When the observatory at 1 World Trade Center (1 WTC) opens May 29 in lower Manhattan, visitors will be treated to a spectacular 360-degree view of New York City and the surrounding area from nearly 390 meters above its bustling streets. All that’s needed to travel to the upper reaches of the building, also known as the “Freedom Tower,” is a 60-second ride in the Western Hemisphere’s fastest elevator system.
The 104-story 1 WTC—which opened for tenants in November—has 71 elevators, five of which will be express elevators with a top speed of more than 36.5 kilometers per hour. They are not the fastest in the world—Taiwan’s Taipei 101 skyscraper elevators race to the top of that 508-meter-tall tower at up to about 60 kilometers per hour—but they are still a full 25-percent faster than the express elevators in 1 WTC’s predecessor, the Twin Towers.
Eight 2.3-ton electric motors installed on 1 WTC’s roof power the high-speed elevators. Each elevator operates using a pulleylike system that consists of a cab and counterweights connected by a cable. Together, 1 WTC’s elevators use about 454,000 kilograms of counterweight to ascend and descend the building’s hoistways, or shafts.
In addition to being speedy, the elevators serving 1 WTC—the tallest building in the Americas—feature several advanced technologies designed to improve ride quality, safety and logistics. Although some of these features are already used in other “supertall” buildings around the world, a closer look at 1 WTC’s lift system reveals just how far elevator technology has advanced since the first Otis Brothers and Co. passenger elevator lifted shoppers from the ground floor of a New York City department store in 1857.
Future elevators are expected to function without cables, but these are years away as engineers develop the means to dead-lift elevator cabs—which weigh upward of 4,500 kilograms apiece—without any help from counterweights. One approach that several elevator companies have pursued over the years is the development of cable-free cars that use electromagnetic levitation to move in any direction.
Race to the top
An elevator needs more than just powerful motors to travel at high speeds. Like bullet trains, fast-moving elevators also require incredibly smooth rails and rail joints to move swiftly. “Over time, train rails have gotten longer in order to cut down on the number of joints a train must pass over and create a smoother ride,” says John Koshak, owner of Elevator Safety Solutions, a Collierville, Tenn.–based elevator and escalator consulting firm. The vertical positioning of elevator rails, however, limits their length to about 4.9 meters, which means any skyscraper requires a large number of rail joints.
Elevators must also account for minute changes in the distance between guide rails that occur as changes in temperature, wind and other conditions cause skyscrapers to sway slightly throughout the course of a day. “These factors mean that you can’t ever have a perfect plane for an elevator to travel in very tall buildings,” Koshak observes.
At 1 WTC, engineers are minimizing elevator car jostling using what is known as an “active roller guide” system. Roller guides keep an elevator’s wheels, known as rollers, in contact with the guide rails as the car ascends and descends. The rollers used at 1 WTC are made of polyurethane so they can absorb slight imperfections in the rail joints and are controlled by a system that pushes and pulls against the rails to prevent any misalignments or imperfections from interfering with a smooth ride, according to Alpharetta, Ga.,–based ThyssenKrupp Elevator, which holds an $87.98-million contract from The Port Authority of New York & New Jersey for elevator and escalator service at 1 WTC. ThyssenKrupp developed the system with roller-guide maker Elsco, headquartered in Owings Mills, Md. These active roller guide systems are “like hugely intelligent shock absorbers, which is hard to do in real time,” Koshak says. “You’ll see these in [supertall] buildings from now on.”
Air pressure is also a concern when designing and building high-speed elevator systems that can scale supertall skyscrapers, which by definition exceed 300 meters. Engineers, architects and builders must take into account how changes in air pressure impact not just the elevator cars and their passengers but the floors they pass as well. As a typical 4,500-kilogram car with a 7,300-kilogram counterweight rushes up its hoistway, it creates a massive air displacement. “With an area of high pressure above the car and low pressure below it, you’re creating a situation where the hoistway doors above the car want to blow out into the hallway and the hoistway doors below the car want to suck into the hoistway,” Koshak explains.
ThyssenKrupp placed aerodynamic aluminum shrouds around the tops of the elevators in 1 WTC to reduce air resistance, drag and wind noise in a way that minimizes air displacement. The idea behind this creative airflow design is that air pressure between the elevator doors and the hoistway doors remain neutral—minimal “whooshing” sound or door rattling when an express elevator passes by a floor without stopping, Koshak says. Air pressure changes that affect people inside the car are more difficult to mitigate, however. ThyssenKrupp’s approach at 1 WTC is to provide extra air pressure inside the cars to compensate for pressure drops, then slowly releasing it to keep passengers’ ears from popping.
