Opal Tower - Sydney Australia 28

[CivilEngAus]

https://www.news.com.au/national/ns...s/news-story/dad28af4aad5ff11cf968102635824f9

https://www.smh.com.au/national/nsw...ns-building-may-collapse-20181224-p50o4b.html

This could be an interesting and developing story in Sydney Australia. A 34 storey near new residential apartment tower in Sydney has been evacuated this afternoon over fears it is in structural distress with cracking noises heard during the day and one or more cracks developing; emergency services are treating it as a major incident.

Given we already have some of the toughest building codes in the world (although little to no registration requirements for engineers) it will be interesting to see how this plays out and what the crack(s) looks like to cause such a major emergency response.

 

[Skogsgurra]

To an EE, this is a completely new world. Interesting, even if I don't get it all. I'm impressed by the way different views are presented and the matter-of-fact tone.

Or is it my positive attitude combined with lack of knowledge that makes me have that opinion?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[CivilEngAus]

https://www.facebook.com/7newssydney/videos/2294825657202307/

Reportedly cracking on level 4.

 

[QSIIN]

CivilEngAus, good link.
Those are some might heavy duty props.

Very curious to hear the findings and the outcome. I certainly hope the investigations and reports are made public.

 

[Sheer Force Engineer]

Hello all,

This is a great discussion. I have had a lot of experience in forensic engineering and reviewing failures on-site across Australia.

Here is my take on what has happened:

Local Column/wall failure has occurred due to insufficient grouting within the grout-bed interface between pre-cast element (wall/column) and in-situ element (beam/slab)

Here is some further explanation:

- Reports have confirmed pre-cast has been used

- Witnesses have indicated that there has been loud bangs/explosion and cracking sounds, even heard from neighbouring building. In my experience with this form of failure, it is brittle, sudden and very loud (if you have ever witnessed a high strength concrete cylinder core test to failure you will get some kind of idea on a smaller scale).

- Reports have now come out that the pre-cast wall/column itself is not to blame

- The images indicate that the cover concrete has "blown out" from the side of the column. This is the first thing to go as it is not confined by any reinforcement or ligatures. This form of failure doesn't mean that the column itself is no good, but rather the column has been over-stressed locally due to reduced contact area where grouting has not made full contact with entire column cross-sectional area.

- Reports saying that the building is "structurally sound" gives a glowing appraisal or the building itself and may be somewhat misleading, however would be technically correct as after local failure and crumbling of the base of the column occurs the contact area actually starts to increase and the grout-bed connection approaches full contact bearing area.

- However after the above has occurred, you end up with anywhere from 5mm to 20mm of "settlement" meaning that every floor above which that column supports will drop the same amount. This would explain the crumbling of the adjacent hebel wall at the level in question. This would also confirm reports that doors have binded in their door frames and some residents required police services to break them out of their apartments.

- Grouting of columns is generally not inspected by the building surveyor or the design engineer, you are leaving it up to the contractor (more specifically the subbie) to get this element right.

- This is frequently the most common "structural failure" I see in the industry at the moment as you rarely get much else go wrong, to be clear our construction industry is of high quality, is robust and we are more advanced than most first world countries. However this particular failure mechanism has been around for over a decade and actually leaves me scratching my head as to why this hasn't been rectified or the approach changed to mitigate risk.

- This has not been in the media previously as the failures I have seen occurred during construction and was able to be rectified while the public are none the wiser. Unfortunately on this occasion, while the grout connection may have been poor, perhaps not poor enough to cause failure under structural self-weight only but required service load to tip it over the edge.

- This is not a local state construction issue, I have reviewed projects where this has occurred in Melbourne, Adelaide and NSW.

 

[human909]

Thanks for the insightful comments SheerForceEng. By the sounds of things you have significant experience in local construction of this nature. Certainly much more than myself.

Just a couple of questions:

to be clear our construction industry is of high quality, is robust and we are more advanced than most first world countries

Could you elaborate further on this? Particularly in residential construction, my perception isn't like this. (Though my perception is based layman percerception rather than on professional assessment.) I'd expect northern Europe to put us to shame in robust residential construction. In infrastructure, new industrial construction and particularly OH&S I do see Australia being robust if not somes over the top for the latter, residential not so much. (All that said, Opal Tower does seem to be one of the higher quality residential builds. I've seen far worse. But despite being 'higher quality', one critical mistake can still ruin it all.)

