Page 205 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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BUILDING CODES, LOADS, AND FIRE PROTECTION*

BUILDING CODES, LOADS, AND FIRE PROTECTION 4.33

is less relevant. In the United States and Canada, the vast majority of cities, states, and provinces now
enforce one of the following model codes:

• International Building Code (IBC), International Code Council, Falls Church, Va.
• NFPA 5000, Building Construction and Safety Code, National Fire Protection Association (NFPA),
Quincy, Mass.
• National Building Code of Canada, National Research Council of Canada, Ottawa, Ontario
In the United States, some vestiges of the previous three model building codes (National Building
Code, Standard Building Code, and Uniform Building Code) may also remain. Regardless of the
code used, two fire-related characteristics of materials influence selection and design of structural
systems: combustibility and fire resistance.

4.12.1 Combustible and Noncombustible Materials
Most fires are either accidental or caused by carelessness. Fires are usually small when they start,
and require fuel and ventilation (air supply) to grow in intensity and magnitude. In fact, many fires
either self-extinguish due to a lack of readily available fuel or are extinguished by building occu-
pants. Furthermore, even though most fires involve building contents, a combustible building itself
may be the greatest potential source of fuel.
By definition, noncombustible materials such as stone, concrete, brick, and steel do not burn and
therefore do not serve as sources of fuel. Although the physical properties of noncombustible mate-
rials may be adversely affected by elevated temperature exposures, these materials do not contribute
to either the intensity or duration of fires. Wood, paper, and plastics are examples of combustible
materials.
Tests conducted by the National Institute for Standards and Technology (formerly the National
Bureau of Standards) indicate that an approximate relationship exists between the amount of avail-
able combustible material (fire loading, pounds of wood equivalent per square foot of floor area) and
fire severity, hours of equivalent fire exposure (Fig 4.6). Subsequent field surveys measured the fire
loads typically found in buildings with different occupancies (Table 4.18). More modern fire load
surveys have been expressed in terms of the potential heat energy of the combustible contents.
A reasonable estimate of the structural fire loading for conventional wood-frame construction is
2
1
7 / 2 to 10 lb/ft . For heavy-timber construction, the corresponding structural fire load may be on the
2
1
1
order of 12 / 2 to 17 / 2 lb/ft . As a consequence, building codes generally limit the permitted size
(allowable height and area) of combustible buildings to a much greater degree than for noncom-
bustible buildings.
However, the potential fuel of the combustible construction or contents is not the only variable
that influences fire severity. Ventilation is another major fire parameter, and its effects have been
included in more modern analytical models of natural fires.
4.12.2 Fire Resistance
In addition to regulating building construction based on the combustibility or noncombustibility of
structures, building codes also specify fire-resistance requirements as a function of building occu-
pancy and size, i.e., height and area. In general, fire resistance is defined as the relative ability of
construction assemblies, such as, floors, walls, partitions, beams, girders, and columns, to prevent
spread of fire to adjacent spaces and to avoid structural collapse when exposed to fire. Fire-resistance
requirements are based on laboratory tests conducted in accordance with “Standard Methods of Fire
Tests of Building Construction and Materials” (ASTM E 119).
The ASTM E119 test method specifies a “standard” fire exposure that is used to evaluate the rel-
ative fire resistance of construction assemblies (Fig. 4.7). Fire-resistance requirements are specified
in terms of the time during which an assembly continues to prevent the spread of fire, does not
exceed certain temperature limits, and sustains its structural loads without failure, when exposed to

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