A Life-Cycle Cost Comparison of Exit Stairs and Occupant Evacuation Elevators in Tall Buildings

Recent changes in the International Building Code (IBC) require a third exit stair for buildings in excess of 420 ft (128 m) high. Additionally, the new code provision allows for occupant evacuation elevators (OEE) to be used as an alternative to the third stair, provided the passenger elevator be protected in such a way to facilitate safe building evacuation. In this study, we evaluate the life-cycle costs of these alternative means of egress, using two prototypical building designs. Building '42F' is a 42 floor, 504 ft (154 m) high building with a total floorspace of 1.68 million ft(2) (0.16 million m(2)). Building '75F' is a 75 floor, 900 ft (274 m) high building with a total floorspace of 3.38 million ft(2) (0.31 million m(2)), including an 8403 ft(2) (781 m(2)) sky lobby. The life-cycle cost of the OEE is compared to two exit stair designs, differentiated by width: 44 in. (112 cm) and 66 in. (168 cm). The wider exit stair conforms with another change to the IBC that requires the increase in width of exit stairs by 50% in new sprinklered buildings. The results of the economic analysis demonstrate that: (1) an additional exit stair is a cost-effective alternative to the installation of OEE on a first-cost basis; and (2) OEE are a cost-effective alternative to the installation of an additional exit stair on a life-cycle cost basis when rental rates are moderate to high and when discount rates are moderate to low.