How Buildings are Classified in the Building Code

By Rob Aman, PE, SE, LEED AP

The planning and design process for private or public building construction is a critical component for a successful project. Every building project faces its own unique set of challenges, including finances, site location, schedule, public approval, environmental impacts, owner satisfaction, and meeting building code requirements. While the decision or need to construct a building typically determines its use and function, the size, shape, height, construction materials, and structural systems utilized tend to develop during the process. The building code plays a role in defining and shaping the building’s aspects by requiring adherence to a method of classification. The current 2021 International Building Code (IBC) requires that all buildings and structures, existing and new, are classified under one of two categories:

• Use and Occupancy (Chapter 3)
• Type of Construction (Chapter 6)

Use and Occupancy

The IBC has ten main occupancy groups as well as multiple subgroups. The occupancy group or subgroup defines the specific use of the building. Subgroups are numbered based on the perceived risk for the building occupants. The lower the subgroup number, the higher the perceived risk. Occupancy groups and subgroups are defined as follows:

In many cases, a building has space that may be classified under more than one occupancy group. These multi-use buildings are identified as a Mixed occupancy type. However, if a building space can be considered as either an Accessory occupancy or an Incidental Use area, it may become part of the main occupancy group.

Accessory occupancies are those that represent a lower hazard or risk as compared to the main occupancy. Their aggregate area per story cannot exceed 10% of the floor area and they generally do not require any separation from the main occupancy. Incidental use areas provide minor support to the main occupancy, such as storage and mechanical spaces. They generally pose a greater level of risk to the main occupancy and therefore have special requirements for fire separation from the main building. Also worth noting is that each incidental use space must be less than 10% of the floor area on the story they are located.

For a building that falls under a mixed occupancy type, two categories may be considered: Separated and Nonseparated occupancies. A nonseparated occupancy allows for multiple occupancies with no separation between them. The area and height limits, construction type requirements, and fire protection requirements are based on the most restrictive occupancy category within the space. A separated occupancy requires physical separation by a fire barrier and/or horizontal assemblies (or floors) between the different spaces. The construction type requirements are based on the height of each separated area and the sum of ratios of the actual area divided by the allowable area for each separated occupancy. Each area is considered separate with regards to fire protection.

Occupancy type determines the vertical live loading requirements for the building structure. For higher-risk buildings such as a fire station, school, or theater, lateral loads (e.g. wind and seismic) and snow will require increases to the loading requirements.

Construction Type

Construction Type identifies the type of materials utilized for constructing a building and classifies the level of combustibility and fire resistance associated with the building elements of a structure. As listed under Table 601 in the IBC, these building elements include the primary structural frame, exterior and interior bearing and non-bearing walls, and the floor and roof construction elements. As indicated in Table 601, these building elements are required to have a fire-resistance rating of 0, 1, 2, or 3 hour, which indicates the amount of time it can continue to confine a fire and also maintain a level of structural integrity.

Non-combustible materials generally include concrete, masonry, and steel building elements while combustible material typically refers to wood framed building elements that do not meet Heavy Timber requirements.

For construction types I, II, III, and V, structural building elements must also be classified as either A or B, resulting in a total of nine types (i.e. Types IA, IB, IIA, etc.):

Many buildings will require or benefit from utilizing more than one construction type, which is determined by factors such as code or durability requirements, architectural design, and construction costs. In these cases, portion(s) of the building are separated by construction type with a fire rated wall or horizontal (floor) assembly, allowing each area to be treated as a separate building for meeting building code requirements. This condition commonly occurs in podium type construction, where a multi-story wood framed apartment building (Type VA) sits above a concrete podium parking or commercial type structure (Type IA).

Impact of Building Height & Areas on Construction Type

In general, the Construction Type(s) allowed for a building, or its separated portions, are limited by the building height and floor area based on its Use and Occupancy. For each of the ten Occupancy groups listed above, Table 503 of the IBC has set limitations on building heights and floor area corresponding to each of the nine construction types. Where portions of a building are required by code to be fire separated by construction type and/or occupancy type, each portion can be treated separately for determining allowable floor areas. In some cases larger building areas with common construction and occupancy type are fire separated into smaller portions to meet the code requirements for allowable areas.

Installation of a fire sprinkler system will also provide additional fire protection in a building, and will provide an increase of the allowable heights and areas listed in Table 503. Generally, a fire sprinkler system is required by the code for the following reasons:

The construction type is the key driver to defining the vertical and lateral force resisting structural systems that will be utilized on a project. Each of the four major materials typically used (concrete, masonry, steel, and wood) have their advantages and code limitations that should be considered during the early planning stages in order to develop an efficient and cost-effective structural system.

While the architect is typically responsible for identifying the occupancy, construction type, and fire protection requirements for a new building or retrofit of an existing building, the role of the structural engineer is to understand the overall structural design and detailing requirements related to each of these elements, identify alternate cost-effective construction solutions, and provide the most efficient and robust structural system based on the project goals and requirements.

Please email Rob Aman (rob@aman-structural.com) with any questions or comments related to this article.