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.

 

[Agent666]

Guessing that the movement referred to is possibly relative movement either side of a crack that can be observed after the fact. Will be interesting to see what went wrong, without the structural drawings and the exact nature of what occurred it's a bit of a guessing game.

 

[human909]

The 1-2mm is a comment from first responders and really needs to be forgotten about. It was made in the context of the building as a whole and immediate measurements from first responders.

It is pretty clear that some floor have moved significantly more than that. Several residents require emergency services to pry/smash open doors to be able to leave their apartment. This implies significant interfloor movement.

Meanwhile it would seem that the developer involved is being a little unprofessional and making big claims which I would be surprise if anybody has enough information about.

'Absolutely no danger': Developer defends evacuated Opal Tower building
"The developer of a new high-rise apartment block in Sydney's west, which was evacuated after residents heard cracking noises on Christmas Eve, says there is "absolutely no danger for residents" and the company is confident the "localised" damage to an internal wall can be fixed."

What we DO know is that some slab floors have moved. And that loud cracking sound where heard throughout the day. This crack was loud enough that some residents fled the building of their own accord. None of that sounds like an easy fix. The developers say it is an internal wall, it is a structural wall.

 

[Tomfh]

Taking a more optimistic view - if it’s some localised crushing of the wall at the support point prior to engagement of the transfer beam (compatibility yielding), maybe it’s now finished and they can just patch it up?

 

[rscassar]

I'm not on board with any of this. 1-2mm by first responders, if the reports of the doors jamming are true than the displacements prolly around the 10mm mark. That video shows the column adjacent to unit 1005 which has spalled, don't know what happened to the column transition or transfer below it at Level 10 floor slab. If they are banning people from the units above and below this column, when I scrolled thru the drawings I counted at least 76 units which you could say fall within the tributary area of that column. From when this news story first occurred at 3pm to someone giving it the "OK. Building is at no risk of collapse at 1:45am". Hopefully it was an independent engineer with 20+ years high rise experience who conducted that 8-hr review. Given the nature of what happened there is no-way that building should have been re-opened. It should be closed until at least two independent parties completely review the design, completely review every site inspection report and site photograph, completely review every concrete cylinder report, every PT extension and every site survey, and carry out survey now that the failure has occurred.

I'm okay with 30% moment disstribution negative moment to positive moment provided that the reinforcement and displacements are accounted for, by the sounds of it, this slab('s) had to re-distribute prolly 3,000+kN in an instant.

 

[human909]

Agreed. Hopefully this will be a big wake up call to the industry and the authorities that supposably oversee it. The state government have already announced an independent investigation.

It would seem that sagging slabs are not entirely uncommon. In a nearby structure "some balconies have sagged as much as 180mm on the outer edges over three years".

https://www.smh.com.au/national/nsw...re-root-cause-of-defects-20181226-p50o97.html

Not dissimilar to the flammable cladding and the Grenfell Tower fire. Until the public and authorities are slapped in the face with the dangers, much of it gets ignored. Due to the nature of joint ownership and here-one-day-gone-the-next companies lots of these problems just end in protracted legal battles. Maybe this will be a wake up call.

 

[civeng80]

My thouthts only here. Destruction of a concrete wall with no lateral loads e.g wind or seismic is very serious.

But how can they say that it moved by 2mm ? The movement is insignificant in this building and in much smaller 1 storey buildings.

 

[human909]

The 2mm comments were made by first responders on the day. They certainly were not made by engineers. They might have been made of the basis of monitoring after responding. AKA it means nothing, but the media will happily report.

 

[rscassar]

Starting to hear that the wall panels were precast wall panels, which I think you can see a vertical panel joint just outside the glazing though it may be a dummy joint. And the spalled concrete looks like it has mesh each face. 2 thoughts:

- Grout beds, grout beds normally get detailed 20mm with 10mm steel shims placed around the panel dowels. The grout beds aren't as good as RC. They settle with load and create localised bearing points at the shims which can initialize cracking and spall the cover concrete, and could account for the vibration and sound that went thru the building. I remember reading about an Adelaide highrise early this year which didn't have the failure of this building but did have the ugly cracking of load bearing precast panels.

- Simplified wall calcs in AS3600. I think wall calcs using this equation must use a minimum e of yDim/6. I'm guessing from photos that the parameters for this wall panel would be xDim=1600;yDim=250;fc=65;Le=0.9*2900 (the f'c is a guess):
simplified wall calc using e=0mm = 7,900kN
simplified wall calc using e=yDim/6 (41.6mm) = 6,600kN
concrete column calc (and assuming the vertical bars are restrained) = 5,100kN.
I hate the divergence between simplified wall calcs and column calcs when Le/r>30.

