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.

 

[hokie66]

Yes, after looking again, I agree it is a slab on grade. But the crack does indicate poor attention to detail, which may be an indicator of the problems in the structure.

 

[PTStressHead]

Agree, this is likely a pavement slab on grade at B3, Probably 120-140 thick with a light top mesh and saw cut control joints. The joint running north/south in image may be a pour break/expansion joint (concrete texture looks to differ. Suspect joint dowels may have bound up and no debond agent applied to slab face prior to second pour, or bonded to column footing below. Completely irrelevant in context. Media are funny.

 

[PTStressHead]

My thought is as to how much care was taken in ensuring that the tensile horizontal reinforcement in the bottom of the panel was anchored and developed within and past the critical nodal zone directly above the column. The columns look to be very close to the end of the walls. A 180 panel is very slim to be detailing 2 layers of N28 rebar at 100 c/c, particularly with the central dowel tubes (probably 70 dia). The bars should be developed past the outer face of the CCT node. Cog anchorages for an N28 are very long, and likely would not fit within cover, and these only give 50% bar development anyway. There would have been severe congestion in this panel, which makes me think the N20's were intentionally substituted as N28s with cogs would not fit.

 

[PTStressHead]

Also in the report photos, you can clearly see that the starter dowels for the panel in the critical column zone are not developed/cast into the column below. They are cogged into the 200 slab A flimsy attempt at a compression splice has been attemped by using 180 degree hook bars from the top of the insitu hob into the column, however I doubt this is effective in carrying much compression and and alleviating the unconfined compressive stress in the upstand concrete. And no transverse confinement reo within the upstand itself. All of this combined with the poor grouting, there goes the factor of safety.

 

[rapt]

You cannot develop any compression using a hook/cog. The only development you get is the straight bit before the bend.

 

[Agent666]

And you probably get no transfer of bond stresses in the straight portion if the dowels are embedded into plastic ducts above the hob beam. Any tension/compression would just deform the plastic, so it stands to reason all of the axial load is potentially bearing through the concrete with lmited contributions from the dowels (as a reinforced member) if they cannot be developed above the base of the panel. Can someone comment if these plastic ducts are deemed acceptable practice in Australia?

 

[Toby43]

Agent666,
Yes, I see plastic grout ducts commonly used in my local
Google "anchorage requirements for grouted vertical-duct connectors..." and you should find a report by FJ Brenes et al. Chapter 8 discusses embedment length issues.

Toby

 

[Agent666]

Thanks Toby43, from that report it states "...galvanized steel ducts were able to resist tensile stresses 25 to 35% higher than those with plastic ducts". So if you are after full development as often required by codes it would seem to me that plastic ducts have no place in the type of detail connecting the panel to the hob wall?

 

[Toby43]

Agent666,
Yes - required embedment lengths for plastic are typically 30% larger than for metal ducts. Whether the geometry was there for this particular detail at Opal, I don't know.

 

[Agent666]

For anyone that's interested Icon have created a blog for communications with residents. Link

 

[Agent666]

Contractors structural repairs action plan (SRAP), I haven't read it so not sure if it contains anything new.

 

[david256]

Video of a new crack in one of the apartments, tenant returned to pick-up their belongings and move out.

https://vimeo.com/312421160

 

[Tomfh]

How can they tell it’s new?

 

[Agent666]

Because they didn't look for it two weeks ago obviously....


I once had a colleague go out and do an insurance inspection for a person who was claiming their driveway was cracked due to a recent very mild earthquake that had occurred a few days earlier. They had never noticed the crack before, but it had grass growing out of it.... claim denied.

I've even found in my own house I noticed a lot more cracks after an earthquake, but only because I'd probably never noticed them previously because I never looked in great detail.


In fact one of the report photos shows the same location of the propping and the exact same crack (right at the beginning of the video). So the crack was probably there 2 weeks ago on the balance of probabilities. More uninformed people influencing the internet/media I say than a valid claim to new damage.

 

[david256]

Agent666, I think you're right the person who took the video was in unit 301 and looking at the interim report it says floor between L3-L4.

Edit : Just had a look at earlier photo's and video of her apartment, they have definitely cut a much larger hole in the ceiling plasterboard since she was in there on the 3rd of January, maybe 1-2 meters larger down past the air-conditioner, where the earlier photos show the cutting stopped before the air conditioning.

 

[NorthCivil]

the only sure things in life are death, taxes, and concrete cracking. all concrete cracks are not created equal. Many are fine. This is why we have an entire technical field of concrete crack control.

 

[MDEAus]

Just on the crack in the video posted by david256, that crack is clearly pictured in the interim report on page 16 in figure 11. Figure 11 only shows around the props but it is the same crack and doesn't appear to have changed since the photo in the report was taken (when compared to the video) and I doubt that it was taken last minute before the report was issued. In other words it would have been there 2 weeks ago.

 

[david256]

MDEAus, here's an earlier photo of her apartment, you can't see the crack but you can see in the video how they have cut a larger hole in the ceiling plasterboard following the crack.

She probably didn't even notice the crack on the 3rd, she was probably in shock.

 

[mangotree]

"The Urban Developer" website has an article by Ted Tabet dated 21st January 2019 that seems to have a new photo of cracks on the upstand beam.
It doesn't state what area or level this photo was taken at.

Here's the link to the article:-

https://theurbandeveloper.com/articles/opal-tower-what-we-know-so-far

 

[MDEAus]

mangotree

Figure 10. Photographs of the damaged hob beam on Level 4 near column C34
before (top) and after (bottom) cracked concrete removed

That is not a new photo. It appears in the interim report as part of figure 10(likely where it was taken from; it's even linked in the article).