Elevators and Value

Elevators use floor area and capital, but they also determine how people move through a tall building. Their number, size and arrangement affect waiting, travel time, service access and the space available for other uses.

Elevator cars and waiting time

Floors 10 ft apart; 8 riders per trip; 60 SF of shaft per car per floor.

Illustrative morning up-peak model, not a traffic study. Riders arrive at the lobby and leave at their floors; 8 riders board per trip and the floors are 10 ft apart. Expected stops S = N × (1 − (1 − 1/N)^8); highest reversal floor H = N − Σ (i/N)^8 for i = 1 to N − 1. Round-trip time RTT = 2·H·t_v + (S + 1)·t_s + 2·8·t_p, with t_v = floor height ÷ rated speed, t_s = 8 s per stop for doors, start and stop, and t_p = 1.2 s per rider each way. Interval = RTT ÷ cars. Handling capacity = 8 riders × 300 s ÷ interval, shown as a share of residents (2 per home). Assumed peak demand is 5% of residents in five minutes; average wait = interval × (0.5 + 0.5 × demand ÷ capacity), so it is half an interval when cars run light and one interval when they run full. The target band of 30–45 s is an assumed design range for this illustration, not a cited standard, and the test is 45 s; the car speeds, the 5% five-minute demand and the 60 SF of shaft per car per floor are likewise assumptions. Cars are drawn at 47 s into the cycle, each trailing the one before by one interval. Core area is 60 SF per car per floor plus 120 SF per car for machine and pit allowance. No code compliance, cost or rent premium is claimed.

A design with too little vertical transport may create queues or make the building inconvenient to use. Adding cars and shafts has its own costs and spatial effects. The right balance depends on building height, occupancy patterns, peak demand, elevator performance and the owner’s service expectations.

Take a 12-floor building with 192 homes and cars rated at 300 ft/min. With 1 car, the round trip takes 120 seconds. A rider waits 117 s on average, far above a 45 s target. The model assumes the morning peak, when everyone rides up from the lobby.

With 2 cars, the interval between cars halves to 60 s and the average wait falls to 44 s. That just meets the 45 s target. The 2 cars take 1,680 SF of shafts and machine space.

With 3 cars, the wait is 26 s, comfortably inside the target. The 3 cars take 2,520 SF.

With 4 cars, the wait is 19 s. The fourth car cuts the wait by less than the third did, and it adds another 840 SF of shaft.

Now make the same building 28 floors tall, still with 3 cars. The round trip grows to 185 s because the cars travel farther and stop more. The average wait is 66 s, above the 45 s target. The cars can move only 4.3% of the 896 residents in five minutes, against an assumed peak demand of 5%.

A taller building needs more cars, faster cars or both. At 28 floors, 4 cars at 600 ft/min bring the wait down to 27 s on a 134 s round trip. They also take 7,200 SF of shaft and machine space, against 2,520 SF for 3 cars in the 12-floor building.

That makes the elevator plan an early design question rather than leftover core space. These figures come from a simplified model with stated assumptions. A traffic study can help compare options for the proposed building, but no simple elevator count guarantees a good experience or a particular rent premium. Vertical circulation also needs to work for service and accessibility, not only the busiest passenger period.