Design Challenges of High-Speed Elevators
This article is an excerpt from ELEVATOR WORLD’s July 1995 issue. We believe it is pertinent as background information to the challenges of designing high-speed elevators for very tall buildings. …Editor
Physiological Problems Associated with High-Speed Lifts
The elevator industry has developed the following physiological limits that standing elevator riders can tolerate without feeling discomfort:
- Vertical acceleration/deceleration: ≤ 1-1.5 mps2
- Jerk rates: ≤ 2.5 mps3
- Sound: ≤ 50 dBa
- Horizontal sway: 15-20 mg
- Ear-pressure change: ≤ 2,000 Pa
All of the physiological elevator design parameters, except ear-pressure changes, can be regulated by proper equipment designs. Ear comfort/pressure changes do not usually affect elevator riders, unless the descent speeds exceed 7 mps, and the vertical travel exceeds 300 m.
When Frank Lloyd Wright revealed his plans for the Illinois Office Tower to the Chicago Daily News (which subsequently published a story reviewing the proposed method of elevatoring the project), the paper immediately received comments from a number of airline pilots questioning the ability of the atomic-powered, 25-mps elevators to serve the project without causing eardrum discomfort in the riding public. Airline pilots are well aware of the problems associated with rapid changes in altitude. Apparently, the inner ear can react adversely to changes in pressure associated with rapid ascents and descents experienced as aircraft change altitude. The same condition can affect elevator riders when elevator speeds exceed about 7 mps or the vertical travel distance exceeds about 300 m. Elderly persons, those with colds, flu or allergies, or those who cannot rapidly clear their ear passages are more at risk. Obviously, if 25-mps, quintuple-deck elevators envisioned by Wright were to really rise and then descend about 1,600 m above grade in just 1 min., the riders would experience considerable pain if they did not sufficiently “clear” their ears en route, or if the cabin presser were not controlled.
Think of the middle ear as a balloon that expands as exterior pressure decreases during ascent, and contracts as exterior pressure increases during descent. As the airline-cabin or elevator-cab pressure decreases during ascent, the expanding air in the middle ear pushes the normal Eustachian tube open (at approximately 4,000 Pa), letting the increased pressure escape down into the nasal passages until the pressure in the inner ear and the cabin, cab or final ascent level is equalized. However, during rapid descent, the passenger must consciously open the Eustachian tube by swallowing, yawning or tensing muscles in the throat, or by closing the mouth, pinching the nose closed and attempting to blow through the nose (known as the “Valsalva Maneuver”) to equalize the pressure. If either the ascent or descent (particularly the descent) is too rapid, and the pressure is not relieved, a painful condition called “ear block” can develop. Ear block can produce severe inner-ear pain and loss of hearing that can last from several hours to several days. If not treated, fluid can accumulate in the middle ear, which can become infected. In extreme cases, eardrum rupture can occur.
Reportedly, the two 2725-kg at 9 mps observation elevators that express 410 m from the ground- to the 103rd-floor observation deck in the Chicago Sears (now Willis) Tower building had to be slowed to 8 mps in order to minimize the problems and potential litigation associated with ear block. Reportedly, one building visitor suffered a broken eardrum sometime after descending from the observation deck via the shuttles when they were running at the original contract speed.
In order to better understand the problem and suggest some solutions that may assist in designing future, mega high-speed, high-travel shuttle lifts, it would be beneficial to review how airlines handle the problem. Most jet aircraft cruise at altitudes of 9,100-12,200 m above sea level, while the cabin is re-pressurized to a maximum of 2,450 m to protect the crew and passengers from discomfort. After takeoff, the cabin is re-pressurized at a nominal ascent rate of 1.75 mps, even though many jets climb at a rate of 15-20 mps. This combination of pressurization and ascent speeds apparently agree with the passengers, and little discomfort is normally experienced. However, because of the difficulty some people have in clearing their inner ears’ Eustachian tubes, the descent process is much more complicated. During descent, the cabin is re-pressurized at a nominal descent rate of 1.75 mps after the aircraft descends to 2,450 m, while the actual descent is accomplished at approximately 2.5 mps. At this rate, it would take approximately 23 min. to increase cabin pressure to that at sea level. Notice that the salient points here are that ascent can be accomplished very rapidly with little discomfort, while descent must be carefully controlled. Have you ever noticed a baby crying on an airplane during descent? The baby cannot consciously clear his or her ears, so when the inner-ear pressure builds up, causing pain, the baby cries in response. Voila! The painful inner-ear pressure is naturally cleared!
The easiest solution to major, high-rise, high-speed lift depressurization problems likely encountered in 100-story-plus, sky-lobby shuttle elevators would be to ascend at approximately 10-15 mps, and to descend at no more than 2.5-4 mps. Another method would be to install sky-lobby breaks at every 75-100 stories. Passengers going to and from higher building floors and sky lobbies would transfer between the sky lobby by using inter-zone shuttles, getting a chance to depressurize and re-pressurize en route to their final destinations. This system of “feeder” shuttles is the reason Ohbayashi Corp. indicated it would take approximately 15 min. for a person to go from the ground to the top floor in its proposed 500-story, 2,000-m-tall Aeropolis 2001 tower planned for Tokyo Bay. The most difficult problem would be designing a series of re-pressurization locks or holding areas to be located at the top elevator sky-lobby terminal. There, the lift passengers would be re-acclimated before boarding the shuttle lifts for the descent. Under this scenario, elevator hoistways or cabs would have to be enclosed and pressurized, along with the adjacent, pre-board airlock. The advantage of this scheme is that the lift passengers could wait for the lifts in a pre-pressurization holding/waiting lock, and then board the lifts for a very rapid descent (speeds of 10-15 mps would not be uncommon) to the grade level. This scheme would also permit the lifts to express to heights in excess of 200 stories without having to transfer between intermittent sky-lobby shuttles (the feeder-lift scheme) or to travel down at very slow speeds. The mega height/speed scheme also dramatically reduces the time it takes to reach the top floors and total passenger transit times.
Psychological Waiting-Time Standards
Over the years, the following design parameters and waiting-time standards have been developed for elevators in “Class A” office towers. The standards are for morning up-peak conditions (elevator ascents) and are designed to mollify the human expectations about what is an acceptable wait for an elevator:
- Sky-lobby shuttle – average interval: ≤ 28-30 s.; group-handling capacity: ≥ 15-25% of combined local zone populations moved; transit-time to destination (calculated at one-half the average interval, plus the total time on the lift): ≤ 60-90 s. – en route from main lobby to sky lobby
- Local lifts – vertical transportation – average interval: 25-30 s.; group-handling capacity: ≥ 12-15% of zone population moved; average time to destination (calculated at one-fourth the round-trip time, plus one-half the average interval): ≤ 60 s. – from sky lobby to local destination floor; and average waiting time: ≤ 20s.
If mega high-rise, sky-lobby shuttle elevator travels and speeds must be slowed to comply with the maximum recommended ear-pressure changes, the waits for elevator descents at the sky lobby(ies) may increase from 30 s. to 2 min. If these potential elevator riders must wait for service at the sky lobbies while the elevators are cycling – or must enter a pre-descent, pressurization air lock before boarding elevators – it will probably be desirable to provide audio/visual screens showing short subjects to minimize the boredom of the wait. Similarly, audio/visual screens may be installed in the shuttle elevators to accomplish the same purpose during slow descents to the ground form the sky lobbies.