High-Rise Office Renovation: Occupant Load and Exit Capacity
An existing 12-storey office tower is being renovated floor by floor. On a typical floor of 1,400 m², the tenant proposes open-plan workstations for 130 people, two meeting rooms of 40 m² each, a 60 m² lunch room, and a server room. The floor currently has two exit stairs, each 1,100 mm wide, and one bank of two elevators. The corridor serving the floor is 1,500 mm wide. The designer argues that because the building is sprinklered and has a voice communication system, the exit width is automatically adequate. During review, the building official notes that the stair doors swing into the corridor and that one stair discharges through the lobby.
1. Calculate the occupant load for the floor and test it against the exit capacity, stating each assumption.
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Compute occupant load by use: business/office area at the applicable floor area per person (commonly 9.3 m²/person for office), assembly-type meeting rooms at their assembly floor area per person (commonly 1.5 m²/person for unconcentrated seating), and the lunch room at its applicable rate. Example: office area = 1,400 − 80 − 60 − server ≈ 1,250 m² at 9.3 = ~134 persons; meeting rooms 80 m² at 1.5 = ~54; lunch room 60 m² at, say, 1.5 = ~40; total ~228 persons. Then apply egress width: the required width per person for stairs and level passages (e.g., 9.2 mm/person for stairs, 5 mm/person for corridors in many Canadian-style codes) times the occupant load, and compare with provided width. Two stairs at 1,100 mm each = 2,200 mm; required stair width for 228 persons at 9.2 mm = ~2,100 mm, so marginally adequate - but the calculation must use the governing factor and the correct per-person value for the actual occupancy mix, not a blanket assumption. Sprinklers and voice communication may permit a reduction in the number of exits or an increase in travel distance, but they do not eliminate the width calculation. Show the arithmetic and the code values used.
2. Identify the egress deficiencies implied by the corridor, door swing, and stair discharge, and state the corrective actions.
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Three issues. (1) Corridor width: 1,500 mm may be adequate for the calculated load, but the required corridor width is the greater of the minimum (often 1,100 mm) and the width needed for the occupant load; confirm by calculation. If the load requires more than 1,500 mm, widen or reduce load. (2) Doors swinging into the corridor: exit doors serving a stair or exit should not reduce the required corridor width when open, and doors in a means of egress must swing in the direction of travel where the occupant load is high (commonly 100+ persons). If the stair doors swing into the corridor and obstruct the required width, they must be re-hung, recessed, or the corridor widened. (3) Stair discharge through the lobby: an exit stair must discharge directly to the exterior or through an enclosed path to the exterior; discharging into the lobby is acceptable only if the lobby is separated from the rest of the floor by fire separation and the path is protected. Otherwise, provide a protected discharge corridor. Also confirm the two stairs are remote from each other and that the elevator lobby is not the sole path.