At the top of the uncompleted Princess Tower, more than 400 metres above Dubai Marina, stairs lead to a dark, windowless room.
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Inside are four, 1.5 metre tall machines, each tied to nine, 19mm wide steel cables. These control the lifts that smoothly and quickly transport people to the various floors of the tower without causing nose bleeds or nausea.
"We do a lot of ride comfort tests," says Daniel Abraham, a project manager for Kone, the Finnish company that designed the lifts. "The unofficial test is to put a coin on its edge [in the lift] and see if it falls over."
Without innovations in "vertical transport" technology, super-tall buildings such as the Princess, which will be the tallest residential-only tower in the world when it is completed next year, would not be practical.
Engineers can design buildings to go higher and higher, but not without methods to efficiently transport people. No one wants to take a 10-minute journey and change elevators twice to get to their office or home.
In recent years, there has been a huge leap forward in elevator technology, as the industry has moved from the equivalent of the horse-drawn buggy to the space rocket.
In the Princess, the lifts travel at up to 6 metres per second - more than 21kph - making the journey from lobby to top floor within one minute. Like car and plane designers, elevator engineers are pushing the envelope.
In the Burj Khalifa, the world's tallest building, lifts can travel at up to 10 metres a second. In the Shanghai Tower under construction in China, the elevators are planned to travel at a record 18 metres a second, or almost 65kph.
But speed is not the only issue. Passengers should not feel as though they are travelling inside a giant bullet. Speed up too quickly or brake too abruptly and the ride will be jarring.
In modern elevators, computers tightly control the speed of acceleration to prevent this.
The lifts also need to handle the slight movements that affect super-tall towers. Sensors warn of high wind conditions and automatically slow the elevators. Guide rails are not fixed, allowing for adjustments in increments by as little as a few millimetres.
"The way to achieve greater heights is through the control system," says Sebi Joseph, the regional director for Otis, the company that designed and installed the elevators in the Burj Khalifa. "That is the soul, the control system."
To help direct "people flow," new systems ask passengers to plug in their specific floor destination when they call for an elevator, instead of simply pressing the up or down button. The "destination control" system then directs them to a specific elevator, calculating how to keep the elevators moving quickly to different levels of the building.
Efficiency is essential for lifts in tall buildings. Shafts gobble up large chunks of space in towers, where every square metre is at a premium. Designers want to keep the number of lifts to a minimum, while still ensuring tenants can get to their destination without long delays.
"When planning a building, the single biggest factor in making them work economically is floor space," said Anthony McCarter, the head of the Middle East building engineering practice for Aecom. "If you can get lifts operating more efficiently you can use less of them."
Many super-tall buildings are now a mix of residential and commercial, adding another level of complexity. Offices create greater challenges - large groups tend to enter and leave at the same time every day, putting pressure on systems.
The Burj Khalifa includes a hotel and restaurants as well as offices and apartments. To handle the traffic, Otis included two "double-decker" lifts, which simultaneously allow access from different levels. Typically, the bottom portion services odd-numbered floors, the top half even-numbered floors.
Otis also developed special mechanisms to stabilise lifts travelling at higher speeds and to adjust to the sway in the world's tallest building.
The system is unique to the Burj Khalifa, says Mr Joseph.
For buildings to go even higher - a 1km tall tower is under development in Saudi Arabia - lift technology will also need to develop further. Most elevator systems are limited to about 575 metres before passengers have to transfer to another, usually via a "sky lobby".
As buildings get higher, seconds can make a huge difference. Lift designers are spending much time and effort on creating systems to make elevator doors open and close more quickly.
The next generation includes multiple lifts operating in the same shafts, and cable-less elevators.
"That will have a radical effect on tall buildings," Mr McCarter says.
The weight of the cables is one of the main challenges as buildings reach for the sky, Mr Abraham says, as he showed visitors around the Princess elevator control room.
Each lift cable spool weighs one tonne and there are nine cables for each elevator. Go higher and the length of the cable, and the weight, increases. "The lift is very light compared to the weight of the cable," says Mr Abraham.
The Princess has 13 elevators for its 763 apartments, including a service lift capable of hauling five tonnes.
"The acceleration is very smooth," says Mr Abraham, as the lift can move from zero to 21kph in less time than many jet planes.
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