Selecting size and number of lifts

The size (capacity and speed) and number of lifts for an office building are chosen so that the peak traffic (morning arrival or evening departure) is handled within an acceptable time and waiting interval.

Steps

1. Population: net rentable area ÷ area per person (about 8–12 m²/person in offices).
2. Peak demand: persons to be moved in the critical 5 minutes (handling capacity 12–15 % of population in 5 min for offices), or the whole population in a given time (e.g. emptying in 60 min).
3. Choose car capacity and speed: capacity 8–20 persons (68 kg each); speed rises with building height (about 1 m/s up to 5 floors, 1.5–2.5 m/s for 10–20 floors, higher for taller).
4. Round trip time (RTT) = running time up and down (2H/v) + door and stop time (number of probable stops × time per stop) + loading time at ground floor.
5. Handling capacity per car = car capacity × (time period / RTT).
6. Number of lifts = required persons per period ÷ capacity of one car; round up.
7. Interval (waiting time) = RTT / number of lifts; should be about 25–40 s for offices.
8. Motor size of each lift from P = (unbalanced mass) × g × v / η.
9. Provide at least one fire/service lift, and connect lifts to standby supply.

Example in brief: 2000 persons, 10-person cars, RTT = 162 s → each car moves 10 × 3600/162 ≈ 222 persons/h; to move 2000 in an hour, 2000/222 = 9.01 → 10 lifts.