Abstract
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 ft2 (0.16 million m2). Building ‘75F’ is a 75 floor, 900 ft (274 m) high building with a total floorspace of 3.38 million ft2 (0.31 million m2), including an 8403 ft2 (781 m2) 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.
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Notes
Passenger elevators must meet specific criteria to be used for occupant evacuation purposes [3].
Because the cost estimates contained in the Exit Stair Cost Analysis document are for July 2007 in Washington, DC, all other cost information presented in this article uses July 2007 as its reference date and the greater Washington, DC metropolitan area as the building’s location.
CostWorks contains information on typical ranges of risers for various story heights. The prototypical buildings in Table 4 have a story height of 12 ft. For a story height of 12 ft, the average number of risers is 21. The minimum tread width for commercial buildings is 11 in. The cross sectional area in the calculation of loss of rental space includes two landing areas, two stair areas, a gap area between stairs, and surrounding wall space. One flight of stairs would have 11 risers. The other flight of stairs would have 10 risers. The landing area would be slightly larger on the side where there are 10 risers. The increase in the landing area is the same as the decrease in the stair area. For the calculation, we would use 11 risers to calculate the length of stairs with no increase in the landing area. For the case where the stair width is 44 in., the stair length is 11 in. × (11 − 1) = 110 in. In this calculation, 1 is subtracted from 11 because the last step of a flight of stairs is on the landing area. The gap between the stairs in our calculation is 4 in. The wall is 8 in. in thickness. With a stair width of 44 in., the width of a landing area is also 44 in. Therefore, the length of the cross sectional area is 8 in. × 2 + 44 in. × 2 + 110 in. = 214 in. The width of the cross sectional area is 8 in. × 2 + 44 in. × 2 + 4 in. = 108 in. The cross sectional area is therefore 214 in. × 108 in. = 23,112 in.2 = 160.5 ft2. In the case Building 42F, where there are 42 floors, the loss of rental space due to an additional stairwell is 160.5 ft2. × 42 = 6741 ft2.
The nominal rate of $100.00/ft2 for building floorspace is based on data compiled from RS Means, CostWorks [6].
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Butry, D.T., Chapman, R.E., Huang, A.L. et al. A Life-Cycle Cost Comparison of Exit Stairs and Occupant Evacuation Elevators in Tall Buildings. Fire Technol 48, 155–172 (2012). https://doi.org/10.1007/s10694-010-0203-8
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DOI: https://doi.org/10.1007/s10694-010-0203-8