Traffic
How many elevators a tower needs, and why the answer is not a rule of thumb
The factors that really determine capacity: building use, occupancy profiles, waiting times and simulation criteria.
2 min read
Two numbers describe whether a building moves people well, and neither of them is the number of elevators. The first is handling capacity: the share of the building population the bank can carry in five minutes. The second is interval: how long a person waits, on average, between one car leaving and the next arriving.
Both come from one thing: round trip time
Round trip time is how long a car takes to leave the main landing, serve its stops, and come back ready to load again. It adds up travel at rated speed, acceleration and deceleration on every stop, door opening and closing, and the seconds each passenger takes to walk in and out. Interval is that time divided by the number of cars in the bank. Handling capacity is car capacity divided by that same time.
This is why adding a car helps less than people expect on a tall building: the round trip time grows with travel and with the number of probable stops, so a taller bank spends more of its cycle moving and less of it loading.
The arrival pattern changes everything
An office tower has an up-peak: almost everyone arrives in the same half hour and travels in one direction, from one landing. A residential tower has two-way traffic spread across the morning, with people leaving from many floors. A hotel has luggage, a check-out peak and service circulation that should not share a car with guests. Same height, same population, three different banks.
What a control system can buy back
Before adding a shaft, the control is worth looking at. Group control coordinates the bank instead of letting each car answer calls on its own. Peak service control changes the pattern during the busy half hour. Destination dispatch goes further: it asks for the floor before boarding and groups passengers going to the same place, which cuts the number of stops per trip and therefore the round trip time itself.
To get a bank sized: floor count, floor-to-floor height, population per floor and building use. Four numbers, and the answer comes back with car capacity, rated speed and the number of shafts.
