Elevator service is judged by two primary criteria: quantitative, or the number of persons who can be moved by the system within a defined peak traffic period, and qualitative, which expresses the calculated time between departing elevators during the same heavy traffic period.
Number of Elevators Required
The number of passenger elevators required for a particular building depends on the number of persons expected to work or live in the building. Traffic is measured by the number of persons requiring service during a peak 5-min period. For proposed buildings, a population estimate is generated on the basis of occupancy trends for that specific building type. Peak-traffic projections are based on the type of tenancy expected for the building. From the population and peak-traffic projection, the demand is established as a peak 5-min traffic flow. While peak traffic in most buildings is a rather complex pattern of two-way and interfloor movement, most models assume a simplified traffic pattern in which traffic is primarily incoming or outgoing. The lack of a complex model is more a result of the poor understanding of the existing model than of the absence of sophisticated measuring devices.
After the peak 5-min traffic flow is established, an estimate may be made of the quantity of elevators required. The ability of a specific system to handle the traffic is tested against the projected traffic level. The 5-min handling capacity of an elevator is determined from the round-trip time.
where V peak traffic, persons in 5 min. Equation (16.2) indicates that the minimum number of elevators required is directly proportional to the round-trip time for a car and inversely proportional to the car capacity.
Elevator-related space requirements may not be minimized through the use of the fewest elevators to serve a particular building, since large groups of highcapacity cabs must be employed to serve a large number of floors. Large groups of elevators increase cost of the overall system by increasing the average number of elevator entrances required for the building. For greatest efficiency and lowest cost, elevator group sizes should not exceed six elevators, with four elevators per group as a more practical approach. This method has the added advantage that passenger trip time—that is, the time it takes an individual to travel to a destination during peak traffic—is reduced due to use of smaller cabs and assignment of fewer floors to be served by a particular elevator group.
After an approximate quantity of elevators is found to meet quantitative traffic requirements, qualitative performance should be reviewed. The criteria for qualitative performance is generally based on the quality of service expected for a specific building, as well as the overall quality level of the project. Qualitative service is typically expressed as interval, or the calculated time between elevators departing the ground floor. Improving elevator service for a building, however, generally results in increased cost.
After the number of elevators has been computed on the basis of traffic flow, the average interval should be checked. It is obtained by dividing the round-trip time by the number of elevators.
Round-trip time is composed of all of the pieces of a projected elevator trip, including starting, running, and stopping of the elevator car, time for opening and closing doors, and time for passengers to move in and out. Often some factor is added to the round trip time to simulate normal use of the system.
Opening and closing of doors may contribute materially to lost time, unless the doors are properly designed. A 3-ft 6-in opening is excellent, because two passengers may conveniently enter and leave a car abreast. A slightly wider door would be of little advantage. Department stores, hospitals, and other structures served by larger passenger elevators (4000 lb and over) usually require 4-ft door openings.
Center-opening doors, preferred for power operation, are faster than either the single or two-speed type of the same width. The impact on closing is smaller with the center-opening door; hence, there is less chance of injuring a passenger. Also, transfer time is less, since passengers can move out as the door starts to open.
Another factor affecting passenger-transfer time is the shape of the car. The narrower and deeper a car, the greater is the time required for passenger entry and exit during peak-traffic conditions likely to be.
(See also Art. 16.9.8 and 16.9.9.)
Elevators in General-Purpose Buildings
For a proposed diversified-tenancy, or general-purpose, office building, peak traffic may be estimated from the probable population computed from the net rentable area (usually 75 to 90% of the gross area). Net rentable area per person typically ranges from 150 to 190 ft2. Some rare organizations occupy space at higher densities but averages for specific floors rarely become as dense as one person to 100 ft2.
Diversified-tenancy office buildings usually have important traffic peaks in the morning, at noon, and in the evening, and very little interfloor traffic. The 5-min morning peak generally is the controlling factor, because if the elevators can handle that peak satisfactorily, they can also deal with the others. In a well-diversified office building, the 5-min peak used is about 12.5% of the population.
For busy, high-class office buildings in large cities, time intervals between elevators may be classified as follows: 26 to 28 s, excellent; between 28 and 30 s, good; between 30 and 32 s, fair; between 32 and 35 s poor; and over 35 s may be unsatisfactory. In small cities, however, intervals of 30 s and longer may be satisfactory.
