Occupancy classification and building use

The first question in any code review is what the building is used for. Occupancy classification sorts buildings and parts of buildings into groups according to the hazard their use presents: how many people are present, whether they are awake and alert, whether they can leave on their own, whether they know the building, and how much combustible material and hazardous activity there is. Almost every later requirement depends on the classification, including the permitted building size, the construction type, fire separations, sprinklers, exits, plumbing fixtures and accessibility. A wrong classification can make the whole code analysis wrong.

This lesson explains the occupancy groups in your country's code, how to classify a building with more than one use, how to deal with accessory spaces and how a change of occupancy triggers new requirements. Construction managers need this knowledge to understand design decisions, to spot problems when an owner changes the intended use during construction, and to manage tenant improvements in existing buildings.

Why occupancy drives the code

Consider three buildings of the same size: a warehouse, an office and a nightclub. The warehouse may have few people but a large fire load from stored goods. The office has more people, but they are awake, familiar with the building and spread out. The nightclub has many people crowded together in low light, possibly impaired, unfamiliar with the exits and focused on a stage. The life safety risk in the nightclub is far higher, so the code requires more exits, wider egress, more fire protection and tighter limits on size and construction. Hospitals and prisons add another dimension: occupants who cannot evacuate on their own, so the building must protect them in place.

Codes therefore classify by use rather than by building type. A single building may contain several occupancies, such as shops on the ground floor, offices above and parking below. Each part is classified separately and the code then decides how the parts must be separated or how the whole building must be treated.

The IBC occupancy groups

The International Building Code classifies buildings and structures, or parts of them, into ten occupancy groups, most divided into numbered subgroups. State and local codes based on the IBC use the same letters.

Classifying a building step by step

Classification is a structured process. Experienced designers follow roughly the same steps for every project.

• List every use: sales floor, storage room, offices, staff room, kitchen, seating area, parking, mechanical rooms.
• Assign each use to an occupancy group using the code definitions and the examples in the code's appendix or commentary. Where a use is not listed, classify it by the group it most closely resembles in hazard.
• Decide which uses are separate occupancies and which are accessory or subsidiary to a main use.
• Identify the major or main occupancy of the building and any others that need separation or special treatment.
• Record the classification in the code analysis on the drawings, with floor areas for each occupancy.

IBC Section 302 requires each building or portion to be classified in one or more groups, and where a structure is proposed for a purpose not specifically listed, it is classified in the group it most nearly resembles based on fire safety and relative hazard. Rooms that are accessory to the main use, such as a storage room in a store, are often classified with the main occupancy, as discussed below.

Mixed occupancies

IBC Section 508 gives three options for buildings with more than one occupancy. Accessory occupancies are portions not exceeding 10 percent of the floor area of the story they are on and not exceeding the tabular area for that occupancy; they need not be separated and are classified with the main occupancy for most purposes. Nonseparated occupancies are not separated from each other, but the most restrictive requirements of all the occupancies present apply to the whole building, and fire protection requirements apply to each portion according to its own group. Separated occupancies are divided by fire barriers or horizontal assemblies rated according to Table 508.4, and the building area is checked so that the sum of the ratios of each occupancy's actual area to its allowable area does not exceed one.

The choice affects cost. Separating occupancies adds rated walls, floors, doors and dampers but may allow a cheaper construction type or avoid a requirement that would otherwise apply to the whole building. Not separating them saves rated construction but imposes the most restrictive rules everywhere. Designers usually test both options early in design.

Special uses and conditions

Some uses attract special requirements beyond their basic occupancy group. Codes have specific provisions for high buildings, buildings with interconnected floor spaces such as atriums, underground buildings, parking garages, covered malls, health care facilities, buildings containing hazardous materials, and assembly spaces with stages and platforms. Construction managers should flag these early, because they often bring smoke control, emergency power, special fire alarm features and commissioning that affect schedule.

In the IBC, Chapter 4 contains special detailed requirements based on occupancy and use, including covered and open mall buildings, high-rise buildings (an occupied floor more than 75 ft above the lowest level of fire department vehicle access), atriums, underground buildings, motor-vehicle-related occupancies, Group I-2 and I-3 facilities, hazardous materials, stages and platforms, and storm shelters. Each section adds requirements on top of the basic occupancy rules.

Occupant load

Occupancy classification answers what the space is used for; occupant load answers how many people the code assumes are in it. The occupant load drives exit capacity, the number of exits, plumbing fixture counts and some fire protection triggers. It is calculated by dividing the floor area by an occupant load factor, in area per person, for the use, unless the design provides fixed seats or the owner proposes a higher number. Lesson 7 uses occupant loads in detail for egress.

Change of occupancy and tenant improvements

Buildings outlive their first use. Warehouses become offices, offices become schools, stores become restaurants and old industrial buildings become apartments. A change of occupancy can bring a building under requirements it was never designed for: more exits, sprinklers, fire separations, accessibility upgrades, more plumbing fixtures, higher floor live loads and energy upgrades. Construction managers on renovation projects should identify proposed changes of occupancy early, because the cost of bringing an old building up to the new use can exceed the cost of the interior fit-out.

In the US, the International Existing Building Code governs repairs, alterations, additions and changes of occupancy where it is adopted. It offers a prescriptive method, a work area method with three levels of alteration, and a performance method. A change of occupancy that moves the building into a higher hazard category, judged by tables in Chapter 10 for means of egress, heights and areas and exposure of exterior walls, triggers upgrades in those areas. Some jurisdictions still use Chapter 34 of older IBC editions or local rules for existing buildings.