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.

 

[mangotree]

Thank goodness for whistle blowers.

Here’s a link to the ACA clip:-

https://www.9news.com.au/2019/02/04...eo-construction-cracks-cement?ocid=Social-ACA

 

[human909]

In this discussion I've been occasionally pulled up when my comments have been about political, legal or media perceptions rather than engineering facts. But the reason why I keep bringing up these issues it they are so involved in the way this is handled. With that in mind I'll go back to them.

I thought their mandate was to produce a public report on cause and comment on proposed repairs (items 1 & 2 of their scope). These repairs seem to be underway of course, and one recommendation was that the design of any repairs was peer reviewed by independent and qualified engineers, which I assume and hope someone is at least doing.

Yes, that might be their mandate of the independent review. But they have now power to control what actions the builder is currently choosing on taking.
The builder has EVERY incentive to:
-reoccupy the building as fast as possible
-perform superficial repairs as fast as possible to hasten reoccupation of the distressed areas
-complete an 'adequate' structural repair to prevent further damage

They have little incentive to allow peer reviews into the process. The peers might decide a more expensive solution is recommended and then they are facing a real PR nightmare. This is boots on the ground kinda of stuff. Better to get in the quickly and do it your way than have independent people try to dictate how you spend your money. Not to mention the independent reviewers would have minimal resources compared to the builder and structural engineers responsible. They have every reason to throw plenty of resources at the issue now.

Otherwise its the guys who made the mistake fixing the mistake

Yep. But not really any different from most consumer or commercial warranties. But I do share and understand your concern.

I have a hard time believing that there are not more design issues throughout the structure than the areas that failed, but I'm a cynic. Too many years of doing peer reviews to know where there's one mistake, that there are generally more if you go looking.

I agree.

To support your argument, it seemed there were quite a few mistakes in construction identified in the interim report which likely had no effect on the failure. I would have little doubt there are many more.

 

[human909]

Thank goodness for whistle blowers.

Here’s a link to the ACA clip:-

Thanks.

Sure the clip focused on plenty of other corner cutting and imperfections in construction. But the smoking gun might just be that they placed the precast panel on level 10 upside down so the appropriate shear reinforcement was not present. Only got about 10 seconds of coverage at 6:28

 

[Agent666]

Thank goodness for whistle blowers.

Funny how the I told you so brigade comes out of the woodwork after the horse has bolted so to speak. I wonder what if anything they did at the time.

We don't know what involvement the design engineers had in the construction repairs that were shown, they seemed to be implying it was a coverup (possibly by the contractor) but really offered no facts on this or rebuttal from the contractor or designer. But I do find it hard to believe the designer wouldn't have been consulted and had the final say on any significant repairs to the structure.

If properly repaired the column would still be absolutely fine I would have thought.

Slab looks a bit suspect, but I've never seen random cracking like that before so can't offer any insight as to what might have caused it, they didn't mention the thickness. Could be honeycombing though that's been patched up rather than cracks. Guy in the video stated they could drop a 2" nail through the cracks to the next level which sounds alarming to say the least and would certainly warrant more investigation if its an accurate reflection of the crack widths. Reporter also mentioned the possibility of adding water to the concrete trucks, but this sounded more like his opinion of what might have happened vs what was actually happening on site.

 

[Chan WK]

The crazing cracks on the 10th floor slab is probably a result of using a concrete mix with cement content above 300 kg/m3 to achieve early strength.

The reason a contractor would be willing to pay more for such a concrete mix is to cut down the waiting time before tendons of the post tension system can be stressed, thereby releasing the supporting formwork of the slab.

Unfortunately, too high a cement content results in uncontrollable shrinkage. Seen this happened in 1993 in an office tower project in Bangkok.

 

[Mongrel]

It's probably worth noting that A Current Affair, like many of our mainstream "news" media, is basically just a televised tabloid. They love vague conspiracy theories and are not known for their extensive, well-researched articles. Controversy gets ratings!
Having said that, they have at least managed to provide video and images, which have been useful it seems.

 

[Agent666]

A few details of the repair thats been agreed to by Cardno/WSP. Details only for strengthening the affected upturn beam are shown, presumably the stell plates sandwiching the hob beam are intended to improve the confinement at the base of the wall.

