How fast should an elevator go?
The Washington Post recently wrote about The surprisingly cutthroat race to build the world’s fastest elevator, looking at the new elevator to the observation deck of the Shanghai Tower, the world’s second tallest building. The elevator reaches a top speed of 18 meters per second, or about 40 miles per hour. Adam Taylor compares this to other elevators:
The Burj Khalifa in Dubai is the only skyscraper in the world taller than Shanghai Tower, but its elevators go barely half the speed. The fastest elevator in the West, installed at 1 World Trade Center in Manhattan, runs at a paltry 23 mph. (10 M/S)
I have been in that elevator at the WTC, as a guest of the manufacturer, ThyssenKrupp. I take issue with that word “paltry.”
Lloyd Alter/ World Trade Center/CC BY 2.0
(Full disclosure: I have been a guest of ThyssenKrupp a lot, because I am fascinated by elevators, moving sidewalks and transport systems in general. Elevators are one of the most energy efficient means of transport, and you get a lot more people housed going vertically than you do horizontally, so they have an important role to play in green building. See more stories in related links at bottom of post.)
There is a fundamental issue that Adam Taylor fails to discuss in the Post article, and that is the difference between speed and acceleration. In fact, the “elevator problem” is basic high school physics,
[the] application of Newton's second law to the forces felt in an elevator. If you are accelerating upward you feel heavier, and if you are accelerating downward you feel lighter. If the elevator cable broke, you would feel weightless since both you and the elevator would be accelerating downward at the same rate.
The elevator could be going 80 or 100 MPH and as long as it is a constant speed, you wouldn’t feel the movement. It’s the acceleration and deceleration that you feel, pressing you into the floor or making you feel lighter. Standard practice in the elevator industry is that 1.5 M/S² is pushing the limits of comfort.
Top speed does matter, as you can see in this example from ThyssenKrupp, comparing what happens at 10 m/s max to 20 m/s max- the slower elevator reaches top speed and runs without accelerating for a while, adding 12 seconds to the trip. The faster elevator just keeps accelerating until it reaches top speed and then has to start decelerating.
Speed also matters when it comes to the issue of ears popping, and our bodies can handle going up better than they can going down. Elevator expert James Fortune writes:
Ear comfort and pressure changes do not usually affect healthy elevator riders unless the descent speeds exceed 10 m/s and vertical travel exceeds 500 meters. For this reason, virtually all of the latest supertall high-speed lifts, with “up” travel speeds of 10 to 20.5 m/s, have a maximum “down” speed of 10 m/s.
And in fact, as Adam Taylor notes, the Shanghai Tower elevator does descend at a maximum speed of 22.3 MPH (9.96 meters per second). There is also the issue of how much time is actually saved by designing for twice the maximum speed; when you add in the reality of stopping, unloading and reloading, it doesn’t add up to much compared to the huge expense.
So really, there is no surprisingly cutthroat race to build the fastest elevator, as a lot of companies are not even competing, because it makes so little sense. And all the comments in the Post that complain about how America has lost its lead in everything and doesn’t know how to build anymore and it’s all Obama’s fault that everything has gone to China miss the point:
An elevator is supposed to get you there in comfort, without your ears popping at high speed and without your stomach rising or sinking thanks to the acceleration. The speed of the WTC elevator isn’t “paltry”- it is pretty much as fast as you can go for that distance without people complaining. And they still build pretty good elevators in America and Germany.