I see no fundamental difference between this and the transfer beam case with respect to shear transfer to the columns.

b) Take the punching shear reinforcing out to some judicious location where stresses are low enough that we don't much care any longer. Maybe [ 0.5 Vc > Vu ] or something. If you think about it, we're already doing pretty much this same thing with the integrity steel which, in my locale, only goes out [ 2 x Ld into the slab ]. This assumes an designer savvy enough to recognize that punching shear integrity reinforcing is a different thing from catenary action of course. We seem to have perennial confusion on that front.

4) Another approach might be:

a) Deal with punching shear with real, steel reinforcing, either stud rails, ties, or stirrups.

b) Double/triple/quadruple up your ties at the perimeter of the reinforcing and place your integrity steel there, spread out over the width of the reinforced zone. I feel that the peculiar detailing required would make this prone to field problems however.

5) Any of these alternate solutions would, naturally, require approval from the authority having jurisdiction.

RE: Integrity Reinforcement in Transfer Slabs

Quote (Ar Engineer)

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.

I, for one, would not shed any tears if we were forced to use transfer beams instead of flat plates for transfer situations above some, reasonable load limit. In fact, so long as my competitors were all forced to the the same, I'd celebrate that. Whenever I have a transfer slab that's getting upwards of about 30" thick, I start doing my darndest to use discrete beams, even if their beams embedded within a thick slab.

RE: Integrity Reinforcement in Transfer Slabs

I have to admit, I haven't been doing this for the two-way slabs I've done. I always had it filed away in the back of my mind as something applicable in high seismic zones. (Which I have avoided with this system.) ACI 352 justifies it from two standpoints: construction failures that have happened (which shouldn't, I would think, if proper procedures were followed).....and (of course) moderate/high seismic zones.

They make this statement about the latter: For Type 2 connections, the design loading conditions may result in general yielding of the top and/or bottom slab reinforcement at the connection. Experimental data42 indicate that under such conditions the punching shear strength may be reduced considerably below the nominal value of 4√f'c Acs permitted by ACI 318, thereby reducing the margin of safety against collapse. Thus, minimum continuous bottom reinforcement as specified by Eq. (5-1) is recommended to support the slab in the event of a punching shear failure.


RE: Integrity Reinforcement in Transfer Slabs

(OP)
Glad we're on the same page here Koot.

Do you think there is any requirement of what ratio of reo bars go in either direction?? Could you justify putting all your reinforcement in one direction for a long, thin column - i.e if you had all, or majority, of the bars perpendicular to the longer dimension of the column and the slab was to punch, can you rely on your normal reinforcement to carry the slab loads coming from the shorter dimension sides to the extra bars in the other direction?

Our standard only mentions the amount of steel required and gives nothing on detailing requirements.

RE: Integrity Reinforcement in Transfer Slabs

I do believe that the reinforcement can, and probably should, be placed about the perimeter in proportion to the length of the sides. For narrow wallumns, I often don't put any bars on the short sides. In addition to congestion issues, plumbing risers often need to be located at the wall ends.

RE: Integrity Reinforcement in Transfer Slabs

Quote:

Do you think there is any requirement of what ratio of reo bars go in either direction?? Could you justify putting all your reinforcement in one direction for a long, thin column - i.e if you had all, or majority, of the bars perpendicular to the longer dimension of the column and the slab was to punch, can you rely on your normal reinforcement to carry the slab loads coming from the shorter dimension sides to the extra bars in the other direction?

ACI 352 specifies that integrity steel should be provided (by their equation) in each principal direction. In other words, you cannot satisfy it by having half in one direction and half in the other.

EDIT: See Section 5.3 of the link below for the integrity steel equation:

http://civilwares.free.fr/ACI/MCP04/3521r_89.pdf

I think there has only been one revision since this version......and it didn't change Eq. 5-1.

RE: Integrity Reinforcement in Transfer Slabs

Quote (WARose)

ACI 352 specifies that integrity steel should be provided (by their equation) in each principal direction.

I've been treating that as a failure, on the code committee's part, to fully consider the range of application in modern concrete design. Given the mechanics underpinning the integrity steel provisions for punching shear, I don't see how one could fail to come to the conclusion that reinforcing running perpendicular to a wallumn would improve intended joint performance.

Quote (WARose)

In other words, you cannot satisfy it by having half in one direction and half in the other.

That's gotta be a typo as the provision says specifically that having half in one direction and half in the other is how they want things. It's my and OP's proposal that putting 100% in one direction would suffice that is contentious.

RE: Integrity Reinforcement in Transfer Slabs

Some additional thoughts:

1) I believe that, as the ratio of flexure to shear in a slab drops, the appropriate Vc value shifts away from 4SQRT(f'c) and towards 2SQRT(f'c). As such, given the usual loading arrangement sand proportions of transfer slabs, I suspect that the we're often overestimating Vc punching shear capacity.

2) When I'm dealing with wallumns, I actually make two modifications that I consider important:

a) I run most or all of the integrity steel perpendicular to the wallumn and;

b) For all but the shortest of wallumns, I concentrate the reinforcing near the ends of the columns where I expect punching shear failure to occur first.

