Occupancy Classification and Fire Safety Requirements
Classify first-everything else follows
Almost every requirement downstream (construction type, sprinklers, egress, separations, fire alarm) is a function of occupancy group and occupant load. Learn the major groups as functional families: Assembly (A-1 through A-5), Business (B), Educational (E), Factory (F-1/F-2), High-hazard (H-1 through H-5), Institutional (I-1 through I-4), Mercantile (M), Residential (R-1 through R-4), Storage (S-1/S-2), and Utility/Miscellaneous (U). Classification is based on the actual use and the occupants' capability to escape without help-not on the sign on the door or the zoning designation.
The high-yield distinctions
Assembly subcategories hinge on occupant load, seating, and whether occupants are standing or seated: small venues versus large concentrated assembly behave very differently. E versus I-4 turns on whether occupants are capable of self-preservation (day care: more than five children under 2-1/2 years old pushes toward I-4). R-2 versus R-3 turns on the number of dwelling units and whether occupants are transient. H occupancies split by material and process, not merely by quantity stored.
Mixed and accessory uses
Where uses coexist, apply the mixed-occupancy rules: either separate them with rated assemblies or treat the building as the more restrictive group with the required safeguards. Incidental uses (small shops, storage rooms, boiler rooms) have their own separation thresholds. Accessory occupancies are subordinate and limited in area and load.
Occupant load
Compute it from the area divided by the table factor for the function, then use the larger of the calculated and the actual/design number. Factors are listed as net (usable floor area) or gross (whole floor). Assembly concentrated (standing) at 5 net and unconcentrated (tables and chairs) at 15 net have been stable for decades-good anchors. Business and mercantile factors have shifted between editions, so verify the adopted table.
Traps: using gross when net is specified; rounding down when the code requires rounding up to the next whole person; classifying by building name rather than use; forgetting that a small assembly inside a larger building may still be an assembly occupancy.
Rule of thumb: classify by hazard and occupant capability, then compute load twice-calculated and design-and design to the larger.