REVOLUTIONARY
LIFT DESIGNS FOR
MEGA-HIGH-RISE BUILDINGS

by J.W. Fortune
Lerch, Bates & Associates, Inc.

Background

For decades, vertical transportation design engineers have been trying to devise methods for placing more than one elevator car (lifting pod) in a hoistway, to reduce the number of dedicated lifts and hoistways required. The adaptation of double-deck and soon triple-deck lifts, sky-lobby floors and local-zone elevator stacking plans are all attempts to place more than one elevator in the same hoistway space. Frank Lloyd Wright's Mile High Building was designed to be equipped with 76 atomic-powered, rack and pinion, quintuple-deck elevators. Even with this novel, futuristic design, the lower portions of the project still had nothing but elevator shafts. Mitsubishi elevator and escalator engineers estimate that almost 30% of the total floor space in a 100-floor skyscraper must be devoted to elevators, hoistways, loading lobbies and machinery spaces. To plan and construct 200- to 500-story mega-high-rise towers, new ways of moving people and goods vertically and horizontally must be devised. The key to this dilemma is to dedicate lift shafts in which more than one transport pod can move vertically at a time, a horizontal run/out travel path at the terminal/sky lobby floors, and lateral dock (unloading/loading) so that a door/loading/ unloading failure does not shut down the system (Figure 1).

Even with the latest development of ACV3F gearless hoist motors, converter/inverter drives and extra-high-strength steel hoist ropes, traction elevators have a limitation of roughly 5000 kilograms of capacity, a 10-15 mps maximum speed and a 600-750 meter vertical-rise practical limit. A high-speed traction, roped, counterweighted elevator is about 90+% efficient and consumes little electricity to lift its rated load.

New transit technologies being considered might employ linear-induction motor (LIM) drives or rare earth permanent magnets and be capable of moving both horizontally and vertically in and out of a building's dedicated transit shaft. The main problem with these advancing technologies is the cost and difficulties of matching the efficiencies of today's common-roped, counterweighted traction lift. Presently, it is difficult if not impossible to construct a vehicle that can overcome gravity and lift itself, as well as a payload in an efficient manner. Even if the technology were available, estimates indicate such a device would consume four to eight times the power of a similar, conventional-roped elevator.

Any 75- to 125-story office tower planned today will likely be designed with one or two sky lobbies that are to be serviced by double-deck or possibly triple-deck, roped shuttle lifts. Utilizing sky lobbies and their shuttles, multiple local zones of lifts may be stacked on top of one another, significantly reducing the number of lift shafts that penetrate the building's lower floors.

Discussion

The twin 110-story New York World Trade Center (WTC) towers are a perfect example of stacked office building applications. Each 33-story portion is serviced by four zones ­ each with six single-deck local lifts, with the local lifts of zone II and III separated by two sky lobbies. Building tenants or visitors desiring elevator transport to the floors of zone II or III must first travel on a sky-lobby shuttle elevator to the upper sky-lobby and then transfer to the appropriate local lift for final transport to their destination. A passenger departing an upper-zone floor must first travel via a local lift to the sky-lobby and then transfer to a sky-lobby shuttle elevator for final travel to the ground floor. The WTC sky-lobby shuttles are some of the largest passenger elevators ever constructed. They have a duty of 4,500 kilograms at 8.0 mps and are equipped with front and rear openings, to ease passenger flow. Each cab has a net platform area of 7.8 square meters and is designed to accommodate up to 50 persons, with 30- to 35-person nominal load. Each tower is equipped with 23 shuttle units, with 12 units capable of serving Zone II's 44th-level lower sky lobby and 11 units having service capability to Zone III's 78th-level upper sky lobby.

When the WTC towers were constructed 20 years ago, it was envisioned that each tower might have an eventual population of about 22,500 persons (rental density figured at 12-m2/person). Because of a shift in population demographics and densities (presently about 23-m2/person), increased rents and more upscale private tenants, each tower presently has a population of about 12,500 persons. Thus, only eight shuttles are actively utilized for transport to and from each sky lobby. Two of the shuttles are now exclusively devoted to the top-level (110th-floor) observation deck or restaurant service; three others are utilized for interzone service between the dual sky lobbies, while the other two units have been converted to exclusive-service functions.

If the WTC towers were constructed today (12,500 persons population), the single-deck sky-lobby shuttles would no doubt be provided as double-deck units (ála Sears Tower and the Petronas Towers) or possibly as triple-deck shuttles (Figure 2).

If the WTC towers were to be duplicated in the first part of the 21st century, the sky-lobby lift designs would probably replace or supplement the shuttle elevators with magnetically levitated lifting pods that could possibly replace the shuttles. The purpose of this paper is to explore some possible solutions for replicating the function of the shuttle elevators while utilizing fewer dedicated vertical hoistways. Two solutions are possible:

1. A transit system where individual transport pods move horizontally under their own power and then board conventional-roped elevators for vertical transport to their sky-lobby destination or transfer floor. The vertical-lifting elevator transport "car" can be either single-deck or double-deck units. A very tall structure can be equipped with multiple-stacked express shuttle lifts, so that the transit pods can continue to "climb" through different sky-lobby/transfer/floors to their final destination (Figure 3). The vertical lifts can either be dedicated to one-way travel (either up or down) or they can transport the pods up to a destination/transfer floor, eject the unit and simultaneously board a down-traveling pod for the return trip (requires front and rear openings). With double-deck lifts, the lower and upper decks can eject and board transit pods at the same time. Otis Elevator Co. is presently offering a system called Odyssey that employs transit pods (Transitors) and conventional elevators for vertical transport. In theory, a building of unlimited height could be practically constructed using this technology. The exchange of up-traveling pods and down-traveling pods between the elevator shuttles is referred to as the "three-car shuffle" by Otis. Each passenger pod could transport 12-25 persons in a seated or standing position.