RE: Integrity Reinforcement in Transfer Slabs

Quote:

That's gotta be a typo as the provision says specifically that having half in one direction and half in the other is how they want things. It's my and OP's proposal that putting 100% in one direction would suffice that is contentious.

That's not how I am reading it......although I may be misunderstanding something here. The provision (ACI 352) says:

5.3.1 Connections without beams-At interior connections, continuous bottom reinforcement passing within the column cage in each principal direction should have an area at least equal to [Eq. 5-1]....in which Asm = minimum area of effectively continuous bottom bars or mesh in each principal direction placed over the support,...

Sounds to me like 100% per direction (as per Eq. 5-1).

RE: Integrity Reinforcement in Transfer Slabs

Quote (WARose)

lthough I may be misunderstanding something here.

If you derive the expression yourself, which is a pretty simple exercise, I'm confident that you'll see it my way. The equation that you referenced is really targeting 1/4 of the total required punching shear capacity on each of four column faces.

RE: Integrity Reinforcement in Transfer Slabs

Quote:

If you derive the expression yourself, which is a pretty simple exercise, I'm confident that you'll see it my way. The equation that you referenced is really targeting 1/4 of the total required punching shear capacity on each of four column faces.

You may be right....but that's running with a assumed model. The commentary of 352 isn't specific enough to know exactly what that entails. Given that, I'd still have to run with the "each principal direction" approach they say.

RE: Integrity Reinforcement in Transfer Slabs

Quote (WARose)

You may be right....but that's running with a assumed model. The commentary of 352 isn't specific enough to know exactly what that entails.

It would be running with exactly the model that they provide in their sketch. No assumptions required. 0.5 = SIN(30). The rest is just accounting.





RE: Integrity Reinforcement in Transfer Slabs

They still say provide Asm in each principal direction.....they say it in the latest rev too. So I don't think it's a typo. I might write ACI and see what they say on this.


RE: Integrity Reinforcement in Transfer Slabs

It's not a typo. Asm in both directions gets you 100% of the punching shear resistance split equally over four column faces.

Fyyyyne... I'll do the derivation. After you tell me that you attempted it for at least 5 min on your own. KootK time is, unfortunately, cheap. But it 'aint free.

RE: Integrity Reinforcement in Transfer Slabs

Ok, I think I follow you now. I'll work up the mechanism when I get a chance.

RE: Integrity Reinforcement in Transfer Slabs

(OP)
Thanks for the link WARose, does a much better job of explaining the clause than our standard does.

Would you care to explain what a Type 1 & Type 2 connection is?

RE: Integrity Reinforcement in Transfer Slabs

Basically Type 2 is seismic. According to them:

2.2.2 A connection is classified as either Type 1 or
Type 2 depending on the loading conditions of the connection as follows:

(a) Type 1: A connection between elements that are
designed to satisfy ACI 318 strength and serviceability
requirements and that are not expected to undergo deformations into the inelastic range during the service
life.

(b) Type 2: A connection between elements that are
designed to satisfy ACI 318 strength and serviceability
requirements and that are required to possess sustained
strength under moderate deformations into the inelastic range, including but not limited to connections subjected to load reversals.

RE: Integrity Reinforcement in Transfer Slabs

Ar Engineer

Have any of the people you are taking advice on for this actually read the AS3600 rule?

The rule in AS3600 does not define Asm as defined above. It defines the total area of reinforcement on the 4 faces, so nominally 1/4 of that reinforcement is required on each face. So the code has defined 4 * Asm!

RE: Integrity Reinforcement in Transfer Slabs

Shooting from the hip as I'm killing time on phone and have read none of the reference documents, but is the 0.5 in the equation for Asm = (1/4)/(sin 30 degrees)? So the Asm needs to be in each direction as stated in the code (according to WARose).

Edit: on rereading, I think everyone is saying the the same total amount of reinforcement. It's the split between the two directions that's in question.

RE: Integrity Reinforcement in Transfer Slabs

(OP)
Rapt, I understand the American standards explicitly define the reinforcement required in each direction however I still believe ours doesn't.

The clause states "The summation of the area of bottom reinforcement connecting the slab, drop panel, or slab band to the column of column capital on all faces...".
The way I see it if you have all your reo in one direction and none in the other you will still satisfy this clause as your sum of reo area in both directions is greater than As.min.

One thing however I did notice was this which complicates things even more for wallumns:



I guess that eliminates putting all your bars in one direction, however, as KootK mentioned placing at least 2 bars within the longitudinal column reo of a 250mm wide column when there is a 100mm riser dead centre at the end of the column is easier said than done.

RE: Integrity Reinforcement in Transfer Slabs

(OP)
For reference here is the clause from AS3600

RE: Integrity Reinforcement in Transfer Slabs

Ar Engineer,

That was never the intent. There were interpretation problems in how it was worded in other codes also as to whether the code was applying to the reinforcement in each direction or on each face, and also what to do at edge and corner columns. We tried to avoid the same interpretation problems. Obviously we failed.