Apart from the grout what are your thoughts on the column to wall load transfer? Does the wall not seem undersized in your experience? Even with perfect grouting it is hard to avoid large stress rises with the centre of the wall barely carrying any load and the transition.

I'm impressed by the way different views are presented and the matter-of-fact tone.
Or is it my positive attitude combined with lack of knowledge that makes me have that opinion?

I would put it down to being a forum of professionals but it seems you are a very long time member here so I presume you are speaking in terms of eng-tips forum. My only explanation is that the uncertainty and lack of information creates an environment for open discussion. In some contexts I can be opinionated, brash, argumentative and forthright. But it is hard for me to do that here given the number of unknowns AND recognising that I'm far from the most experienced person in the room. If most people consider themselves in a similar position then you can get constructive discussion. (In contrast if everybody has a fixed opinon and everybody thinks they are the smartest in the room then welcome to unconstructive arguments!

Very curious to hear the findings and the outcome. I certainly hope the investigations and reports are made public.

The planning minister has said that reports will be made public, but politicians change their minds all the time. Not to mention he might not have a job in 4 months which could be later than when the final report is finished.

 

[Sheer Force Engineer]

Could you elaborate further on this? Particularly in residential construction, my perception isn't like this. (Though my perception is based layman percerception rather than on professional assessment.) I'd expect northern Europe to put us to shame in robust residential construction. In infrastructure, new industrial construction and particularly OH&S I do see Australia being robust if not somes over the top for the latter, residential not so much.

In terms of concrete technology, we are very advanced. Ironically enough even though they are swimming in sand over there, we taught Dubai how to use their sand in their concrete mixes to achieve high 80MPa plus concrete.

We have been leaps and bounds ahead of majority of the world in post-tensioning of slabs.

If your talking finishes/fixtures quality and also energy saving initiatives (insulation, glazing technology etc.), we are far behind, but unfortunately this is driven more by the dollar and developers than the skill of our local contractors (we don't have much legislation to force the issue). You can also say that we aren't as advanced as other nations when it comes to structural steel construction (it is done here but we don't have anywhere near the volume per capita compared to say the US).

Apart from the grout what are your thoughts on the column to wall load transfer? Does the wall not seem undersized in your experience? Even with perfect grouting it is hard to avoid large stress rises with the centre of the wall barely carrying any load and the transition.

I agree, it does look "lean" and may be a possible reason for some local failures no doubt. I believe however for the apparent "Funnelling effect" of the load to be the root cause (i.e. half the walls supported load being asked to go through a small cross-section due to corner support from column below) we would see more significant failure of the wall itself (it wouldn't have stopped failing and potentially would have continued to collapse...I don't see anywhere else for the load to go).

It is definitely a critical junction and something that would have little redundancy and margin for error if its working close to its the theoretical limit. May be a combination of a critical junction (perhaps one of a handful of very highly stressed connections) that has exposed less than satisfactory construction practices due to its critical nature.

What you are identifying is indeed a structural engineering "headache" but not out of the norm for most large scale buildings. The engineering firm WSP (who mind you were also the original structural engineers on the design not just the current engineers looking at the problem) are a huge reputable organisation and I don't see them getting a junction like this wrong (I have reviewed a lot of their projects in Australia and yet to find anything "dodgy") however stranger things have happened and may not be something to rule out just yet!!

 

[human909]

Thanks Sheerforce for answering my questions so well! What you have elaborated on also aligns with my layman perceptions.

Also in support of your assessment that it is likely grouting issues, there have been clear statements to the media that no flaws have been found in the precast panel construction (or something to that effect). Those statements puzzled me, but if the main damage is to grouting then that explains it.

The engineering firm WSP (who mind you were also the original structural engineers on the design not just the current engineers looking at the problem) are a huge reputable organisation and I don't see them getting a junction like this wrong (I have reviewed a lot of their projects in Australia and yet to find anything "dodgy") however stranger things have happened and may not be something to rule out just yet!!

I agree that it is pretty elementary to get such junctions wrong. Thanks for clarifying that in your opinion such oversights are unlikely to have been made.

 

[Ingenuity]

The engineering firm WSP (who mind you were also the original structural engineers on the design not just the current engineers looking at the problem)

I thought Bonacci Group were the structural enguneer on the original building.

Of course Bonacci may have been purchased/acquired/merged by WSP in recent times...

 

[QSIIN]

I had thought Bonacci were the original engineers too. Either way, just because it's a large "tier 1" firm doesn't mean much; I've seen some questionable details from both big and small groups, not pointing any fingers, though haven't had much to do with either of these two.

