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]

Agent666,
That is my interpretation of the current state of play. I am out of the game now, so am just giving my impression of what is happening. It didn't work that way when I was practicing, but from what I understand now, site visits/inspections by the design engineer on commercial and residential buildings are increasingly rare. A good reason to stick to industrial structures.

 

[degenn]

This image is from Google maps. It clearly shows the wall at L10 is load bearing with no column as shown in the DA plan. The wall is sitting on the L9 column with a limited bearing area.
The wall above L10 is supported appears supported on 3 columns, so is statically indeterminate. Depending on relative stiffnesses (including creep and shrinkage) the actual load at this transfer may not be what simple load takedown may indicate. It would be useful to know if all three go directly to the foundation or there are additional transfers.

 

[human909]

The wall above L10 is supported appears supported on 3 columns, so is statically indeterminate.

I count only two on the architectural plan. There appears to be a third column at the intersection of Grid lines 20 and 2 but the panels doesn't extend this far. The entry and internal wall is here. My interpretation anyway, I'm open to here others.

I wonder if the transfer slab and precast walls form a very deep beam (as opposed to precast walls *supported on* transfer beam), and the wall failure at the columns is effectively a web bearing failure, which the wall panels may not have been designed for.

Well you could consider it a deep beam if you wanted, it wouldn't be 'wrong'. It is just a designation that helps one apply appropriate analysis. But in this case I don't think describing the slab/wall as a beam is particularly useful. The key loads are mostly vertical compression loads on the wall, moment is minor in comparison.

Also calling the slab a transfer beam would imply that the slab has been designed provide stiffness in bending to the vertical loads imposed by the walls/columns. I don't think this has been done as you would need a significantly deeper slab to be stiff enough to carry the weight of 20 floor above. Instead, I am assuming that the design and analysis treatment of the slab is simply to support the bearing load of the wall/columns. That is my interpretations.

A corollary of this is that the wall is essentially only bearing on two points. These points being the ones above the columns. These points are also where the visible damage has been seen to have occured. ie, concentrated stress at two points along the wall.

(as opposed to precast walls *supported on* transfer beam)

If the design did treat the slab as a transfer beam then I'd suggest that is a key failing. The slab just isn't thick enough to provide the necessary stiffness to bear much load.

A quick rough and ready FEA of the wall to column-slab-wall behaviour and stress distribution. (Not loads are arbitrary, what matters is the distribution.)

(Note the above assumes perfect and continuous attachment between the slab and the wall. This is a generous approach and certainly not conservative. But even with this approach the stress concentrations are obvious. More realistic modelling of would result in even higher stress concentrations.)

 

[QSIIN]

As RAPT has pointed, often the major issue of precast is the connection design and detailing. I've seen precast panels with heavy Reo and ties (therefore technically "column" design) have only 4 heavy bars pass through the joint, and all other bars terminate in the panel. "The concrete takes the load". I hardly think this satisfies strain compatibility, and other fundamentals assumed by the code. Additionally, improper connection details can mean plane sections won't remain plane, therefore the whole analysis is pointless.

Many design offices merely put all the loads onto their RAM transfer models and take the output as gospel, without even scrutinising the results. Graduate level engineers are out there designing these major elements.

Not to mention that most site inspections are conducted by grad level engineers, if conducted at all. Generally contractors push for photos only... Details are easily missed.

 

[QSIIN]

@human909

Great image. What's the bet that the total G & Q load is applied to the transfer as a UDL, and that the wall is simply designed for the same UDL stress distribution? Not many would be doing that sort of FEA analysis..

 

[human909]

@human909
Great image.

Thanks! I appreciate that. It only took me a few minutes. But it just puts a visual to what I was trying to say in words.

What's the bet that the total G & Q load is applied to the transfer as a UDL, and that the wall is simply designed for the same UDL stress distribution? Not many would be doing that sort of FEA analysis..

IMO, any engineer that considers the transfer as a UDL is failing in their task badly. It could never be a UDL or anything close to that due to varying stiffnesses. You could avoid FEA by considering the bearing forces through the slab as a 45degree flow which is how some pile cap analysis is approached. As far as what most would do. I am the ignorant one here. My direct experience in concrete design is pretty minimal. All that said it would not shock me if that is exactly what has happened.

Just forget the calculations and ask yourself does that wall look appropriately wide compared to the column widths above and below it?

My gut says that a skinny wall supporting some fat columns doesn't add up!

