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What Determines How Many Elevators a Building Will Have?

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Answer by Mayur Kanaiya, Architect

Number of elevators (along with their size & speed) are primarily determined by design of the building, usable areas of each floor, number of floors, and height of each floor. The elevator system required is calculated (or estimated) accordingly based on:

1. Population to be served (represented by handling capacity of an elevator)
2. Passenger waiting time waiting for an elevator
 
Both are interrelated.
 
Population to be served: One needs to know the number of people the elevators are to be planned for. Figuring the number of people can be a tricky business, firstly one has to calculate the total population in a building which could be unknown if it's a speculative project, secondly, not all the people would want to go to the same floors at the same time.
 
Let's take the example of an office building. Office buildings can be single or multi-tenanted. They could have different offices at different floors or a single office occupying the whole building. In both cases, the number of people occupying each floor would be different and, to add to the complexity different, people or groups of people would have different office timings. You also need to factor in things like people working at home, on holidays, vacant posts etc.
 
Population of each office will depend on the type of office; call centers generally pack in the most people, whereas corporate floors are sparsely populated. In case the building is not know specifically, (as is not in most cases), there are standards set by various building authorities based on actual studies of population of various types of buildings. Each country has building codes or national standards as a recommended guideline.
 
Once the population is estimated, you need to figure out who wants to go where and at what time? Offices generally have peak traffic at morning going up to various floors, from the main lobby, between floors (and/or down) during lunch time and going down in the evening. In apartments, it can be totally different.
 
This is where traffic analysis comes in, to figure out peak travel numbers between various floors at various times, rate and duration of peak travel time (could be hundreds in a five minute period or sometimes spread over an hour). Traffic analysis also takes into consideration dispersal of population by all other modes of transportation such as stairs and escalators.
 
Once this is known, you need to then decide what the waiting time should be for an elevator.

Passenger waiting Time: This is a very important number (if not the most) and a major point in any building performance indicator. Normally people are more impatient in office buildings than residential ones; hence acceptable waiting times in apartments tend to be almost double of office buildings. Waiting times are again recommended by standards and codes based on user studies done over many years for different types of buildings. For example, a waiting time of less than 20 seconds is excellent and 40 seconds is poor for an office building and could be up to 90 seconds for apartments.
 
You could design an elevator system to serve highest number of people with high waiting times or least number of people with high waiting time or any of the other combinations depending on type of building.
 
Ideally, elevators should be designed to serve maximum number of people with minimum waiting time at the worst peak times so as to disperse the population fastest.
 
Unfortunately, one would then end up having a large number of elevators, which would be economically nonviable as:
- they cost a lot
- they put higher demands on electricity (to run) and have higher maintenance and running cost
- they would occupy a huge floor area which could otherwise be usable
 
The building design, too, plays an important role; in buildings with large floor plates, one group of lifts alone cannot meet all needs. These buildings are often divided into zones based on type of traffic and are served by separate groups of lifts to optimize the lift system.

There are recommended standards for these too, in terms of distance of lift banks or groups to reduce travel time from one location to the eventual exit.
 
In skyscrapers (over 40 floors), either double deck lifts are used or lifts are stacked in sky lobby arrangements where shuttle groups serve traffic between main entrance floor and sky lobby. Local lift groups start from sky lobbies and some all the way from main lobbies too. This is an effective method to optimize the number of elevators.

Last but not the least, number of elevators depends on the type of elevator itself. It plays a very important role in calculation of 'Round Trip Time' used to determine the number of elevators. Round Trip Time is basically the average time taken by a lift to complete one trip, from the main lobby to all the floors of the building and back, during up-peak traffic.
 
Waiting interval in seconds = Round Trip Time (RTT) / number of lifts

Round Trip Time calculation can get quite complex and I won't get into, Refer Steve Denton's answer to How do you optimize the cost of installing an elevator system in a building so that the number of people waiting at any one floor is minimum?and The Theory and Practice of General Analysis Lift Calculations

It's basically a calculation of the following:
  • The average number of passengers assumed to load into a car during up peak traffic
  • The average highest reversal floor
  • Effective population of the building
  • The average number of stops made by the lift during its round trip
  • The average time taken for a single person to load or unload the lift
  • Delay in time consumed by making a single stop
  • The time to travel a single floor and open/close the doors
  • The time taken for the lift to travel between two adjacent floors at rated speed

On Elevator standards used, from Elevator - Wikipedia:

The mechanical and electrical design of elevators is dictated according to various standards (aka elevator codes), which may be international, national, state, regional or city based.

Whereas once many standards were prescriptive, specifying exact criteria which must be complied with, there has recently been a shift towards more performance-based standards where the onus falls on the designer to ensure that the elevator meets or exceeds the standard.


Some of the national elevator standards include:
  • Australia - AS1735
  • Canada - CAN/CSA B44
  • Europe - EN 81 series (EN 81-1, EN 81-2, EN 81-28, EN 81-70, EN 12015, EN 12016, EN 13015, etc.)
  • USA - ASME A17

Because an elevator is part of a building, it must also comply with standards relating to earthquake resilience, fire standards, electrical wiring rules and so forth.


The American National Elevator Standards Group (ANESG) sets an elevator weight standard to be 2200 lbs.


Additional requirements relating to access by disabled persons may be mandated by laws or regulations such as the Americans with Disabilities Act.


References:
Elevator
http://www.peters-research.com/i...
Lift Passenger Traffic Patterns: Applications, Current Knowledge and Measurement
The Theory and Practice of General Analysis Lift Calculations
CIBSE Guide D: Transportation systems in Buildings. 
Nation Building Code (of India)
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