Why Are Elevators Safer Than You Thought?

In a day, how much do you step into an elevator? A lot of people live and work 10, 20 or even more stories above the ground. Yet rarely do you spare a thought about the complicated electromechanical systems that lifts you up and down, lest your mind wander to scary thoughts of a Tower of Terror-like plunge into the subbasement.

Elevator Cables Do the Hard Work

The instrument that makes elevator cabinprincipally sturdy are the steel cables holding them up. How many cables are on every cabin? There are six to eight. So even if all but one fail, the elevator will still be held up safely, as each steel cable is capable of holding more than the weight of the elevator cabin.

Building codes set the factor of safety to 12 for a particular building which means the combined strength of the ropes must be suitable to hold 12 times the mass of a fully loaded cabin. In effect, each rope can hold more than the weight of the cabin.

In 1945, a B-25 bomber collided into the Empire State Building, cutting through all of the cables on an elevator. The lone passenger survived the fall from the 79th floor because the cables beneath the cab decelerated her descent and cushioned her landing. The planes that collided into the World Trade Center on September 11, 2001, also cut the elevator cables, which sadly resulted to casualties.

Elevators have two to three types of brakes. If there is an error in the safety chain, a clamp closes on the pulley above the cabin, preventing the elevator from moving. Unlike a vehicle brake, which has to be depressed to engage, the elevator brake is clamped down unless power is supplied to release it. This means that any loss of power, either due to a system error or an electrical grid failure, will set off the motor brake.

A safety break is also attached in an Elevator connected to the underside of the car. This is the modernization that made the passenger elevator possible when it was unveiled at the 1853-54 World’s Fair in New York.

How does safety break works? If the sensors detect that the cabin is speeding downward, it jams a metal brake from beneath the cabin into a channel in the guide rails, the metal rods along which the elevator travels. Friction builds between the wedge and the rail, which brings the car to a halt at a comfortable rate.

There is another fail-safe. On the opposite ends of the cables that attach to the elevator cabin, there is a set of counterweights. These weights weigh a little bit more than an empty car and slightly less than a fully loaded car. If all other safety system failed and you were the only person in the car, these weights would make the elevator ascend rather than descend. It would happen slowly at first, accelerating as the ascent continued. A cabin that is fully loaded would experience a slowly de-accelerating descent.

In either case, if the counterweights reached the top or bottom of the shaft, they would meet a cushion that would bring the elevator cabin to an abrupt but hopefully survivable stop.

As an interesting closing note even with that being a distant possibility, you should always remember that (statistically) elevators are still safer than taking the stairs!