Introduction:

Occupancy Groups are defined by building code in order to help categorize and classify the use of spaces. These influence and control so many different things such as maximum allowable area to build, along with how the building must be separated from other buildings, how many people can be in rooms and much more.

When figuring out a building location and size, the occupancy group of the structure, along with its construction type go hand in hand.

Definition

Ancillary: having a subordinate, subsidiary, or secondary nature, or serving as a supplement or addition.

Accessory: a thing which can be added to something else in order to make it more useful, versatile, or attractive.

Automatic Door Closing: A door that is normally held in the open position but is released to close on activation by a smoke detector or other type of fire alarm system. An automatic closing door must be self-closing.

Occupancy Group

The occupancy group of a building or interior space is its use – every single building or portions of a building get classified according to the space’s main use/function. Uses are grouped into occupancy classes based on similar life-safety characteristics, fire hazards, and combustible contents.

• If a particular program does not seem to fit any of the categories, the Architect should consult with the local building official for a determination of the occupancy classification of that project.

Life Safety

The idea behind Occupancy Groups is essentially that some uses are inherently more dangerous than others. For example, there are different risks associated with factories compared to restaurants.

In order to achieve equivalent safety in building design, each Occupancy Group has different fire protection requirements that are meant to address the risk of that program. This includes area and height limitations, restrictions on types of construction materials, and means of egress.

• Specific design conditions may also require further more stringent fire protection requirements such as underground buildings, atriums, etc.

Occupancy Group Affects

1. Maximum area
2. Maximum floors allowed
3. How the building must be separated from other structures.
4. Calculation of the occupant load
5. Egress design
6. Interior finish requirements
7. The use of fire partitions and fire barriers
8. Fire detection and suppression systems
9. Ventilation and sanitation requirements
10. Other special restrictions particular to any given classification

International Building Code (IBC) Classifications

The IBC classifies 10 Occupancy Groups, with 6 of these being further divided into smaller categories to distinguish different subgroup requirements.

1. Assembly
2. Business
3. Educational
4. Factory and Industrial
5. Hazardous
6. Institutional
7. Mercantile
8. Residential
9. Storage
10. Utility

Mixed Use Occupancy

It’s common to encounter building programs that require multiple different occupancies. When a building has two or more occupancies, it is considered Mixed Occupancy.

Fire Barriers

Different occupancies must be separated from each other by a Fire Barrier. You will find a table (IBC Table 508.4) in the code which specifies the value for the barrier ranging from between 1 and 4 hours. The barrier will increase the level of protection between occupancies as the relative hazards of the program increases. This value influences the design and detailing of the separation wall or system.

• Unseparated mixed occupancies are allowed in many cases, where there is no separation between the Occupancy Group. The maximum allowable area and height of a building or a portion of a building is based on the most restrictive allowance for the occupancy groups under considerations.

Sprinkled

When a building is equipped with an automatic sprinkler system, the required ratings are generally reduced by 1 hour. This can mean that a fire-rated partition listed as requiring 1 hour separation, is not required when the building is equipped with a sprinkler system. This can help offset the cost of a sprinkler system by reducing the design requirements of the separation.

Maximum Area and Height Requirements

For multistory buildings of mixed occupancy, each story must individually comply with the sum of ratios as listed below. In addition, for non-sprinklered buildings more than three stories above grade, when the area of each story is expressed as a ratio of its actual area to its allowable area, the sum of the ratios cannot exceed 3. When the building is sprinklered, it cannot exceed 4.

• The allowable area and height must be based on the more restrictive allowances for the occupancy groups under consideration for the type of construction of the building.
• When the area of each separate occupancy is expressed as a ratio of its actual building area to its allowable building area, the sum of the ratios in each story must not be greater than one.
  • For Example: Occupancy A is .32 of its allowable building area. In other words, occupancy A takes up 32% of the allowable building area. Therefore the actual building area of occupancy B may not be more than .68 of its allowable building area, or 68%. 
  • .32 + .68 = 1.00
• Each separate occupancy much comply with the building height limitations based on the type of construction

Mixed Use Occupancy Exceptions

In the IBC, there are two variations of the concept of mixed occupancies: accessory occupancies and incidental uses. Each of these have their own particular requirements.

Accessory Occupancies

An accessory occupancy is a space or room that is ancillary to a main occupancy but that does not exceed 10% of the floor area of the story in which it is located. An accessory occupancy does not need to be separated from the main occupancy with a fire barrier.

Exceptions

There are many exceptions to this rule, examples listed here.

• Most kinds of Group H occupancies must be separated from other occupancies.
• Dwelling units and sleeping units in group I-1, R-1, R-2, R-3 occupancies must be separated from accessory with fire partitions

Real Use Accessory Example

A small gift shop in a hospital would be considered an accessory occupancy. Therefore, it would not require 2-hour occupancy separation that would ordinarily be required between an M occupancy and I-2 occupancy.

Incidental Uses

An incidental use is a program space that is ancillary to the main occupancy and has the same classification as the nearest main occupancy, but poses a greater level of risk than the occupancy group. The incidental use must be separated from the main occupancy by a fire barrier, be equipped with an automatic sprinkler, or both. Similar to accessory, an incidental use cannot exceed 10% of the floor area of the story on which it is located.

Real Use Incidental Use Example

A small incinerator room in an apartment building would pose a greater risk to the apartment buildings than the residential units themselves. Therefore, it would need to be separated with a 2 hour rated partition and be fully sprinkled.

• Some exceptions allow a sprinkler system to substitute for a fire barrier. The incidental use area would then be separated from the main occupancy by a smoke barrier, and the sprinklers are required only in the incidental use area. Doors would need to be self closing or automatic closing.

Changes in Occupancy

Not incredibly common, but you may encounter new owners or through the passage of time, a building needing to change its occupancy. This has become more common to talk about in recent years as a gluttony of office buildings becomes considered for residential habitation.

It’s important to make sure that the new occupancy a client wants to switch to will still comply with the old occupancy requirements in terms of construction and egress/fire. If it perhaps is more hazardous of a program, there may need to be additional Fire Walls or other architectural elements to bring the occupancy into compliance. This is one of the things that makes changing occupancy potentially more expensive than what it first seems.

Locating a Building on the Site

There are many design factors that go into figuring out where a building should be located on a site. Additionally, many urban area plots may be controlled or at least influenced by the Occupancy Group and Construction Type. IBC Section 602 lists requirements for the siting of buildings relative to the adjacent property lines based on these.

• Purpose of this is to help limit and prevent the spread of fire from one building to another
• If the structure located on the property line has building openings such as windows, provisions in IBC Chapter 7 regulate the maximum allowable area of those openings.

Two Typical Approaches

There are two typical approaches in the planning and initial design:

1. Building must be placed a certain distance from the property line (program dictates, maximizing footprint, area, etc) then the code specifies the minimum fire-protection rating and limitations on openings.
2. To minimize cost of fire rated walls, the allowable open areas, or both, then the minimum setback can be determined and complied with.

For obvious reasons, the larger the site, the easier this section is to comply with. Urban areas can require a lot of program juggling to make the building in compliance, while still creating useful and functional spaces.