hancock elevator “plunge?”


hancock sofitel

Juicy story yesterday about the failure of an elevator cable in the John Hancock Tower’s 96th floor express shaft. According to the Tribune:

On the return trip, the elevator started going down faster than they were expecting, said one of the students, who didn’t want her name used for privacy reasons.

“It was really bumpy — it felt like a flight into Chicago,” she said.

Married couple Jaime and Maña Montemayor of Mexico City were on a business trip and had just finished dinner with a large group. After getting in the elevator they suddenly heard a loud “clack clack clack clack clack,” said Jaime Montemayor, 50.

Then dust particles began seeping into the elevator, and they panicked. “I knew something wasn’t OK,” said Maña Montemayor, 49.

Initial press coverage talked about the elevator “plunging” 84 floors, finally becoming stuck between the 11th and 12th floors, and the fact that the elevator was in a “blind shaft,” meaning firefighters had to cut through “concrete walls” to access the cab.

Scary stuff if you’re stuck in the cab, for sure.  And some commenters have noted that safety brakes should have engaged if the elevator did, in fact, ‘plunge.’  But I think there’s a slightly more benign explanation, especially given some confusion about the sequence of events.

The express elevators in the Hancock are fast–really fast–1,800 fpm, which makes them as speedy as any others in the U.S.  The ride down, especially for those on their first trip, is pretty dramatic and, frankly, bumpy.  And its elevators, like every other one in the U.S., has multiple hoisting cables, designed to ensure that, if one fails, there’s plenty of redundancy.

The Washington Post’s version of the story gets the timing a bit different:

It whizzed past all the usual stops, falling and falling and falling 84 floors before coming to an abrupt stop somewhere between the 11th and the 12th.

Then came the dust and dirt, floating into the elevator from the ceiling.

And then came the panic.

If that’s accurate, then it’s entirely possible that nothing went wrong until the cable failed.  The safety brakes may have engaged then–or, alternatively, since there would have been plenty of redundancy, it’s possible that the elevator never exceeded its safe travel speed, and only got jammed when the snapped cable ended up getting snagged in the guide rails.  A falling cable is a pretty grave hazard, but the “clack clack clack” jibes with the loose end banging against the other, remaining cables as it fell.

Once the elevator was stuck, getting the passengers out was a definite problem.  Express elevators typically are placed in ‘blind shafts’ that bypass floors they don’t serve.  Every elevator door costs roughly the same as a small car, so if there’s no reason for the elevator to stop on, say, the garage floor on 11, there’s no reason to spend the money.  My initial thought was that the ‘concrete’ that firefighters had to dig through may have been a shear wall, which would be a pretty heroic job, but Chicago Fire Department photos show that it was actually a concrete masonry wall–not structural, and not reinforced–that they had to get through:

Screen Shot 2018-11-19 at 8.31.40 AM

All of that is scary enough, especially if you’re the one trapped in the cab for 2-3 hours.  But, as often happens, the press coverage veered pretty quickly toward the sensational.  A cable snapping in a high-rise elevator is a vanishingly rare occurrence, whereas a century ago this happened with alarming regularity–and universally fatal consequences.    It’ll be interesting to see what the final report on the incident says, but if, in fact, the above scenario plays out then the fact that no one was even hurt–and that an event like this is so unusual–speaks to just how safe modern (or, even, half-century-old) elevators are.

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