Hacker News Reader

Comment by throwawayboise | Hacker News Reader

Agree. Unstable land/foundation may be a contributing factor but my guess is that shoddy/cheap concrete work will be the root cause.

View on HN

It would be quite hard to fake enough cube samples for that and unlikely to be worth the bother. The steelwork failing is more likely. Pre-stressed conc is basically an explosion waiting to happen, which makes demolitions interesting. Another possibility is concrete cancer but the map cracking is well known and easily recognised. That last is unlikely and easy to rule out with some simple chemistry. However, Miami is on the coast so the extra ingredients are there.

There are loads of possibilities but at the moment I think foundation failure is most likely. Unless you pile down to bedrock or build a gigantic raft of conc. then you need very stable land for a building that size. Another other option is to build it flexible but that was obviously not the case. The geology there is likely to be a nightmare. Miami is a lot of reclaimed land, low lying and next door to the Atlantic.

The sight of the collapse is something I won't forget in a hurry. The secondary collapse on the right in the security camera footage took about three seconds. That's insanely fast. The failure was absolutely catastrophic. It wasn't simply a cascade - the whole structure seemed to fail simultaneously from top to bottom and that is not normal.

To your point, the top of last section dropped as if it was free-falling into empty space for 3 seconds.

Shoddy concrete work seems to be a running gag in the south. I had a related experience when the NOLA Hard Rock Cafe collapsed onto the street seconds after I walked down said street. It wouldn't surprise me if the concrete work in the collapsed condo was of a similar (lacking) level of quality.

If you mean this: https://en.wikipedia.org/wiki/1031_Canal that is rather different - a non catastrophic, partial collapse of an unfinished structure. This was a catastrophic collapse of a well used structure. People died in both so please don't think I am trivialising the NOLA tragedy but it was different.

When you use concrete in large amounts you take standard samples called cubes as you pour and these are subjected to standard tests. In the UK we use a 10x10x10 cm cube and then it's allowed to cure for a standard time in standard conditions and then crushed. There's a bit more to it than that but basically, it is quite easy to demonstrate "standard" concrete and there is not much money to be made with rubbish materials. There are exciting possibilities involving admixtures but that is down to workmanship. Shoddy concrete (whatever that is) is very unlikely but a popular meme.

Conc. for a structure like that may be pre-stressed or at least reinforced. Pre stressed is where you put rods or creatively shaped steel through it and literally squeeze it. Conc is great in compression (pushed/squeezed) but rubbish in tension (pulled apart). Imagine a beam supported at both ends and think of pushing down on it in the middle. It will try to bend in a curve. The top surface will try to get smaller and hence is in compression and the bottom will try to stretch and is in tension. If you drill a hole along the length of your beam near the bottom and stick a steel rod through it and put some washers and nuts on and crank on some compression you now have a sort of model for pre stressed concrete. The steel rod is ace in tension and gives the concrete beam a lot more strength. Embedding steel mesh ("rebar" or reinforcement bars and the like) have a similar effect but ...

The thing about pre stressing over adding mesh/rebar is that you are locking away a huge amount of energy into a static structure. There is a huge gain to be had in reduced thickness of conc and hence weight which means its a double win but you also need to ensure that the "rods" (in our model, not quite reality) never fail. If they do fail then it's normally rapid not just a gradual fail. The effect of prestressing can happen by accident in "normal" meshed conc. Most digger drivers that have used a pile driver attachment to break large chunks of conc. will have tales to tell of large lumps of conc suddenly exploding. Fishermen often tell tales of the one that got away ... 8) However it can happen.

So I hope I've shown that shoddy concrete itself is unlikely. There are far more ways for a structure to fail than that. Workmanship is possible but for my money the ground basically dropped away from under the structure and it sort of exploded and imploded simultaneously. I can't put this any other way but it seemed to fail completely in every way possible, simultaneously.

It is the most horrible thing to watch and will haunt us forever.

this is true, if you take standard samples and test them. but the Hard Rock New Orleans may have bogus inspections and testing.

>Tweeter reported three other on-site inspections in July, August and October, but the GPS records show her vehicle stopping at the Hard Rock on just one of those dates.

the Hard Rock New Orleans could have been insufficient supports changed after the city approved plans [2].

>The last set of plans approved by the city were dated June 2018. Those called for the sturdier 16-gauge metal decking. Those plans were revised to go with the thinner, lighter 22-gauge decking in February 2019, shortly after the emails between the construction team and New Millennium. The city confirmed it never received those revisions.

[1] https://www.wwltv.com/article/news/investigations/city-was-c...

[2] https://www.wwltv.com/article/news/investigations/david-hamm...

The detail is very interesting, thanks.

Maybe what you say rules it out anyway, but can concrete deteriorate over time? Eg maybe it was up to strength at the time, but somehow weakened over the 40 years? I don’t know anything about concrete...

Reply on news.ycombinator.com