Building Design and Construction Handbook - Merritt - Ventech!

16.38 SECTION SIXTEEN
is tested against the projected traffic level. The 5-min handling capacity of an
elevator is determined from the round-trip time.
300
HC � P (16.1)
T
where HC � handling capacity of car, persons in 5 min or 300 s
P � car capacity, persons
T � round-trip time of car, s
The minimum number of elevators n required can then be computed from
V VI
n � � (16.2)
HC 300P
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.