For express elevators, which make no intermediate stops, intervals of 30 to 35 s may be considered acceptable.
Car speeds used vary with height of building: 4 to 10 stories, 200 to 500 ft / min; 10 to 15 stories, up to 700 ft /min; 15 to 20 stories, up to 800 ft /min; 20 to 50 stories, up to 1200 ft /min; and over 50 stories, up to 2500 ft /min. Practically speaking, 200-ft /min elevators are generally not economically advantageous and have been replaced by 350-ft /min elevators for most passenger applications.
Elevators should be easily accessible from all entrances to a building. For maximum efficiency, they should be grouped near the center. Except in extremely large buildings, two banks of elevators located in different parts of the structure should not serve the same floors. Since one cannot guarantee equal use of the two groups, each group should be designed to handle 60 to 65% of the traffic.
Elevators cannot efficiently serve two lower terminal floors, inasmuch as cars stop twice to pick up passengers who are typically picked up once. The extra stop increases the round-trip time and decreases the handling capacity, resulting in the need for more elevators to satisfy the same traffic criteria. If there is sufficient traffic between the two lower floors, escalators or shuttle elevators should be installed, and one of the levels should be assigned as the sole terminal for the tower passenger elevators.
When laying out a local-travel elevator group, groupings should not exceed four elevators in line. This arrangement can be exceeded for groupings of express elevators where elevator arrivals can be preannunciated. Elevator core configurations must take into account the need for smoke control at elevator lobbies, as well as code limits on dead-end corridors. Lobby widths should be 9 to 12 ft, depending on the size of the elevators.
It is necessary to divide elevator groups into local and express banks in buildings of 15 floors and more, especially those with setbacks and towers, and in low buildings with large rental areas. In general, when more than six elevators are needed, consideration should be given to dividing the building into more elevator groups.
In addition to improving service, the division into local and express banks has the advantage that corridor space on the floors where there are no doors can be used for toilets, closets, and stairs.
While the decision to include a dedicated service elevator is often market driven, office buildings of less than 250,000 gross square feet typically ‘‘swing’’ a passenger elevator for off-peak deliveries and moves. Buildings of up to 500,000 or 600,000 gross square feet frequently have only a single service elevator, whereas larger buildings are provided with two or more separate service cabs. Where dedicated service elevators are provided, at least one should be hospital shaped, with the capability of carrying end-loaded 9-ft-long gypsum wall board. A 12-ft ceiling allows easy movement of carpet rolls and long conduit.
Elevators in Single-Purpose Buildings
Elevator requirements and layouts are similar, in general, for both single-purpose and diversified-tenancy office buildings; but several different factors should be taken into consideration: Single-purpose buildings are occupied by one large organization.
Generally, the floors that are occupied by the clerical staff are not subdivided into many offices; the net rentable area is about 80% of the gross area. Population densities are higher than for general-purpose buildings. Depending on the kind of business to be carried on, population density varies from 100 ft2 per person for some life-insurance companies to about 300 ft2 per person for some attorney’s offices.
Although traffic peaks occur at the same periods as in the diversified-tenancy type, the morning peak may be very high, unless working hours are staggered. The maximum 5-min periods may be 13.5 to 16.0% of the population, depending on the type of occupancy. If traffic volumes are high, occupancy of the building should be carefully balanced against elevator requirements. Although many floors may be connected with an eight-car elevator bank, the time wasted on elevators becomes excessive as a result of the number of stops made during each elevator trip.
In the past, system designers specified more elevators to meet interfloor traffic demands of single-purpose buildings. The advent of microprocessor-based controls, however, has dramatically improved system response to complex traffic patterns.
With such controls, an elevator system designed to handle the incoming traffic rush will also provide satisfactory service in response to interfloor traffic.
Elevator service in single-purpose buildings is frequently hobbled by location of a cafeteria or similar high-density, facility at some level above the ground floor. If such facilities are served by the office passenger elevators, the total elevator requirement can increase by 15 to 20% as a result of the inefficiency introduced by the cafeteria.
Elevators in Government Buildings
Municipal buildings, city halls, state office buildings, and other government office buildings may be treated the same as single-purpose office buildings. Population density often may be assumed as one person per 140 to 180 ft2 of net area. The 5-min maximum peak occurs in the morning and may be as large as 16% of the population.