 

[SAIL3]

many of the reports mention the "building industry"...are they a separate entity, self-regulating and where does the practicing engr fit into the system?....the engineering society seems to have a reduced influence on the the quality of the design until something goes wrong.....what are the qualification requirements for a certifier and who certifies the certifier?.....no system is perfect but the objective should be to reduce the probability of error wheather it is human or a result of corruption of the system.....here in the USA, the independent/peer review for major bldgs, while it can be a PITA for the design engr, has allot of merit as it introduces an independent oversight....I had expected all hell to break loose after the cladding problem but as time goes on there seems to be only a muffled response...

 

[rapt]

QSIIN,

And the 4 bars through the joint are really dowels and their connection and development is debatable if it exists at all!

I have also seen details where the wall reinforcement terminates before it reaches the dowels as there is too much conflict with the two sets of reinforcement, so there is no continuity of reinforcement!

 

[degenn]

Assuming vertical stress will flow through a joint at 45 degrees means that there are very large horizontal stresses in the lower edge of the wall in order to pull the forces back to vertical. I’m not sure the wall would have this horizontal reinforcement along the lower edge. A detailed FE will show these forces but the code column forces through a slab does not include a check. Traditional design assumed the column above and below were of similar shape so the issue was not as significant as for a thin wall on a much bigger diameter column.

 

[Tomfh]

Human909,

By deep beam I simply meant the thing you modelled, which as your excellent image shows gives you really high “web bearing” stress at the support.

 

[Tomfh]

QSINN,

yes my first thought was the princess and the pea.

Many people would indeed just provide a normal mattress.

 

[degenn]

This plan of level 3 shows what looks like 3 supporting columns below the grid 20 wall.
It is statically indeterminate how these three will share the load.

 

[mangotree]

Hi, I was wondering what everyone thinks of this theory:-
The “transfer beam” is a concrete upstand beam that it acts both as a beam and as the planter wall. (When you look at the balcony photo, the top of the beam that runs along the apartment wall should align with the top of the planter hob that runs into it. However, they are now misaligned as the top of the upstand beam is now lower than the top of the planter.)

The other photo showing a crack has some kind of white sleeve or conduit sticking out of the mess, that could indicate that this beam was post tensioned. Perhaps the post tension wires failed. I remember reading resident’s accounts of banging and screeching sounds.

Another thing I was contemplating was whether the temporary anchors for the “Doka” self climbing protection screen could affect the integrity of the post tensioned slabs. They have videos showing systems that anchor from the top of the slab and also from the slab edge. Could these possibly affect the post tensioning?

https://www.youtube.com/watch?v=vvQPXtvCuQ0

https://www.youtube.com/watch?v=LazbCGDiRWM

 

[human909]

This plan of level 3 shows what looks like 3 supporting columns below the grid 20 wall.
It is statically indeterminate how these three will share the load.

That isn't a plan of level 3 is is a plan of level 5-9. Zoom in on the image and it says 5004 and 5005.

Also the wall you are calling is not a precast structural wall as far as I am aware. It is an internal wall and presumable not structural. (

I've seen the only precast walls have been adjacent to the garden openings.

Here is an overlay of level 9 and level 10, it shows TWO columns underneath a 6m wall. (3 columns over 6 meters would be highly problematic for practical floor layouts)

Also note the discrepancy in the wall opposite to the one which we know has cracked. I would presume that final structurals would have meant a longer precast wall and moving the apartment entrance to the opposite side of the column.

 

[human909]

I'm becoming an all too frequent poster. I am sick and on holiday so that is my excuse. I hope some people are finding my contributions informative and not too frequent!

what are the qualification requirements for a certifier and who certifies the certifier?.....no system is perfect but the objective should be to reduce the probability of error wheather it is human or a result of corruption of the system.....here in the USA, the independent/peer review for major bldgs, while it can be a PITA for the design engr, has allot of merit as it introduces an independent oversight....

That is the second time you have asked that very good question and it hasn't been properly answered!

I would prefer somebody with better insight to answer it, as am ignorant on the specifics. I'll attempt to briefly answer it and not be too inaccurate! Certifiers in NSW state seem to be an administrative box ticking role. In my work I've never seen independent (outside party) reviewing of structural engineering, to my knowledge it isn't common. As others have reported site visits from the responsible structural engineers can be quite uncommon in residential and commercial construction.

many of the reports mention the "building industry"...are they a separate entity, self-regulating and where does the practicing engr fit into the system?....the engineering society seems to have a reduced influence on the the quality of the design until something goes wrong.....

The big players in the building industry are:
-the developers who are the financiers running the show (Generally they buy the land and sell the building. They engage the architects, structural engineers and a principal builder.)
-the principle builder, for big build these are usually an 'engineering company' though often they are more about project and contractor management than engineering. For small builds the builder may not contain any engineers.
-the practicing structural engineer is often just a small player in the scheme of things with little ownership and oversight over the build.
(I'm sure you can join the dots here of where potential problems can arise.)