And I cannot see where there is any benefit by even trying to put it all in one direction. Can you please explain!

It would be nice and would make code writers jobs a lot easier if engineers would think logically about how a building is acting and detail accordingly!!!!!!!

RE: Integrity Reinforcement in Transfer Slabs

At this point, the slab is doing its level best to collapse so 'thinking logically' is not worthwhile advice as obviously the original thinking wasn't right.

The slab will need to be demolished (or patched with grout if it's a Sydney apartment block) so any load path that hangs the slab up for a few hours is enough. Does the slab retain its awesome capacity for redistribution at this stage? Maybe a one-way load path with integrity bars along the long column edge would be sufficient. A case for full-scale testing if not already done.

RE: Integrity Reinforcement in Transfer Slabs

steveh49,

Sorry, I must be missing something. Where has an existing Sydney apartment block with transfer problems been mentioned in this.

The logic to me would be that the designer consider how much load is coming in on each side of the column. With a transfer slab, there is a good chance that most of the transferred load will come in through one face. So that direction would require more integrity reinforcement.

In this situation, even punching shear calculations probably should recognize the difference, but code formulae cannot or do not indicate this. That is where the "engineering logic" has to come into design. Codes cannot cover every specific situation. Engineers are responsible for a building design, design codes are not. Design Codes give limits and guidance. There are many situations where the designer has to use his engineering understanding to apply the rules logically to his or her situation. Unfortunately many engineers these days expect the design code to do it all for them.

RE: Integrity Reinforcement in Transfer Slabs

I understand this to be a new requirement, not covered by commentary and probably won't be for some time (ie years). So designers are going to have to guess at what the limits are. But this is a fail-safe that is only activated after a failure of the primary load-carrying system. Something has gone wrong by this stage that may also affect the performance of the integrity reinforcement (loading/design/construction/material-supply issue). On top of this, post-failure performance is not a primary learning area for engineers. All of this adds up to a need for more prescription in the code until it is better understood IMO.

Have there been any collapses in Australia that would have been prevented by this integrity reinforcement? If there's no history of it, I can't see people rushing to become experts when there's so much else to do and learn.

Edit: You can just put the reo where it's convenient apparently. A SRIA newsletter says to put it all in one direction if there's a penetration that prevents it in the other direction.
https://www.google.com/url?sa=t&rct=j&q=&a...

RE: Integrity Reinforcement in Transfer Slabs

AS3600 (clause 2.13 in 2009 and AS1170) and BCA has required that Robustness be checked previously. They have just has not provided any guidance on what should be done. That does not therefore mean that engineers should have been ignoring it. Designers were required to research the requirements for themselves and produce designs that provided adequate robustness.

WE have now provided MINIMUM requirements for slabs at columns. Unfortunately the minimum requirements for Beams have not made it into this version of AS3600. That does not mean it does not have to be checked for beams. Designers are required to check for Robustness in all cases to check if more than the Minimum is required.

And for Transfer Members, they have to decide what to do, especially single span transfer members which cannot be made robust by adding continuous bottom reinforcement to cater for catenary action as that is not an option. Some other solution is required. I tried to get an extra Importance Strength factor added for this but it is still under debate so is not in there.

The comment that

"post-failure performance is not a primary learning area for engineers"

is not an acceptable solution or reason for not doing it. If it is necessary (and it is) then you are required to research it and provide adequate solutions. The next comment will be that recompense is not adequate. You are required to produce designs that are in accordance with BCA and need to charge accordingly to provide designs that meet these requirements.

Same with fatigue.

As I said above, too many designers are treating the code as a Cook Book.

There should be a commentary out early next year.

Where there is a penetration on one face as shown in that diagram, it is pretty obvious that there is no reinforcement required on that face. But I would disagree that it is then treated as one way. It would be similar to an edge column condition.

RE: Integrity Reinforcement in Transfer Slabs

Additional thoughts based on subsequent discussion:

1) In my opinion, a fundamental understanding of how integrity reinforcement is meant to work is necessary precisely so that designers can extend and modify such code provisions rather than just following them rote. In this case, I believe that a fundamental understanding of the mechanics should be steering designers away from an integrity reinforcing layout that would provide equal capacity at each of the participating sides of a column. More on this below.

2) As shown below, the name of the game with integrity reinforcing is creating the ability to develop a secondary punching shear frustum further out than the original, inadequate frustum. As such, another set of concrete struts develops just outside of the original failure surface. In order to keep the demands on those struts minimal, it would behoove the designer to distribute integrity reinforcement about the column in proportion to the side lengths rather than simply splitting it equally among the four sides. With this in mind, it would make no sense to me to take a 10" x 48" column and pound half of the integrity steel through the short dimension where, surely, you'll just overload the extended failure frustum. It would be vastly more preferable, I think, to spread that demand about the column as much as possible so that concrete shear stresses remain low. This is one instance where I feel that a 50/50 distribution of integrity steel would be ill advised for a wallumn.

