Building Anatomy: Types and Classifications

Christopher J. Naum examines building types and classifications to anticipate variables in structural integrity and resiliency to the effects of extreme fire behavior for firefighters.


Today’s evolving fireground demands a greater understanding of buildings, occupancy risk profiling (ORP) and building anatomy by all companies operating on the fireground. The identification, assessment, probability, predictability and intrinsic characteristics of building performance under fire...


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2. Second arabic number – Columns, beams, girders, trusses and arches, supporting bearing walls, columns or loads from more than one floor

3. Third arabic number – Floors

Type I. Fire resistive. In this type of construction the structural elements consist of non-combustible materials, usually steel or concrete, that afford a fire-resistance rating that provides a given fire protection performance endurance against the effects of fire.

• These specific ratings are determined by the model building codes for a specific type of construction

• These specific ratings apply to the roof and floor assemblies as well as any exterior or interior bearing support walls

• Interior partitions are required to be constructed with approved non-combustible materials

• The fire-resistance ratings are provided by different designs that meet minimum performance

Type II: Non-combustible. The same requirements that apply to Type I construction also apply to this type of construction, with some differences.

• This type of construction may not afford any fire-resistance rating for the exposed structural elements

• If any fire protection of the structural elements is provided, it is at a lesser rating than that required for Type I construction; in this type of building the structural elements are usually made of steel, bolted, riveted or welded together

• This type of construction is susceptible to expansion, distortion or relaxation of the steel members, resulting in early collapse during a fire

• Again, interior partitions are required to be constructed with non-combustible or approved limited-combustible materials

Type III: Ordinary. In this construction type, all or part of the interior structural elements may be combustible. Exterior walls are required to be constructed with non-combustible materials. They can have a fire-resistance rating, depending on the horizontal separation and whether they are bearing or non-bearing walls.

• This category usually is divided into protected and unprotected subtypes; the building will have masonry exterior walls and wooden structural members and combustible interior construction

• The building generally will not exceed six stories and most often will be two or three stories in height

• Floor and roof supports are usually wood, but other materials, such as steel bar joists, may be found

• Floor and roof decking most frequently will be plywood or composition board

• Common walls between buildings may share wall sockets for floor joists and roof rafters

Type IV: Heavy timber. Heavy-timber structural members – columns, beams, arches, floors and roofs – are unprotected wood with large cross-sectional areas.

• A minimum dimension of eight inches for structural wood supports (columns, beams, arches and girders) is required

• All other exposed wood must have a minimum dimension of two inches; concealed spaces usually are not permitted

• These buildings consist of masonry (non-combustible) exterior walls and structural members of substantial timber construction

• Commonly, this type of construction is found in older factories and mills; however, there is a resurgence in their use in various new occupancy types

• Wood floors generally will have a minimum thickness of three inches and may be oil-soaked from years of oiling heavy machinery

• Roof supports will be wood with minimum dimensions of four by six inches, and a minimum roof decking thickness of 11/8 inches

Type V: Wood frame/combustible. This type of construction uses structural members entirely of combustible materials, usually wood, and is divided into two subgroups: protected (structural elements protected as required) or unprotected (no fire-resistance requirement).

• Post-and-beam construction has a wood frame of substantial dimension and is sided with a lightweight covering such as wood boards or plywood covered with aluminum or PVC siding; this type of construction is commonly used for barns, sheds and other storage buildings, but also may occur in dwellings and other occupancies