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Comment by secabeen | Hacker News Reader

The most important section though is buried in a late paragraph:

>Engineers said it seemed unlikely that having less rebar would trigger a collapse in and of itself, even factoring in significant deterioration over many years. An inherent safety factor built into most projects would mean that a slight reduction in steel content would not necessarily lead to disaster.

This certainly suggests that there may have been some shortcuts taken during building, but this is not a smoking gun, and doesn't even start to answer the questions of "why now, after 40 years?"

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Like aircraft disasters, most civil/structural engineering disasters have more than one "cause". There is not always a satisfying answer or a single "smoking gun". A bunch of things usually have to go wrong at the same time for a building to collapse catastrophically. Building codes and engineering designs have a lot of assumptions built in, including an assumption that construction does not follow the drawings exactly. But other assumptions include lack of corrosion to a degree, regular maintenance, proper materials specifications, no subsequent modifications, a particular load pattern, etc.

Even then, most buildings are not really designed for much more than 60 or 70 years. Of course, most buildings are also supposed to be designed in such a way that even if it fails, it does not result in a cascading failure. The fact that this particular kind of disaster (apparently spontaneous collapse) is so rare is what makes it newsworthy.

Yeah, I've read _Normal Accidents_, and I follow AdmiralCloudberg's aircraft disasters. There isn't always a satisfying answer, but there often is, or at least one factor that's most damning.

> why now, after 40 years

If this is a case of wear leading to total failure, isn't this like asking why we take the turkey out of the oven on Thanksgiving after 3 hours? Because that’s how long it takes.

> An inherent safety factor built into most projects would mean that a slight reduction in steel content would not necessarily lead to disaster.

On the other hand, it can lead to disaster, which is why that safety factor exists. We're not doing that just for fun.

The space shuttle SRBs were designed with two o-rings, a primary and a secondary. When the primary kept showing signs of burn through, instead of treating that as a problem, it was said that because the secondary o-ring was not burned through, the safety factor was sufficient. But the primary was not designed to burn through. The safety factor did not account for burn through in normal operation, but risk assessments obscured that fact.

>"why now, after 40 years?"

Huh? Cyclic fatigue yo. Incredibly simple and well known killer.

Less whatever type of supports than spec'd to distribute the fatigue across, earlier things fall apart or break from it.

I am partial to the climate change theory.

Rising sea level means higher soil expanding/shrinking with the tides, leading to fatigue

But we're talking 4 inches over the life of the building. I'm not sure if buildings are positioned that precisely (if I had to guess, I'd guess it's right around there), the tidal range around Miami looks like it's around 16 inches, and hurricanes and tropical storms will do much more than sea level rise.

I could be wrong, it just seems like there are bigger factors at play, and I'd be surprised that it was build so close to safety margins.

Possible. I wonder if there was some unexpected external force; a car crashed into the critical pillar at moderate-to-high speed (as high as you can get in a parking lot) or something like that.

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