July 07, 2005

Traction Elevators and the WTC

I wsa just cruising my blogroll, and I came across a post over at SayUncle's that had a line in it that I thought I should comment on.

After both buildings were burning, many calls to 911 resulted in advice to stay put and wait for rescue. Also, occupants of the towers had been trained to use the stairs, not the elevators, in case of evacuation.

There's a very good reason that people are trained to use the stairs. It's because the elevators are designed to STOP when there is a fire. If a smoke detector goes off in the machine room or the hoistway, modern elevators will immediately stop answering calls, and proceed to their lowest level (or the lowest level easily accessible by firefighters), open the doors and park there. People that were currently in the car can then get out, but the machine will not respond to any more calls. There is a special system installed that allows firefighters to utilize the conveyance with a set of keys, but once in this state, the elevator will not move without a "Fire Service" key or the whole system being reset after the alarm has been cleared.

In the case of the "Heat detector" being tripped, a shunt-trip will kill all the power to the elevator machine and the controller, to keep the systems from being overly damaged when the sprinklers go off. When that happens, noone uses the elevator.

I checked the NIST findings one the WTC disaster, and found an interesting fact right there on page 26 of the "Life Safety (Evacuation, Emergency Response, Active Systems)" section of the report.

After aircraft impact, only two elevators out of 198 were operating inside the two WTC towers. WTC 1, from the lobby to the 16th floor. WTC 2, from the lobby to the 40th floor.

I'm surprised that those two were actually running, due to one specific safety device that's installed on all traction elevator's. Somethign that I did not outline in this post is what's known as a "Compensating Shieve". This is a weighted pulley that hangs from ropes that attach to the bottom of the car, and the bottom of the counterweight. It's there to keep the car from jumping up at the end of an "up" run, and generally keeps tension on all the ropes no matter how lightly the car is laden.

Attached to the compensating shieve assembly, there is usally a little lever that keeps a switch in a closed position. There are little tabs attached to the box that the CS rides in (to keep it centered under the car, and pointed in the right direction) that will engage that lever if the CS moves too far up or down. If there is a slack cable situation, the switch will trip from the upper tab, and if the "ropes" stretch too far, it will trip from the lower one. If this switch is openned, it will kill all power to the drive motor, and the brakes will set, thereby stopping the car (No matter where it is in the hoistway, potentailly trapping passengers inside.).

I'm certain that when the airplanes crashed into the towers, this was the safety that disabled all the elevators. The cars "jumped" in their hoistways, and the safeties did what they were designed to do.

Anther interesting tidbit from page 7 of the obove report:

Functioning elevators allowed many (roughly 3,000) survivors to self-evacuate WTC 2 during the 16 minutes prior to aircraft impact. All but one of the 99 elevators in WTC 1 were not functioning, and survivors could only use the stairways.

So, people who do what I do, had a big part in saving the lives of around 3,000 people. If that doesn't give me a little pride in my job, nothing will.

I can only imagine how many more lives could have been saved if the firefighters could have used the elevators between the impacts, and the collapse of the buildings. Total death toll would likely have been in the hundreds instead of the thousands.

Anyone out there still wonder why I'm an Elevator Man?

Posted by Johnny - Oh at July 7, 2005 08:54 PM | TrackBack
Post a comment

Remember personal info?