The number of elevators, along with their size and speed, that a building will have is determined by several factors. The design of the building, number of floors, usable areas of each floor, and height of each floor all factor into how many elevators are installed.
There are two main factors that determine which type of elevator system will be required. One is the population to be served and the other is passenger waiting time.
Population to be Served
First the architect needs to know the number of people the elevators will be transporting. This can be a tricky figure to calculate. They figure it out by calculating the total population of the building, which can sometimes be unknown. Once the population is estimated, they need to figure out who wants to go where and at what time. For office buildings, peak traffic time is normally the morning, lunch time, and evening. However, for an apartment building it can be completely different.
This is where traffic analysis comes in. This will help figure out peak travel numbers between various floors at various times and duration of peak travel time. Traffic analysis will also take into consideration people using other modes of transportation such as stairs or escalators. After all this data is collected, what the waiting time should be needs to be decided.
Passenger waiting Time
This is one of the most important numbers when figuring out building performance. Typically people are more impatient in an office setting than residential ones, so waiting times tend to be much higher in apartment buildings. There are standards set in place based on previous studies for what is an acceptable waiting time. Usually office buildings want to keep wait times under 20 seconds, while apartment wait times can reach up to 90 seconds. Ideally, elevators should be designed to serve a maximum number of people with minimum waiting time at the worst peak times to help disperse the population quickly. Unfortunately this is usually hard to achieve because it would mean having a large number of elevators and that is financially not always an option. Commercial elevators can be expensive, require more energy to operate, and occupy a huge floor area, decreasing the amount of usable space.
Round Trip Time
Lastly the number of elevators that will be in a building depends on the type of elevator itself. This plays a very important role in the calculation of round trip time. Round trip time is used to determine how many elevators are needed. It is 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 peak traffic.