I had expected all hell to break loose after the cladding problem but as time goes on there seems to be only a muffled response...

The issue here is similar to asbestos in buildings, it will take a good decade to get sorted. The scale of the issue is so large that dealing with it in a timely fashion is difficult except in the most dangerous implementations. Also delaying the process are lawsuits working out who is responsible.

As a side comment. In society, commerce and industry Australia generally has more extensive oversight and government protections than the USA. But in building industry Australia seems to have regressed considerably in the last 30 years. The general lay public are not blind to this. Which is why this single incident has trigger such a big media and political response.

Hi, I was wondering what everyone thinks of this theory:-
The “transfer beam” is a concrete upstand beam that it acts both as a beam and as the planter wall. (When you look at the balcony photo, the top of the beam that runs along the apartment wall should align with the top of the planter hob that runs into it. However, they are now misaligned as the top of the upstand beam is now lower than the top of the planter.)

The other photo showing a crack has some kind of white sleeve or conduit sticking out of the mess, that could indicate that this beam was post tensioned. Perhaps the post tension wires failed. I remember reading resident’s accounts of banging and screeching sounds.

You raise some interesting points. However I'm not sure there are transfer beams in play with the precast slabs. For a couple reasons:
1. No transfer beams can be seen in the photos at the top of the wall. (Levels 15/16) If one is necessary below, then one would likely be needed above.
2. To my knowledge there has only been mention of transfer beams once by the Wood & Grieve Engineers. “The large structural offsets at the base of the towers created a particular challenge. The difficulty was in the coordination of transferring sewer and storm water services through the deep transfer beams and large transfer slabs. (They are not the primary engineers, they just chose to speak to the media about the plumbing)
3. The deep transfer beams and slabs can readily be seen at the top of ground floor. Presumably they have larger spans on the ground floor that was original detailed in the architectual drawings. Larger columns are evident.

.
All that said I had noticed that item that looks like a conduit. I can't explain it, beyond assuming it isn't a conduit. Post tensioning a wall along that axis would likely give poor performance. All reports have said that it is precast walls that have been damaged.

 

[civeng80]

Human, sorry to hear you are sick and hope you get better soon.

I am finding this post and your commnets very interesting and informative.

 

[mangotree]

Thanks for your response human909. I really appreciate that and hope you get better soon!
I was thinking that perhaps there was a sequence of failures that started off with the post tensioning that led the floor to crack and drop 40-50mm, which in turn damaged the precast wall above it.
This is based on the tendency of designers to want to make joints and other building elements to “line up” for visual neatness. Eg. Top of the planter to line up with the top of a beam.
I haven’t seen construction drawings, so my theory is just based on a guess.
Many thanks again human909.

 

[human909]

Human, sorry to hear you are sick and hope you get better soon.

I am finding this post and your comments very interesting and informative.

Thanks. I'm a little embarrassed to admit on this forum, but I am actually fairly fresh into the industry and my day to day work isn't directly structural. So my real world structural design experience is not extensive. But I'd like to consider myself as thoughtful so I try to contribute. I am more than open for people calling "bull-shit" on any of my comments, because I'm sure many people posting here have more experience than me!

(To me, debate and discussion is the way I learn. I've learnt plenty already from others in this thread. My knowledge of concrete barely extends beyond what I learnt in my degree. I'm mostly a steel guy, currently working in a niche industry in bulk materials handling and it is booming. I don't do enough formal structural work, but I've recently renegotiated my employment terms and intend to change that.)

 

[rscassar]

The other photo showing a crack has some kind of white sleeve or conduit sticking out of the mess, that could indicate that this beam was post tensioned. Perhaps the post tension wires failed. I remember reading resident’s accounts of banging and screeching sounds.

I doubt that upstand was used to transfer the wall loads above as it looks about a 450mm upstand supporting 25 levels, if it where used as a beam it should have had heavy-ish shear ties whereas it doesn't appear to have any ties present.

That upstand may have been built at 32 or 40MPa with single layer of reinforcement, and I'm guessing the panels would be detailed somewhere between 60 and 80MPa, so the discontinuity between wall concrete grade and upstand concrete grade could be 1 point of failure.

The white thing sitting on top of the planter upstand doesn't look like a PT tendon. Tendons, even when they pop off the cover concrete and become exposed are more of a metallic grey appearance. And there shouldn't be a tendon placed in the top of the planter wall.

 

[rscassar]

I would also like to see a survey of the building verticality, I went to the Kings game yesterday and had a bit of a look from a distance and it looked a little tilted when trying to gauge it against the neighbouring buildings. It is hard to gauge verticality by eye, I look at every tall building and think it's tilted.