Page 208 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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BUILDING CODES, LOADS, AND FIRE PROTECTION*

4.36 CHAPTER FOUR

4.12.3 Fireproof Buildings
In the past, the term fireproof was frequently used to describe fire-resistant buildings. The use of this
and terms such as fireproofing is unjustified and should be avoided. Experience has clearly demon-
strated that large-loss fires (in terms of both property losses and loss of life) can and do occur in fire-
resistant buildings. No building is completely fireproof, given the wide range of uncertainties that
influence the development of and effects of real fires, and the previously mentioned limitations of
fire-resistance ratings.
(Fire Protection Handbook, National Fire Protection Association, Quincy, Mass.)

4.12.4 Effect of Temperature on Steel
The properties of virtually all building materials are adversely affected by the temperatures devel-
oped during standard fire tests. Structural steel is no exception. The effect of elevated temperatures
on the yield and tensile strengths of steel is described in Art. 1.12. In general, yield strength decreases
with large increases in temperature, but structural steels retain about 60% of their ambient-temperature
yield strength at 1000°F.
During many real building fires, temperatures in excess of 1000°F develop for relatively brief
periods of time, but failures do not occur in structural steel members inasmuch as they are rarely
loaded enough to affect full design strength. As a consequence, in some instances, bare structural
steel has sufficient load-carrying capacity to withstand the effects of fire. This is not recognized in
the standard fire tests, however, because in the standard ASTM E119 tests, the temperatures are con-
tinuously increased while structural members are loaded to design capacities. Based on these tests,
when building codes specify fire-resistant construction, they require fire-protection materials to “insu-
late” structural steel elements.

4.12.5 Fire-Protection Materials
A variety of different materials or systems are used to protect structural steel. The performance of
these are directly determined during standard fire tests. In addition to the insulation characteristics
evaluated in the tests, the physical integrity of fire-protection materials is extremely important and
should be preserved during installation. Required fire-protection assemblies should be carefully
inspected during and after construction to ensure that they are installed and maintained according to
the manufacturers’ recommendations and the appropriate fire-resistant designs.
Gypsum. Gypsum, in several forms, is widely used for fire protection (Fig. 4.8). As a plaster, it is
applied over metal lath or gypsum lath. In the form of wallboard, gypsum is typically installed over
cold-formed steel framing or furring.
The effectiveness of gypsum-based fire protection can be increased significantly by addition of
lightweight mineral aggregates, such as vermiculite and perlite, to gypsum plaster. It is important
that the mix be properly proportioned and applied in the required thickness and that the lath be cor-
rectly installed.
Three general types of gypsum wallboard are readily available: regular, Type X, and proprietary.
Type X wallboards have specially formulated cores that provide greater fire resistance than conven-
tional wallboard of the same thickness. Proprietary wallboards, such as Type C, also are available with
even greater fire-resistant characteristics. It is therefore important to verify that the wallboard used
is that specified for the desired fire-resistant design. In addition, the type and spacing of fasteners
and, when appropriate, the type and support of furring channels should be in accordance with
specifications.
(“Design Data—Gypsum Products,” Gypsum Association, Washington, D.C.)
Spray-Applied Materials. The most widely used fire-protection materials for structural steel are
lightweight mineral fiber and cementitious materials that are spray-applied directly to the contours
of beams, girders, columns, and floor and roof decks (Fig. 4.9). The spray-applied fire-resistive mate-
rials (SFRM) are based on proprietary formulations. Hence it is imperative that the manufacturer’s

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