3) For situations that are, effectively, three sided punching shear (edge column / sleeve), not all of the sides will be of equal value when it comes to providing fail safe reliability against ULS punching shear. In my opinion, it is clear that the bars that are able to pass through the column and extend out the other side are of much greater reliability than those that must be dubiously developed within the column. This is a second instance where I feel that a 33%X3 distribution of integrity steel need not be rigorously applied.

4) Codes vary on this a bit but many reference the desire to create the ability to form a rebar net to hold up a slab post-punching shear. That, in addition to the primary mechanism of last ditch punching shear resistance. In this sense, there is some tacit acknowledgement of catenary action. In the Canadian code, one is allowed to "lap" integrity reinforcing with the distributed bottom steel mat to achieve this. As such, it seem more sensible to distribute punching shear reinforcing such that it is lapping with available rebar of a similar, available capacity to itself. For a 10"x48" wallumn, it seems unwise to dump a gaggle of 30 M integrity steel through the short side of a wallumn where it would be lapping with, say, 15M@300 bottom bars. Much better to distribute the integrity bars along the 48" length in hopes that slab reinforcement capacity is of a similar order as the integrity bar capacity. This is a third instance where I feel that a 50/50 distribution of integrity steel would be ill advised for a wallumn.

5) With respect to the appropriateness of standard punching shear provisions to transfer slab situations goes, I agree that designer discretion is required. That said, I believe that:

a) If the slab is stiff enough to convincingly behave as a two way slab, then I think that conventional punching shear provisions would apply and that the eccentricity of load could be handled as it normally is, as a load producing moment on both the punching shear failure frustum and on the supporting column(s).

b) I feel that the design process itself will often steer designers towards the right choice for #5a. If column moments and eccentric punching shear demands prove onerous to deal with, then a designer should either stiffen the slab or start considering one way transfer element design options and the shear transfer mechanisms appropriate to that.


RE: Integrity Reinforcement in Transfer Slabs

KootK, you mentioned in your first post to this topic that integrity reinf action is different to catenary action, and in your most recent post again implied that catenary action isn't the primary goal. Can you provide the references this is based on? I looked up one of the references given in ACI352 and it's all about catenary action. As such, it's not expected to work in some circumstances such as general overload of the slab (ie all bays overloaded like a crowd rather than local overload).

Aside from the alternative to catenary action, it would be interesting to see whether there have been large-scale tests verifying the performance. The article I read referred to testing of slabs with 4mm reinforcing bar (12.9 sq.mm per bar). Some other tests were mentioned but details not given.

RE: Integrity Reinforcement in Transfer Slabs

Quote (steve49)

Can you provide the references this is based on?

I'm afraid not really as I don't have any of that stuff easily accessible at the moment. In general:

1) It seems to me that the centenary implications vary from code to code. And I'm not sure how purposeful that variation is.

2) Virtually all codes say that the integrity bars should be continuous. This makes one feel as though that should mean continuous between columns (catenary) but I suspect that main thing is continuous over columns. Not that having both doesn't sound pretty great.

3) With many, modern, irregular slab layouts, running the integrity steel column to column is a nightmare. It messes with the basic bottom mat horrendously in terms of layout and effective depth and, as a result is often best stuffed above the bottom mat which has it getting pretty close to mid-depth rather than "bottom" in many cases.

4) As I mentioned in my previous post, the Canadian code allows lapping with the basic bottom mat (I think), which implies a pseudo-adherence to catenary action. As I believe you intimated, catenary action based on integrity level reinforcing doesn't usually check out. Nice as a bonus, of course, but not enough there-there for primary action.

5) The diagram in my previous post, which I believe to be the latest and greatest on this topic (could be wrong), definitely seems to stress the use of integrity steel primarily as a punching shear fail safe rather than a primarily catenary setup. They don't even mention catenary action. This is the one reference that I can provide. See below.

Quote (steve)

I looked up one of the references given in ACI352 and it's all about catenary action.

Can you direct me to this? I'd love to check it out.



RE: Integrity Reinforcement in Transfer Slabs

Here's the Canadian version for any interested parties. We were fashionably early to this particular party.


RE: Integrity Reinforcement in Transfer Slabs

(OP)
Good to see that, once again, our standards are almost a direct copy of the Canadian standards with extra safety factors thrown in to satisfy our conservative quota

RE: Integrity Reinforcement in Transfer Slabs

steveh49,

There are 2 parts to this disussion.

- Robustness rules require that you provide a capacity for the structure to survive the removal of a support. One solution is to provide sufficient continuous reinforcement through the bottom of the support for the member to survive the removal of support by catenary action. Obviously this does not work in single span members or cantilevers hence my comment above regarding single span transfer beams.

- As punching shear is a brittle failure condition, technically we should be providing an alternate load path to that collapse to ensure robustness. In this case the support has not failed, the connection to it has. So again catenary reinforcement will help. But also, in the testing for Studrail back in the late 1980's (I think), it was found that continuous bottom reinforcement through a column slab joint, while it did not increase punching shear capacity as such, it made the failure much more ductile. The collapse load was about 2 - 2.5 times higher. They found this when they compared the effect of continuous rails through the column compared to rails that stopped at the face of the column on the 4 faces. From this, it was decided that continuous bottom rei9nforcement thought the column should be mandatory.