Population cannot be used as the sole basis for determining the number of elevators needed for buildings occupied by doctors, dentists, and other professional people, because of the volume of patient and visitor traffic. Peaks may occur in the forenoon and midafternoon. The maximum occurs when reception hours coincide. Traffic studies indicate that the maximum peak varies from two to six persons per doctor per hour up and down.
Since crowding of incapacitated patients is inadvisable, elevators should be of at least 3000-lb capacity. If the building has a private hospital, then one or two of the elevators should be hospital-type elevators.
Hotels with transient guests average 1.3 to 1.5 persons per sleeping room and are typically populated based upon 90% occupancy. They have pronounced traffic peaks in morning and early evening. The 5-min maximum occurs during checkout hour and can be about 12.5 to 15% of the estimated population, with traffic moving in both directions.
Ballrooms and banquet rooms should be located on lower floors and served by separate elevators. Sometimes it is advisable to provide an express elevator to serve heavy roof-garden traffic. Passenger elevators should be of 3000-lb capacity or more to allow room for baggage carriers. Intervals for passenger elevators should not exceed 50 s.
Service elevators are very important in hotels. Hospital-shaped elevators are often preferred for handling linen and food service carts as well as baggage.
Typically, hotels are provided with one passenger elevator per 125 to 150 rooms.
The service elevator quantity is 50 to 60% of the passenger elevator quantity. The ratio of rooms per elevator is lower for better-quality hotels and higher for more modest facilities.
Elimination of noise and vibration from medium- to high-speed elevators is virtually impossible, so hotels should be carefully planned to ensure that guest rooms are not adjacent to elevator hoistways. Rooms that adjoin elevator hoistways may generate complaints throughout the life of the building.
Multistory residential buildings do not have peaks so pronounced as other types of buildings. Generally, the evening peak is the largest. Traffic flow at that time may be 6 to 8% of the building population in a 5-min period. Building population should be estimated in consideration of the market for which the building is designed.
If only one elevator is selected to satisfy traffic conditions for a building of modest height, residents will be forced to use the stairs at times the elevator is out of service for repairs. Where the elevator is considered more than an amenity, two elevators should be provided. Market conditions may require that a separate service elevator also be provided in some urban settings. Typically, a 2500-lb elevator with a 9-ft clear ceiling height can be relied on to carry most furniture.
As is the case with hotels, the potential for noise and vibration should be considered in location of elevators in living units.
Department stores should be served by a coordinated system of escalators and elevators. The required capacity of the vertical-transportation system should be based on the transportation of merchandising area and the maximum density to which it is expected to be occupied by shoppers.
The transportation area is all the floor space above or below the first floor to which shoppers and employees must be moved. Totaling about 80 to 85% of the gross area of each upper floor, the transportation area includes the space taken up by counters, showcases, aisles, fitting rooms, public rooms, restaurants, credit offices, and cashiers’ counters but does not include kitchens, general offices, accounting departments, stockrooms, stairways, elevator shafts, or other areas for utilities.
The transportation capacity is the number of persons per hour that the verticaltransportation system can distribute from the main floor to the other merchandising floors. The ratio of the peak transportation capacity to the transportation area is called the density ratio. This ratio is about 1 to 20 for a busy department store. So the required hourly handling capacity of a combined escalator and elevator system is numerically equal to one-twentieth, or 5%, of the transportation area. The elevator system generally is designed to handle about 10% of the total.
The maximum peak hour usually occurs from 12 to 1 pm on weekdays and between 2 and 3 pm on Saturdays.
The type of elevator preferred for use with moving stairs is one with 3500-lb capacity or more. It should have center-opening, solid-panel, power-operated car and hoistway doors, with at least a 4-ft 2-in opening and a platform 7 ft 8 in by 4 ft 7 in.
Traffic in a hospital is of two types: (1) medical staff and equipment and (2) transient traffic, such as patients and visitors. Greatest peaks occur when visitor traffic is combined with regular hospital traffic. Waiting rooms should be provided at the main floor and only a limited number of visitors should be permitted to leave them at one time, so that the traffic peaks can be handled in a reasonable period and corridors can be kept from getting congested. In large hospitals, however, pedestrian and vehicular traffic should be separated.