Both large and small consultants are subject to the tight deadlines, fees and pressure placed on them from the contractor, and would have similar potential QA issues, generally speaking of course. And of course, both firms only look to 3600-2009 for wall and column design.

Touching again on the precast, if the panel design itself is sound, the fact that the connection/joint has failed is still a major issue, in my opinion. Any structure could be pretty much built like Jenga blocks for gravity, with dowelled connections everywhere, but subject them to wind loads* and the whole system fails. The fact that this is happening under gravity alone is concerning, regardless of how well the panel itself is performing.

*(and EQ, assuming the engineers haven't ignored them, because you know, "we don't get earthquakes in Australia")

 

[Sheer Force Engineer]

I thought Bonacci Group were the structural enguneer on the original building.

I had thought Bonacci were the original engineers too. Either way, just because it's a large "tier 1" firm doesn't mean much; I've seen some questionable details from both big and small groups, not pointing any fingers, though haven't had much to do with either of these two.

No, unfortunately fake news strikes again which looks like the Sydney Morning Herald has since retracted but damage may already be done it seems to Bonacci Group reputation...

https://www.smh.com.au/national/nsw...stigate-opal-tower-crack-20181225-p50o6i.html

If you scroll down to the fine print you will read...

"An earlier version of this article incorrectly listed Bonacci Group as the structural engineers."

 

[Tomfh]

Channel 7 news just showed large quantities of propping being installed behind plywood privacy hoarding, with apparently more propping being brought in from overseas. They had quite a few pallets of props there already.

 

[Tomfh]

SheerForceEng, you seem very keen to lay the blame on the workers, eg saying tier 1 firms like wSP almost never make mistakes, and saying its most likely just a poor grouting issue and that the consultants would therefore have no responsibily there.

Assuming it is merely just poor grouting (and we don’t know that) it’s not ok for consultants to leave it up to workers to ensure critical load bearing elements like these are grouted. If you need 80MPa grout across the whole bearing area for your walls not to explode then you need to check it’s gone in and that it’s the right stuff.

 

[human909]

My interpretation of SheerForceEng's comments certainly haven't been that. I don't think he has been unduly blaming workers and absolving the consultants.

He has a view and backed it up with clearly explained reasons. To me none of it is about blaming one party or another. It is about finding the immediate and associated source(s) of the failure.

Regarding the lack of oversight that occurs at grouting connections. I'd expect SheerForceEng would agree with you. He has already mentioned that he is puzzled why this failure area hasn't been addressed.

 

[QSIIN]

Here's a screenshot of the propping from the channel 7 video.

 

[Sheer Force Engineer]

SheerForceEng, you seem very keen to lay the blame on the workers, eg saying tier 1 firms like wSP almost never make mistakes, and saying its most likely just a poor grouting issue and that the consultants would therefore have no responsibily there.

Hi tomfh, appreciate your input. I don't wish to lay blame unwarranted to any party (bearing in mind that this discussion is filled with hypotheses that precede my comments which you can either look at as being a blame game or a discussion amongst interested parties and enthusiasts based on limited information that is out in public currently). I simply came to my conclusion based on the information that I know is currently being circulated in the public domain mixed with my past experience and dealing with these very failures on projects across Australia....

- Photos show that local failure at the wall base has occurred, but not total failure of the wall itself, I believe a design error (i.e. under-sizing of the wall strength or thickness) would mean the wall would have continued failing rather than crumbling until a point of larger contact area was reached.

- An error in design would be picked up by now during these investigations noting that there is also a third party engaged by the developer checking the design. If a design error existed in this local area a couple of days should be enough to spot check this. The potential poor grouting issue is not readily detectable and in fact if my theory is correct they may never find out as the failure has now occurred and full bearing is now achieved, we may never know weather there was adequate grouting or not unless another lightly loaded column (that hasn't failed) is found and investigated to have poor grouting undertaken on it. This is the reason why the engineers, builder and developer are scratching their heads and sadly not able to give the residents the answers they need in my opinion.

- Playing the averages, in all the constructed failures I have been involved with investigating approximately 15% are design related with the remainder being a mixture of workmanship/materials or a structure being loaded beyond what it had been designed for with its intended use.

Assuming it is merely just poor grouting (and we don’t know that) it’s not ok for consultants to leave it up to workers to ensure critical load bearing elements like these are grouted. If you need 80MPa grout across the whole bearing area for your walls not to explode then you need to check it’s gone in and that it’s the right stuff.