ACI has gone away from this in its latest rules in 2014 ACI I think and allows continuous PT through the column to provide this capacity. While it will provide the caterary action, it does not provide ductility for punching shear. That is why we did not follow the latest ACI318 logic.

And AS3600 is not an exact copy of CSA on this either, but we preferred the total reinforcement logic to the reinforcement per face logic. CSA allows top prestress to be included, even though it is top reinforcement and does not provide this ductility as it has already been used up in the punching strength calculations, though it does contribute to caterary action!

RE: Integrity Reinforcement in Transfer Slabs

KootK, the article I mentioned is Mitchell & Cook "Preventing progressive collapse of slab structures" Journal of Structural Engineering, 1984. Snippet below. Blink twice if you're happy to receive by email.

Rapt, is the loading for the support-removal case specified? I'd have gone for dead + short-term-live, but perhaps dead plus long-term-live is justifiable. Short-term-live isn't given in any ULS load combinations in AS1170.0.



RE: Integrity Reinforcement in Transfer Slabs

My left eye is twitching up a storm. Mitchel's ours. Very high probability that's the CSA source doc .

RE: Integrity Reinforcement in Transfer Slabs

steveh49,

Logically it would be the Permanent Load case. So SW + SDL + .4LL for normal buildings.

ABCB has put out a handbook on Structural Robustness. It is a free download.

Handbook-Structural-Robustness.pdf

RE: Integrity Reinforcement in Transfer Slabs

(OP)
Not willing to put this one to bed just yet

Rapt,

The purpose of integrity reo is as a fail-safe in the event of failure so you would expect either miscalculation of design loads, or design loads to be exceeded to cause a failure (assuming perfect execution on the construction end). Wouldn't it then make sense to take your maximum ULS loads when considering integrity reo? Anything less than 1.2G+1.5Q seems like it won't work considering the structure would've been seeing loads around 1.2DL+1.5LL to cause failure in the first place.

I'd assume once the building was evacuated it would be safe to assume DL+0.4LL but by the time that happens, the integrity reo has already failed and you'll be left looking at a bunch of rubble wondering where all that live load went.

RE: Integrity Reinforcement in Transfer Slabs

Ar Engineer,

Integrity Reo N* is defined in AS3600 as the ultimate load. So yes, 1.2D + 1.5Q

We were talking about Robustness (Support Removal) calculations for that case.

RE: Integrity Reinforcement in Transfer Slabs

(OP)
Where does the difference between removing a support and a support failing come from that allows for a much lower load case to be considered?

I'm not familiar with structural robustness so I may be missing a basic concept here

RE: Integrity Reinforcement in Transfer Slabs

(OP)
I'm thinking in the case of your support punching you are still providing your loads to that support via your integrity reo and
in the case of removing your support you are providing a new load path to surrounding supports?

Is this correct?

RE: Integrity Reinforcement in Transfer Slabs

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).

RE: Integrity Reinforcement in Transfer Slabs

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

RE: Integrity Reinforcement in Transfer Slabs

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.

RE: Integrity Reinforcement in Transfer Slabs

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!

RE: Integrity Reinforcement in Transfer Slabs

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

RE: Integrity Reinforcement in Transfer Slabs

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

RE: Integrity Reinforcement in Transfer Slabs

(OP)
Think I can help with a few of your points

Quote (aaronPTeng)

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)

Quote (aaronPTeng)

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

RE: Integrity Reinforcement in Transfer Slabs

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

RE: Integrity Reinforcement in Transfer Slabs

Quote (aaronPTeng)

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.

RE: Integrity Reinforcement in Transfer Slabs

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.

RE: Integrity Reinforcement in Transfer Slabs

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.

RE: Integrity Reinforcement in Transfer Slabs

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.

RE: Integrity Reinforcement in Transfer Slabs

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.

RE: Integrity Reinforcement in Transfer Slabs

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

RE: Integrity Reinforcement in Transfer Slabs

4

Quote (rapt)

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.

RE: Integrity Reinforcement in Transfer Slabs

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

RE: Integrity Reinforcement in Transfer Slabs

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.

RE: Integrity Reinforcement in Transfer Slabs

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.

RE: Integrity Reinforcement in Transfer Slabs

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?









RE: Integrity Reinforcement in Transfer Slabs

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

RE: Integrity Reinforcement in Transfer Slabs

Yes, If adding robustness is the underlying rationale for adding bottom integrity steel then it makes sense to consider if PT is an adequate substitute.

RE: Integrity Reinforcement in Transfer Slabs

StructEng23

Answering your questions in order

- Commentary some time next year, hopefully. Along with at least one more amendment.

- Would have been wonderful to have 9 years. But someone else would have had to do the 2009 commentary to free us up to do it. As I said, I am not able to comment on the cause of errors, 2009, Commentary or 2018, but they cost us a lot of time and frustration as well, finding and fixing them.