For vehicular traffic or a combination of vehicular and pedestrian traffic, hospital elevators should be of stretcher size—5 ft 4 in to 6 ft wide and 8 or 9 ft deep, with a capacity of 4000 to 5000 lb. Speeds vary from 100 to 700 ft /min for electric elevators, depending on height of building and load. For staff, visitors, and other pedestrian traffic, passenger-type elevators, with wide, shallow platforms, such as those used for office buildings, should be selected (see Arts. 16.11.2 and 16.11.3).
Elevators should be centrally located and readily accessible from the main entrance.
Service elevators can be provided with front and rear doors, and, if desired, so located that they can assist the passenger elevators during traffic peaks.
In low-rise buildings, freight elevators may be of the hydraulic type (Art. 16.10), but in taller buildings (higher than about 50 ft) electric elevators (Art. 16.9) generally will be more practical. Figure 16.17 shows the components of an electric freight elevator.
In planning for freight elevators, the following should be considered:
1. Building characteristics, including the travel, number of floors, floor heights and openings required for a car. Also, structural conditions that may influence the size, shape, or location of the elevator should be studied.
2. Units to be carried on the elevator—weight, size, type, and method of loading.
3. Number of units to be handled per hour.
4. Probable cycle of operation and principal floors served during the peak of the cycle.
5. Freight elevators are not permitted to carry passengers.
For low-rise, slow-speed applications, especially where industrial trucks will be used, rugged hydraulic freight elevators generally will be more economical than electric freight elevators.
Classification of Freight Elevators. The ‘‘American National Standard Safety Code for Elevators, Dumbwaiters, Escalators and Moving Walks,’’ ANSI A17.1, defines three classes of freight elevators. Class A applies to general freight loading.
This is defined as a distributed load that is loaded or unloaded manually or by hand truck and no unit of which, including loading equipment, weighs more than onequarter the rated load of the elevator. Class B elevators may handle only motor vehicles. Class C elevators may be subjected to heavy concentrated loads and fall into one of three subclasses. Class Cl applies to elevators that carry industrial trucks, Class C2 to elevators for which industrial trucks are used only for loading and unloading the cars and are not carried by them, and Class C3 to elevators carrying heavy concentrated loads other than trucks.
Car Capacity of Freight Elevators. The size of car to be used for a freight elevator is generally dependent on the dimensions of the freight package to be carried per trip and the weight of the package and loading equipment. Power trucks, for example, impose severe strains on the entire car structure and the guide rails than do hand trucks. As a power truck with palletized load enters an elevator, most of the weight of the truck and its load are concentrated at the edge of the platform, producing heavy eccentric loading. Maximum load on an elevator should include most of the truck weight as well as the load to be lifted, since the truck wheels are on the elevator as the last unit of load is deposited.
The carrying capacity per hour of freight elevators is determined by the capacity or normal load of the elevator and the time required for a round trip. Round-trip time is composed of the following elements:
1. Running time, which may be readily calculated from the rated speed, with due allowance for accelerating and decelerating time (about 21⁄4 s for ac rheostatic control with inching, 13⁄4 s for multivoltage), and the distance traveled.
2. Time for operation of the car gate and hoistway doors (manual 16 s, power 8 s).
3. Loading and unloading time (hand truck 25 s, power truck 15 s). Wherever practical, a study should be made of the loading and unloading operations for a similar elevator in the same type of plant.
Operation of Freight Elevators. The most useful and flexible type of operation for freight elevators is selective-collective with fully automatic doors. Attendant operation requires an annunciator. When operated with an attendant, the car automatically answers the down calls as approached when moving down and similarly answers up calls when moving up. The elevator attendant, when present, has complete control of the car and can answer calls indicated by the annunciator by pressing the corresponding car button. The addition of fully automatic power-operated doors means the elevator is always available for use, unless taken out of service by the attendant.
The standard hoistway door is the vertical biparting, metal-clad wood type. For active elevators and openings wider than 8 ft, doors should be power-operated.
Automatic freight elevators can be integrated into material-handling systems for multistory warehouse or production facilities. On each floor, infeed and outfeed horizontal conveyers may be provided to deliver and remove loads, usually palletized, to and from the freight elevator. The elevator may be loaded, transported to another floor, and unloaded—all automatically.