2. A futuristic, completely new technology not yet developed would be similar to the "sky train" pod system indicated in Figure 1. For lack of a better term, this method of moving people shall be referred to as the "Transpod System." It is composed of a number of self-powered pods similar to the Otis Odyssey system but with the capability for each to also move vertically within their own dedicated shafts. As building demand increases during peak periods, more transit pods would become active, and the headways between pods would be reduced, increasing the group system's handling capacity. The initial transpod systems would probably be set up to replicate sky-lobby shuttles. Later systems could be configured to provide express and then local service right to a tenant's floor or suite.

Analysis

The analysis for the sky-lobby shuttles are indicated on attached Summary Charts 1 and 2. Utilizing existing Zone II (44th Floor) and Zone III (78th Floor) building populations, we analyzed how many single-deck (existing), double-deck or triple-deck elevator units would be required to handle the evening down-peak traffic. Next, we looked at the same zone population, evening down-traffic demands and design criteria utilizing a horizontal-traveling, inject/eject transit-pod type system which uses conventional elevators for vertical transportation. Such a system has recently been introduced by Otis and is called the Otis Odyssey system (now available). Finally, we performed the same analysis with a futuristic, transpod-type of individual car operation where independently powered, ropeless multiple cars (pods) can move vertically and horizontally within dedicated hoistways.

The lunchtime down-peak or evening-down peak period is selected for sky-lobby lift service designs because it generally has the most intense people handling capacity requirements. In an office building, we normally figure that the morning up-peak period lasts about one hour (12 five-minute periods) when the building tenants arrive at the ground floor and require vertical transport to their work location. Translating the five-minute arrival periods into population percentages, each average five-minute period is then a 1/12 (8.3%) indicated group handling capacity requirement. The peak five-minute period is normally about 1-1/2 to 2 times the average or a required 12-15% group handling capacity requirement for the morning up-peak shuttle service to the sky-lobby floors.

The lunchtime/evening down-peak group handling capacity requirements can be much more intense, as it is assumed that the majority of the building tenants would depart during a 45-minute peak period (nine five-minute periods). This equates to an 11% average handling capacity or about a 15-20% five-minute down-peak requirement. The local elevators, feeding tenants down to the sky-lobby floors during the evening exiting period, will often fill to their "bypass" capacity (set at 50-70% of the load) and, after making two to three local stops, express down to the sky-lobby floor, bypassing any further calls. At the sky-lobby, the passengers depart to catch their shuttle cars or "pods," and the local car immediately closes its doors and is dispatched to pick up another load. Because the local cars make few hall stops but a greater number of round trips during the down-peak, they can easily handle 15-20% of the zone traffic during this period. Therefore, the sky-lobby shuttles or transport pods must be designed to also handle these peaks. If not, all building tenants being fed to the sky-lobby levels cannot be handled in a timely manner and backup queues will begin to form.

Conclusions

The various methods of handling the sky-lobby traffic utilizing the existing WTC populations, traffic patterns and populations indicate that double-deck or triple-deck shuttles can clearly save elevators and shafts over the present single-deck configurations as shown in Chart 3.

The Otis Odyssey system (Figure 3) can only save dedicated sky-lobby shuttles by utilizing double-deck transport lifts (two pods can load and unload simultaneously) because the pod sizes are presently limited to about a 25-person capacity. The pod weight and size would also be restricted by the existing WTC shuttle capacities and hoistway sizes. Calculations with and without hoistway doors and car gates are shown because we don't know if they will be required by the model building codes.

An added advantage of the Odyssey or transpod system is the ability to accommodate various types of pods (Figure 4). Odyssey could minimize the number of full-stop goods/ firefighters' lifts required in a mega-high-rise building (local goods lifts would still be required), while the transpod system could eliminate the need for them entirely!

The transpod system would also use 25-person capacity pods and require far fewer vertical shafts, because it would be a simple matter to add more pods to the system as the demand increases to a minimum headway of 30 seconds. (This is our estimated safety time between pods transitioning vertically.)

The results of this analysis indicate that an integrated vertical and horizontal building transit system could revolutionize mega-high-rise building construction while making the building cores far more efficient than present elevatoring technology dictates.

Originally published in "Multi-purpose High-rise Towers and Tall Buildings," Proceedings of the Third International Conference "Conquest of Vertical Space in the 21st Century." Reprinted with permission from the Concrete Society.

James W. Fortune graduated from Pasadena City College with an A.A. degree in architecture and from California State Polytechnic University with major in architecture and a B.S. degree in industrial technology. After a stint in the U.S. Navy, he served with Westinghouse Elevator Division in Los Angeles, California for three years, then joined Lerch, Bates & Associates ­ Denver, Colorado Headquarters ­ in 1971 as a staff engineer, later becoming project manager and regional vice president. In 1979, he relocated to Los Angeles as the consulting firm's vice president and West Coast zone manager and, in 1985, became the company's East Coast and West Coast vice president, while relocating to Denver Headquarters office. He obtained his MBA degree from the University of Denver in 1989 and was elected president of Lerch, Bates & Associates in 1994.