- I explained the logic behind the clause above in about June/July and the difference between it and CSA/ACI. It is not to provide overall Robustness. Simply a more ductile punching shear failure by providing Bottom Reinforcement at the column. Overall Structural Robustness still needs to be checked and provided in accordance with 2.1.3 and the Building Code of Australia. AS3600 does not give specific provisions on how to provide Robustness, just the motherhood statement in 2.1.3. Designers have to resolve the Robustness issue themselves and PT tendons can be used for that. Unfortunately, the building code also needs a rethink on its Deemed To Comply Robustness provisions.

- The clause is very specific on what is required. It is not CSA23 or ACI218, so you cannot read those clauses to interpret it.

- No, it was not added for disaster situations as I have implied several times in earlier comments. It is simply to provide a more ductile punching shear failure mode rather than the brittle failure you get with no bottom reinforcement through the column core. Unfortunately the default detailing rules in AS3600 before 2018 basically allowed a designer to provide no continuous reinforcement through the bottom of a slab/beam at an internal column and for PT slabs at an edge column.

The clause specifically says N* is the "column reaction from the floor at the Ultimate Limit State"! 1.2G + 1.5Q is the Ultimate Limit State for gravity load and always has to be checked for Flexure, one way shear and Punching Shear. Why is interpretation required? If it was required only for Earthquake or Fire, it would only be in those code sections or would have been limited to those situations.

Agreed Earthquake and Fire are other ultimate limit states but their loading tends to be less, though detailing is as or more important. Earthquake design would require bottom reinforcement there anyway, in any RC or PT frame structure. How many PT designers are adding bottom reinforcement at columns for earthquake (and top reinforcement at mid span)? We specifically made Earthquake more prominent in 2018 moving it from an Appendix to a main section to try to get designers to actually apply it.

RE: Integrity Reinforcement in Transfer Slabs

Re Ingenuity's Northbridge picture, the prestress in that case was unbonded. Do not start thinking that bonded PT will act in the same way.

There are special provisions for prestress in special moment frames in ACI318. The main one for us is that bonded tendons must be unbonded over plastic hinge regions and the strain limited to .01 under the design displacement as the fully bonded tendons would fracture due to their relatively low ductility.

Fully Bonded PT will act very differently to unbonded PT in these situations.

RE: Integrity Reinforcement in Transfer Slabs

RAPT I know you disagree with my last statements. However I have one for you.

If we are after a ductile failure shouldn't N* be higher than 1.2 DL + 1.5LL? As our punching shear around our column should already be designed for this load combination. In testing I have seen (And an old building that failed many years ago that was tested) it proved to be much stronger than this. So if we are getting a failure at a Higher load then N*, then how is reinforcement in the bottom at N* going to provide ductility?. If it has a punching shear capacity of 2xN* or 3xN* (columns may be for a tower and nice and big) then reo in the bottom at N* aint going to help much?

On the logic stated above, shouldn't the design load for this reo match the load at which punching failure will occur based on the column size and slab thickness. Not the actual 1.2 DL + 1.5 LL. And as we all know this would then be near impossible to feed through the columns.

Your above comments implies you and the committee have spent so much time on this topic to think about it, which is good. Would be great for you to point out to us the research/ testing that verifies this and formed part of the conclusion.


"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

RE: Integrity Reinforcement in Transfer Slabs

AaronPTeng,

Punching shear is generally a non-ductile failure. It reaches capacity and collapses without warning. Providing this amount of bottom reinforcement is supposed to provide sufficient ductility to make the failure more ductile, providing sufficient warning against collapse. It does not increase the punching capacity significantly, simply reduces the likelihood of sudden collapse after punchiung failure.

AS3600 has to assume that a designer can calculate N* correctly for his column. That is the load that it is designed for so is the load it needs to be ductile for. If the designer is incapable of calculating N* correctly, then that person should read my comment about finding another profession. A design code cannot completely protect us against incompetence or all of our buildings would be mass concrete. Read AS3600 clause 1.1.1 Note 2.

It is not overload we are attempting to save, though this will help, it is sudden collapse. ACI352 refers to papers suggesting we should design it for 2 * G. We decided 1.2G + 1.5Q would be adequate and may be lower than 2 * G in domestic/parking structures.

In the Darwin problem buildings, my understanding is that some of them have punching capacity (unfactored) Nu of half or less of the applied N*. So either the designer did not check punching shear or could not calculate N*. You cannot blame AS3600 for either possibility. None actually collapsed though at least one had obviously failed. They were RC slabs so presumably had some bottom reinforcement through the column cores, even if they were lapped bars rather than continuous. Without being involved directly we cannot know how much.

I was fist told about the German Studrail tests that showed this in the early 1990's but do not have copies of the results and no longer have contact with the researchers who showed the effects (If Dr Nadia C???? is lurking she can comment). My understanding was that they showed punching capacity little changed either way , but where the rails were continuous through the column core, collapse load was about 2.5 times the collapse load where the rails for the studs stopped at or just inside the column face.

