Did I Uncover a $100M Mistake with the Millennium Tower “Fix”? | Hacker News Reader

Did I Uncover a $100M Mistake with the Millennium Tower “Fix”?

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80 pointsgarrettgarcia15 comments

Long video, but I watched it until the advertisement at the end.

I don't live anywhere near S.F. and my engineering background is limited (undergraduate degree, nothing professional), but I agree that the video author does have a valid and very important concern.

It would've been a lot nicer if he shortened the video and if he showed his calculations for the steel bar that the all-thread is transferring the load to.

Watched the video. It's very interesting. It has the background information on the sinking Millennium Tower and the pending remedy for it. It describes and analyzes the pending remedy in detail. It's worrying that the downward force is hinging on such a small steel plate. It looks like a disaster waiting to happen. It's also surprising very little calculation has been submitted by the engineering firm for the proposed remedy. The people involved seem amateur.

Interesting to me is that the potential failure mode this engineer thinks is possible is actually very similar to what caused the Hyatt Regency walkway collapse, https://en.wikipedia.org/wiki/Hyatt_Regency_walkway_collapse.

Interestingly it looks like the engineering firm that later identified the issue with the walkway is the engineering firm that designed the Millennium tower fix.

The same engineering firm that did the original design leading to the sinking.

TLDR (IANA engineer):

The project to prevent the tower from sinking further, relies on four small steel plates that are supposed to bear a large amount of load. These plates are encased in concrete and therefore cannot be checked or maintained after installation. The video creator can't find any evidence that the project engineers calculated whether the plates can bear this load, and his own calculations suggests that they cannot.

Also the steel cable strength calculated in the submitted proposal exceeds the safety margin of the cable’s yield strength.

It's not a steel cable. The idea is that the bolts are sacrificial components that stretch when the building sinks further and can be jacked back up to correct the sinking and these bolts need to be inspected and possibly replaced routinely.

That in itself is not damning. The real question is how do they intend to inspect the threaded connection to the concrete? To avoid frequent inspections you would want to over engineer it so it lasts for hundreds of years even if the building will be torn down in 15 years but the steel bar is ridiculously undersized and should be considered sacrificial as well but it cannot be replaced nor inspected.

What is really disappointing about this whole project is that nobody stopped and thought about building a model for dynamic and static load tests to prove that it works in reality. After all, this has never been done before on a building of this scale.

Iirc steel rust was one of the factors contributing to the collapse of the Morandi Bridge in Italy. And because corroded elements were covered in concrete, could not be inspected with naked eye until concrete started cracking, which was too late.

I've read about this and it surprised me that concrete-encased steel could not be inspected in the case of the Morandi bridge. X-ray, or measuring how conductive it is should work.

While it is true that the cables were enclosed in concrete and not directly accessible, there were all the signs that they needed intervention.

The "same" cables on the "near" pylon #11 (the one that collapsed was #9) had been replaced in years 1992-1996 because of severe corrosion.

Everyone was expecting that the same replacement would have been soon made on the pylons #9 and #10 but the inspections (circa 1991) that were made (boring holes to visually inspect the cables in a few points) and, although these inspections (obviously) showed some relevant corrosion it was underestimated, and anyway no correction measures were taken in a timely manner (please read as in the next 25 years).

That the bridge was overloaded (by traffic as much as 4-5 times the amount it was designed for originally) and that it needed interventions was a known fact and starting 2006-2009 there were several projects about demolishing and rebuilding it that were never finalized/approved.

A new inspection made in 2015 evidenced the very poor state of the cables, the results of this inspection were once again underestimated or even falsified.

A project was made for replacing/reinforcing the cables in 2017 (20+ years after the works on #11 and 2 years after latest inspection) and the bid for it was published only a few months before the bridge collapsed in 2018.

It was a story of incompetence, sloppyness and greed.

I’m surprised that there are no prediction markets going on the Millennium Tower.

There kinda are. Consider what nearby business headquarters exist in its potential wake.

The bigger they are, the... something something

I guess we could short Salesforce, but I would like something more legible.

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