 

[Tomfh]

Oh wow. Precast!

 

[human909]

Oh and a backflip. Now doesn't that fill you with confidence....

https://www.theage.com.au/national/...ilding-for-up-to-10-days-20181227-p50ofi.html

Hundreds of residents of Opal Tower at Sydney Olympic Park were told on Thursday that everyone who lived in the building would be "relocated" to allow "comprehensive investigations" into the damaged pre-made concrete panel.

"The building is structurally sound and the temporary relocation is a precautionary measure to allow engineers to work around the clock to comprehensively investigate and remediate the site in the quickest time frame possible, without further disruption to residents," the builder, Icon, said in a statement."

What horseshit. You wouldn't be evacuating the entire building [AGAIN] if you didn't think there was a safety risk. Yet a few days ago it was rapidly declared safe in under 8 hours.

 

[Agent666]

Some more fine journalism (the blame games started), I'm glad all the experts in the public are onto the case at the end.
[URL unfurl="true"]https://www.nzherald.co.nz/business/news/article.cfm?c_id=3&objectid=12182960[/url]

Something looks odd about that concrete in the top photo, like it resembles masonry block texture. Could be an issue with the concrete strength/mix. Not even sure it's concrete from the closeup, sort of looks like grey polystyrene?

 

[rscassar]

That bit is Hebel and most defiantly won't be carrying load. It's been crushed

 

[waross]

Hebel can refer to:
...........
A brand of aerated autoclaved concrete from CSR Limited

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[human909]

It is just a close up of the internal damage as already posted. It isn't structural. The media are just being particularly poor.

It is just the top part of the wall in this photo.

 

[QSIIN]

@rscassar

My immediate thoughts as well, hard to say until we get more information but I agree, there is a disconnect between chapters 10 & 11 in AS3600.

I personally have seen several design firms detail blade elements as CL10 "columns" but use CL11 "wall" capacities, and they don't even question the disparity. Just boost the concrete to 65 or 80MPa and just cheer at the extra capacity without Reo or ties.

Although the new 2018 3600 has some stricter rules on walls/columns, I believe 3600 should follow ACI and have walls and columns under the umbrella of "compression" members that must be designed for a minimum moment. Slenderness cannot be ignored. Therefore a magnified moment must be designed for.

If the magnified moment doesn't induce tension in the cross section, then the simplified method might be appropriate, but you simply can't ignore slenderness.

 

[Tomfh]

In my opinion it would be a real shame if you had to design every wall as a column.

By all means reduce the capacity of the simple wall calcs if using simplified method (I’ve often considered them generous), but let’s not throw the baby out with the bathwater and demand complex analysis for everything. AS3600 has a nasty habit of doing that. Every edition gets worse.

 

[QSIIN]

Im hardly saying every element needs to be designed as a column, rather that compression elements need to be designed appropriately, and if slenderness is an issue, then it must be addressed.

Confinement in compression members cannot be understated and simply upping the concrete strength to avoid ties is reckless, in my opinion.

Using an interaction diagram, considering biaxial bending, and moment magnifier hardly constitutes "complex analysis". In actuality, the moment magnifier is the simplified method to avoid complex second order analysis.

 

[rapt]

Tomfh

How long does it take to design a column compared to a wall.

Part of the problem is that simplified methods which were developed in times when materials were a lot lower in capacity, eg concrete strength of 50MPa maximum up to 2001 and 65MPa up to 2009 are now being used for very high concrete strengths and with details which are completely inappropriate (central mesh reinforcing) for very tall walls. Something had to be done to limit the application of the very simplified methods.

Other things that have become more complex are crack control (because reinforcement strengths increased to 500 and now 600Mpa) and the old simplified rules simply no longer apply.

With computers, there should be no problem with removing some of the inappropriate design methods that were included in the code in previous generations because hand calculations were simply too hard to do.

 

[human909]

Im hardly saying every element needs to be designed as a column, rather that compression elements need to be designed appropriately, and if slenderness is an issue, then it must be addressed.

That is certainly the way that I interpreted your comments and I agree.

The other thing is that I'm very surprised that such discontinuous connections be designed without deeper analysis. Even without potential stress concentrators of shims like rscassar mentioned, you are always going to get a very uneven load distribution in the wall. Stress analysis approaches should readily lead you to that conclusion.

 

[Tomfh]

Qsinn,

It’s more complex than I’d want to do when checking the simple capacity of a wall!

But in general yes I agree with you that these simplified methods can go too far, and give capacities beyond what seems reasonable. I believe stricter limits should apply, and I don’t believe walls should be giving significantly higher capacities than an identical column.