ACI352.1R-11 section 6.3 covers this (and has a diagram amazingly similar to Kootk's first on this topic!!), and specifically says that only bottom reinforcement can provide this and precludes the use of draped tendons to replace the bottom reinforcement. ACI318 and CSA 23 obviously did not listen to them. We did not completely listen either as we did not require this reinforcement to be continuous in the bottom or you would be adding 500t to your building, not 100t. As I said earlier, I would ignore bonded tendons at the face of the column in the plastic hinge zone anyway due to their relatively low ductility under the strains we are talking about.

RE: Integrity Reinforcement in Transfer Slabs

Rapt, are you saying that the reinforcement area based on N* provides the desired ductility in cases where punching occurs at significantly greater column load than N*?

I echo the request for the research that this is based on to be named here in advance of the commentary. We're supposed to self-educate but it sounds as though AS is different from other codes. Either AS is from other research than we're likely to find without a pointer, or the AS committee is using the same experiments and drawing different conclusions. Either way, self-education doesn't seem feasible from the available information.

From the outside, I don't understand why the commentary is significant additional work. The science is presumably settled when the code is published and, I would expect, written up by the people responsible for each change.

RE: Integrity Reinforcement in Transfer Slabs

steveh49,

Not sure how much more I can say. It provides a much more ductile collapse mechanism. It does not increase Vu significantly. It is not going to save you if you have grossly under-designed punching shear which would be the case if V* >> phi Vu.. I think I said or implied all of that in my last post!

You obviously did not read my last paragraph!

After this post, I will never again be commenting on anything to do with AS3600 development. I have gone outside the rules in trying to help. I heave learned my lesson. Never again!

RE: Integrity Reinforcement in Transfer Slabs

We're asking about the case of V*<<phi.Vu.

RE: Integrity Reinforcement in Transfer Slabs

Personally I don't believe ductility is the best terminology to explain the integrity reo requirements.
i.e. 1.2G+1.5Q=950kN
Predicted Punching Design Capacity = 1000kN
Predicted Post Punching Design Capacity = 1000kN
Actual Punching Failure = 1200kN (Over-strength)
Actual Post punching Capacity = 1100kN
Result = Pancaking with no ductility
If ductility is the intent, then "Capacity Based Design" principles would need to be applied to ensure Post punching had a Design Capacity greater than the Punching Over-strength.
As stated in my previous post, I feel that integrity reinforcement is a cheap safety net to account for the inherent uncertainties in what we do. As for its practicality in major transfer slabs, I can't comment as I have no 1st hand experience in high-rise construction.

Regards
Toby

RE: Integrity Reinforcement in Transfer Slabs

Quote (rapt)

After this post, I will never again be commenting on anything to do with AS3600 development. I have gone outside the rules in trying to help. I heave learned my lesson. Never again!

Out of curiosity, what are the proper channels to contact the AS3600 committee?

As engineers we are often required to defend our designs and decisions - I don't think it's unreasonable for engineers to question and interrogate the logic behind code clauses and/or the research it's based upon. But the question remains, without a coincident code commentary, who/what will shed the light?

Engineering judgement comes into play here, but it's obviously to the benefit of everyone if these decisions are well informed.

RE: Integrity Reinforcement in Transfer Slabs

Quote (rapt)

After this post, I will never again be commenting on anything to do with AS3600 development. I have gone outside the rules in trying to help. I heave learned my lesson. Never again!

I just want to go on record as saying that I do not relish this outcome. In my opinion, the value of your contributions to content in this area vastly outweigh the emotional cost of any abrasiveness endured in the production of that content. That said, if you feel that you've been breaking the rules in this space, I'll accept your decision on that basis alone.

RE: Integrity Reinforcement in Transfer Slabs

Toby,

Ductile Punching shear failure is definitely the correct term for the slab bottom reinforcement requirements, as the opposite to the normal brittle punching shear failure mechanism, no matter what your personal feelings. We are not looking at overall structure robustness for a lost support, just trying to avoid sudden brittle collapse at punching shear failure, essentially stopping us from losing a support, or at least delaying it.

So your column in question has phi Vu = 1000. But assuming material over capacities, its actual capacity = 1200KN.

Applied Load = 950KN.

Why does it fail in punching if the applied shear force is only 80% of the capacity, phi Vu = 1.25 V*?

If this does fail,
- Why does it fail? Is there another cause increasing the shear load or reducing the capacity that I do not know about?
- What is the slab load transferring to the column at failure, 950KN, or something much higher, > 1200KN presumably to cause failure? If much higher, why is the loading 25% or more greater than the maximum design load?

If the actual slab loading transferring to the column is still 950KN or less. Then why is it going to "pancake" if the collapse capacity = 1100KN?

The bottom reinforcement does not have to substitute for the column head capacity, it has to transfer the slab load to the column. The maximum value of this should be the ULS factored gravity load. Under earthquake it should be less than that. Under fire loading, it should be less than 70% of the ULS gravity load, and the reinforcement will be reduced to about 70% of the normal ultimate strength due to temperature increase as long as the cover is adequate, so still ok.