Your comment has probably hit the nail on the head regarding why I don't understand why there is no code coverage or regulatory guidance considering this issue has been around for over ten years now.

Regarding presence of the design engineer on-site, you could argue that every piece of the structure is important not just select critical locations. The standard across the board in Australia is that generally the design engineer does not have a full time site presence to hold the builders hand through every step of the construction process. Notwithstanding this status quo, this junction is particularly hard to inspect and I will try to explain why in my elaboration below...

There are two methods to grout column connections which are adopted in Australia, Option 1 is quicker and easier to build and is the obvious builders proffered, however impossible to visually inspect as an engineer. Option 2 is far more labour and time intensive however results in a better outcome for the structural integrity of the connection itself and the quality however is used far less and not as common, as the builder generally decides what construction methodology they want to use even sometimes against the engineers recomendation.

Option 1

1. Plastic packers/shims are placed on surface of slab/beam
2. Pre-cast column is placed on packers
3. "Ram Pack" grout is then used around the column perimeter to grout up the contact surface i.e. a dry or semi-dry grout mix.

The above procedure when completed makes it impossible to visually inspect weather full contact area has been achieved. IT is also tricky to get the grout all the way to the centre of the column/wall if it is of a large cross-section.

A visual inspection can show that the full perimeter is covered (which it generally is) however voids may exist internally which are not visible. This is a primary reason why, looking at the photo of the failure, I theorise that the cover concrete has blown away because that is where the higher stress is most likely generated (perimeter of the wall/column) until the wall then locally fails and therefore closes up the potential voids within internally.

Option 2

1. Plastic packers/shims are placed on surface of slab/beam
2. Pre-cast column is placed on packers
3. Exterior perimeter of column is sealed up temporarily with one breather hole for grout access
4. A "flowable" grout mix is then pressure injected into the gap between column and slab,
5. Usually a breather hole or two is cast into the column itself via grouting tubes which would allow the grout to extrude out of the column therefore indicating that the joint is fully "filled". Generally flow of grout from the breather holes is allowed to continue for several seconds to ensure no air pockets remain.

 

[Tomfh]

The hypothetical design error is a stiffness compatibility problem between wall and transfer stiffness, such that walls end up locally yielding at the hard points, but not fully failing. As you put it “crumbling until a point of larger contact area was reached.”

Human909s December 30 post shows that concept.


Maybe it was that. Maybe it was that and was exacerbated by poor grouting. Or maybe it was just poor grouting alone. Or maybe something else entirely. Hopefully we find out.

 

[Tomfh]

WSPs statement today:

https://www.wsp.com/en-AU/news/2018/opal-tower

Guy Templeton, President and Chief Executive Officer of WSP, Australia & New Zealand says, “We have found no other areas of damage to the extent of that in one part of level 10. There are fewer than 20 parts of the building with a similar configuration to the connection between pre-fabricated and in-situ poured concrete that was damaged on Level 10. Two of these areas, both on level four, show evidence of some but lesser damage.”



As a precautionary measure, propping is being installed to support level four and this will be completed by Icon Co by the end of today.



The building is structurally sound overall.

 

[Tomfh]

Bonacci reference may have originated here?:

https://www.skyscrapercenter.com/building/opal-tower/18049

 

[degenn]

If a wall is designed as a column then its capacity is limited to about 0.45f’c. If designed a a simple wall it would be less. The load at the time of failure would be only about 0.75 of the design load. The failure therefore occurred at about one third of the expected capacity.
It seems unlikely that construction only can account for this.

 

[human909]

The hypothetical design error is a stiffness compatibility problem between wall and transfer stiffness, such that walls end up locally yielding at the hard points, but not fully failing. As you put it “crumbling until a point of larger contact area was reached.”
Human909s December 30 post shows that concept.

Exactly. Local yielding resulting in stress redistribution. Thanks for highlighting this, I was about to do it myself! Though I'm starting to be persuaded against my own arguments.


As much as I would like to continue pushing the line of thought I have been, there are a couple other things that support the grout description of the failure. Most notably the quote from you own post.

"There are fewer than 20 parts of the building with a similar configuration to the connection between pre-fabricated and in-situ poured concrete that was damaged on Level 10. Two of these areas, both on level four, show evidence of some but lesser damage."

They focus really does seem to be on the connection BETWEEN the elements. So that likely implies grout. (alternatively shimming, dowels etc, but grout makes good sense)

SheerForceEng, you have convinced me for the moment! In time we will hopefully get more details.