Building Construction, Fire Resistance, and Means of Egress
Construction type and fire resistance
The five construction types describe how much structural fire resistance a building provides: Type I (noncombustible, protected, highest fire resistance), Type II (noncombustible), Type III (exterior noncombustible, interior combustible permitted), Type IV (heavy timber), and Type V (combustible). The A/B suffix sets the fire-resistance rating: Type IA commonly 3-hour structural frame and 2-hour floor construction, IB 2-hour, IIA 1-hour, IIB unprotected, IIIA 1-hour, IIIB unprotected, VA 1-hour, VB unprotected. Fire resistance also lives in exterior wall ratings tied to fire separation distance, opening protectives (fire doors, shutters, fire-rated glazing with the correct rating and labeling), shaft and stair enclosures, and fire/smoke dampers in penetrations.
Egress as a system
The means of egress is exit access, exit, and exit discharge. Check the components as one continuous path: occupant load and required number of exits, egress width, common path of travel, dead-end corridors, travel distance, door swing and clear width, panic hardware, stair geometry, handrails, exit signage, and emergency illumination.
Numbers worth memorizing (verify in the adopted edition): egress capacity of 0.2 inch per occupant for stairways and 0.15 inch per occupant for other components; stairway width minimum 44 inches; handrails 34–38 inches above tread nosings; dead-end corridors generally 20 feet, extended to 50 feet in sprinklered buildings of many groups; common path of egress travel commonly 75 feet unsprinklered and 100 feet sprinklered for Business; travel distance for Business 200 feet unsprinklered, 300 feet sprinklered.
Traps: measuring travel distance along the path rather than straight-line; forgetting that adding sprinklers increases allowable travel; missing that required exit count is set by occupant load, not by the two-exits assumption; accepting a fire door with an unlabeled or incorrect rating; counting a door that swings against egress travel.
Worked example: A sprinklered 12,000 sq ft Business floor at 150 gross yields 80 occupants. Two exits are required; the two doors provide far more than the 80 × 0.15 in = 12 in of required width, so the governing problem is geometry-common path and travel distance-not capacity.