The only thing that needs clarification in the rule is distribution to faces. ACI's rule does this the way I suggested it should be done, based on the force coming onto each face. I will get that clarified, either in the code or the commentary. Otherwise there is nothing wrong with the rule as it is!

I assume all of this also applies to steveh49 and aaronPTeng's questions also.

RE: Integrity Reinforcement in Transfer Slabs

RAPT, please don’t go.

RE: Integrity Reinforcement in Transfer Slabs

All i was trying to say was that in this case, the ductile mechanism would need to be reliably stronger than the brittle mechanism (owing to the fact that it is a secondary mechanism that only comes into play post brittle punching)

RE: Integrity Reinforcement in Transfer Slabs

I think the question is simply whether this reinforcement is intended as a foolproof way of achieving a collapse load greater than N* in the event that the initial punching capacity was too low for some reason (incorrect design, construction error etc).

Then the question moves to whether the reinforcement (sized based on N* rather than Vu) is effective if slabs are correctly designed and built with punching capacity 50% or more in excess of N*.

RE: Integrity Reinforcement in Transfer Slabs

Maybe this is obvious, and you guys are going on just for the sake of going on, but it feels like most of you think the intent of the integrity reinforcement is to improve punching shear capacity. But rapt is saying, and I agree with this, it has negligible effect on the punching shear capacity, but rather the failure mechanism. The integrity reinforcement is intended to improve the ductility of the punching shear failure. Yes the slab still fails in punching shear, yes it still requires major repairs and is likely started via a brittle initial failure. But the integrity reinforcement is intended to provide SOME ductility to avoid a total brittle collapse.

RE: Integrity Reinforcement in Transfer Slabs

All

I think we all agree there is something that needs to be done for ductility. Also agree with others engineering discussion is based on numbers and not emotion. If there is no trail to the roots of a solution then its always going to be hard to ask people to blindly follow. Also this is not just a debate on the current AS 3600. Its a code that is constantly under development. There are always additional considerations which are fair and reasonable to discuss. Otherwise we would be sitting around a camp fire hitting each other with sticks in a cave.

As per my previous question and Steveh49, Toby 43 comments

You wont get ductility if N* is much less than Vuc. Other parts of the code relate ductility to section parameters and not load.

I look forward to seeing some research and testing come out around this topic and hopefully a more practical solution is developed otherwise industry wont incorporate it.

Enjoy all!

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

RE: Integrity Reinforcement in Transfer Slabs

ArronPTeng , steveh49 and Toby43

I assume the above post is from all 3 of you, or at least you all agree on it.

I cannot understand your conclusions, which I assumed would be based on logic considering your desire for an engineering discussion. Contrary to you comment regarding a "trail to the roots of a solution", both Kootk and I have given you the name of a reference document covering this. ACI318 has had a clause at least since 2011 covering it and possibly earlier, CSA 23 has had it since 1994 that I know of and possibly earlier. The Canadians possibly "got into it first" as someone suggested several months ago, because they were heavily involved in the development of Studrail and would have seen the test German results I saw for it in the early 1990's and possibly had their own corroborating tests.

It is not like compression ductility in over reinforced sections requiring capacity to control it. As suggested above in several posts by several people it simply provides a mechanism to provide support for the applied loads on the floor to transfer to the column in the event of a punching shear failure. It allows the connection to behave in a more ductile manner in the case of a punching failure instead of the normal very brittle failure mode. The methodology requires reinforcement to be supplied to support that load, V* as defined in AS3600. if you think it requires sufficient reinforcement to support the shear head capacity, Vuc, that is ok. You can provide the extra reinforcement. AS3600 requires you to provide a minimum based on V*. You can always add extra based on Vuc if you want to.

It does not supply a Robustness solution in the case of the loss of a support, the designer still needs to consider this as an extra design requirement.

The research and testing has been done since the 1980's and possibly earlier. That is why it has been in some codes for so long. AS3600 has now caught up and it is now a mandatory requirement in AS3600-2018. It has been discussed by sub-committees and the main committee developing AS3600 and went out in draft form for Public Comment. It passed through all of that and is now included in the code. It is not optional. If you do not provide it then you are designing outside AS3600 and outside the Building Code of Australia. And presumably outside the requirements of your PI insurance and any contracts you have signed with clients.

RE: Integrity Reinforcement in Transfer Slabs

Rapt,
I wholly agree in the need for integrity reinforcement, I'm just not convinced on the "ductile" part of it. Most of the tests I've read about about (Mitchell and Cook, Melo and Regan) firstly break the slab by getting it to punch, and then proceed to re-apply load (through some form of jack) to assess the post-punching capacity. Their is a load drop in this scenario (see image), whereas, I suspect if they used kentledge to load the slabs until punching failure, then the post punching capacity would quickly be exhausted aswell. A real structure would not experience a load drop at a column unless their was an alternate load path, thus I feel ductility cannot be assured unless capacity design principles are adopted.
So as I have stated earlier in this discussion, I most definitely see the need for it, but more so to minimise the risk of disproportionate collapse, in the event that the inherent uncertainties related to punching shear, cause failure.


Regards
Toby

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