I'm not sure if this qualifies for the independent review that was requested, given Cardno represent the body corporate but perhaps as good as its going to get for the repairs.

 

[hokie66]

So the too skinny walls will be turned into fat walls. "Pay me now, or pay me later."

I would hope the state appointed professors will continue to insist on the overall independent review. We shall see.

 

[MDEAus]

Agent666,

Has it been agreed to by Cardno? The update says it's been issued to them for agreement but not whether it's been agreed to. It strikes me that the remediation work that's detailed has the wall the correct way around, given the shape of it and dowels on one end how would you even install it upside down?

If I'm drafting a fix for something I would show it in its as built state rather than its as designed. Now this might just be a 'media release only' draft to not release any info that might be damaging to one of the parties involved and the actual working draft could be different.

 

[rapt]

Those repairs would appear to agree with my guess a couple of weeks ago that the hob was poured at slab concrete strength and the walls were much higher strength plus the connection detailing was awful so there were serious development and bursting problems. probably introduced in a conversion from insitu walls to precast at some stage during the construction.

The concrete strength problem was confirmed to me a couple of days ago.

 

[tornear]

given the shape of it and dowels on one end how would you even install it upside down?

I think what is meant by 'They placed the panel on L10 upside down' is that the reinforcement cage was placed in the mould upside down, and the rest of the hardware and ducts went in to design. As you point out, it is hard to lift a precast panel without lifting points on the top edge, not to mention getting it into place with the ship-lap joints inverted.

 

[Agent666]

Has it been agreed to by Cardno?

I'm guessing they have been party to early agreement on the concept involved, the remedial work has been in the works for a while and I can't imagine WSP not keeping them in the loop and getting agreement at each step. So while they still need to 'sign off' on it, I guess its possibly a done deal.

The concrete strength problem was confirmed to me a couple of days ago.

Interesting, I see some media reports today stating that the final 'official' report will be issued shortly as well.

 

[mangotree]

Just curious- is there something that’s stopping them from making the loads go directly down via new intermediate columns through the whole garden slot areas. (To effectively continue the columns down throughout all the levels)

Rather than sticking with the transfer beam scheme.

In terms of the visual impact of the elevation, it would appear more logical as the garden slot wall is presently treated as one large plane. To add little piddly pier-like columns that go up for one level then stop, risks looking odd when the void goes up several stories.

I guess the wall option “looks” better as it provides a continuous line through the wall width, but it does eat up quite a bit of someone’s floor area.

Edit: Perhaps they can ask the designers to work around their solution eg. maybe create maybe a niche wall-look so the add on piers/ columns don't look out of place...

 

[Tomfh]

Re. the concrete strength. Was it as per the design?, or did they place the wrong strength concrete?

 

[Agent666]

Mangotree, that would possibly just be admitting that the original transfer beam structural scheme was the wrong one perhaps..

What you are suggesting would be how I would have approached the design situation in the first place, column elements at the end of the wall. Heavily axially loaded walls make me nervous given the type of wall buckling failures observed over here in the Christchurch New Zealand earthquakes. Our codes were changed as a result, anything over I think 0.3f'cAg and you need confinement as per a column (i.e. a lot more confinement!).

 

[rapt]

I got the impression it was built as detailed, but cannot be sure.

RE heavily loaded singly reinforced walls and high concrete strengths and confinement, we have addressed this in the latest AS3600-2018, much to the displeasure of some consultants.

 

[hokie66]

rapt,
It should be to the pleasure of engineers. Architects always want things skinny. I think I read that these bearing walls and upstands were only 150 thick. A minimum of 200 in multi-storey construction would be a good code requirement.

 

[rapt]

Hokie,

I think they were 180mm.

Anything more than lightly loaded walls (equivalent to about a 3-4 storey building) and any wall in a limited or moderately ductile earthquake design are now required to have reinforcement on both faces. No central reinforcing. That should force 200mm hopefully. And above a minimum stress level must be confined as for columns and above 50MPa concrete must be confined as for columns.

 

[Retrograde]

and above 50MPa concrete must be confined as for columns.

Is this requirement only for limited or moderately ductile earthquake design, or for any wall with greater than 50MPa concrete?

 

[rapt]

All walls.