An elevator can ascend as fast as the technology will allow without creating any passenger discomfort caused by changes in pressure. The 530-meter-tall CTF Finance Center in Guangzhou, China, is expected to have elevators that ascend at speeds up to 72 kilometers per hour, says James Fortune, principal at elevator consultancy Fortune Shepler Saling in Galveston, Texas. Elevators generally do not descend faster than 36 kilometers per hour, however, because anything faster can lead to ear-popping.
One of the main structural differences between the Twin Towers—built in the late 1960s and early 1970s—and 1 WTC is that the former were held together by a steel exoskeleton, whose vulnerability was revealed on September 11, 2001. The new building has a hollow concrete core that serves as a structural backbone for the building. The elevator hoistways run through this core, protected by a one-meter-thick concrete wall, according to ThyssenKrupp.
The emergency elevator hoistway in 1 WTC is kept at positive pressure to prevent smoke from entering during an emergency. The elevator itself features an auxiliary door that leads to a separate corridor where responders can go if the main part of a floor is too dangerous for them to exit the elevator’s main door.
Occupants of high-rises and skyscrapers are typically cautioned to never take an elevator during a fire or other emergency. This rule does not apply in extremely tall buildings, however, because most occupants would be unable to walk down 100 or more floors in time if all of the elevators were recalled to the ground floor, Fortune says. He advocates what he refers to as “lifeboat” operations in high-rise buildings where elevators could be switched to an evacuation mode that enables responders to take them to designated floors, such as sky lobbies, to rescue building occupants. Burj Khalifa in Dubai, United Arab Emirates—the world’s tallest building at nearly 830 meters—is prepared for such evacuations. It has pressurized, air-conditioned refuge floors and 10 elevators available for lifeboat evacuations, according to Fortune.
Calling all cars
Traffic management is a crucial part of elevator use in high-rise buildings. Supertall and “megatall” (exceeding 600 meters) structures are essentially buildings stacked on top of buildings with sky lobbies in between each one, explains Jay Popp, executive vice president, international, at elevator consulting firm Lerch Bates in Littleton, Colo. The use of sky lobbies is done to improve the efficiency in moving people to their desired floors and cutting down on waiting times. “In general, best design practice is to serve about a maximum of 60 floors with what we term local elevators, or elevators that serve a specific segment of the building,” he adds.
The elevator systems at 1 WTC rely on a kiosk setup in the lobby that determines which elevator a particular visitor will ride. ThyssenKrupp’s destination dispatch system is used in 63 of the building’s elevators to direct individuals to the appropriate car. Those headed to the same floor are grouped together for faster service. People working higher than the 64th story in 1 WTC, for example, first take an express elevator to the sky lobby on the 64th floor, where they can then catch a lift to the building’s higher floors. Destination dispatch systems can be linked to building security so a tenant or visitor can swipe a badge at a turnstile and automatically be directed to the appropriate elevator traveling to their intended floor.
High-rise elevators rely on ropes and cables to move up and down their hoistways. Although this design limits their motion to a single axis, it is not likely to change until someone develops a motor that can dead-lift an elevator car without the need for counterweights. Elevator companies have pointed to motor-propelled magnetic levitation—already used for a handful of high-speed trains—as one way to replace rope–counterweight systems.
ThyssenKrupp unveiled its idea for cable-free maglev technology—called MULTI—last December. In addition to allowing elevator cars to move in any direction their rails guide them, multiple cars could be added to same hoistway to improve efficiency. The company is prototyping its MULTI elevator and plans to install it in a test tower in Rottweil, Germany, for testing in 2017. (Earlier projections had testing beginning next year.) Otis announced in 1996 it was working on its own multidirectional elevator system, called the Odyssey, but shelved the project within a few years after the company shifted its focus to a more conventional design that uses flexible steel-reinforced belts instead of stiff metal cables.
The problem with these magnetic designs has always been heat, Koshak says. “To get the strength of the magnetic attraction needed to lift a heavy load you need a tight assembly of rails and magnets with a tremendous amount of electrical current to drive it all,” he explains. This creates a lot of heat that must somehow be released, otherwise the counterweight, hoistway and car all get too hot.
Despite these challenges and the fact that other manufacturers have explored different cableless designs over the years, “MULTI is the most technically advanced to date and has a real chance of making it into production,” Popp says. Still, the first buildings to use MULTI, if the system makes it to the market, will most likely restrict the technology to conventional up/down elevator hoistways at first, he adds. That way, if the technology does not work out, building owners could still install a more conventional lift.