Building Design and Construction Handbook - Merritt - Ventech!

7.49 EFFECT OF HEAT ON STEEL
STRUCTURAL STEEL CONSTRUCTION 7.129
A moderate rise in temperature of structural steel, say up to 500�F, is beneficial in
that the strength is about 10% greater than the normal value. Above 500�F, strength
falls off, until at 700�F it is nearly equal to the normal temperature strength. At a
temperature of 1000�F, the compressive strength of steel is about the same as the
maximum allowable working stress in columns.
Unprotected steel members have a rating of about 15 min, based on fire tests of
columns with cross-sectional areas of about 10 in 2 . Heavier column, possessing
greater mass for dissipation of heat, afford greater resistance—20 min perhaps.
Columns with reentrant space between flanges filled with concrete, but otherwise
exposed, have likewise been tested. Where the total area of the solid cross section
approximates 36 in 2 , the resistance is 30 min, and where the area is 64 in 2 , the
resistance is 1 hr.
The average coefficient of expansion for structural steel between the temperatures
of 100 and 1200�F is given by the formula
C � 0.0000061 � 0.0000000019t (7.81)
in which C � coefficient of expansion per �F and t � temperature, �F.
Below 100�F, the average coefficient of expansion is taken as 0.0000065.
The modulus of elasticity of structural steel, about 29,000 ksi at room temperature,
decreases linearly to 25,000 ksi at 900�F. Then, it drops at an increasing rate
at higher temperatures.
7.50 FIRE PROTECTION OF EXTERIOR
Steel members, such as spandrel beams and columns, on the exterior of a building
may sometimes be left exposed or may be protected in an economical manner from
fire damage, whereas interior steel members of the same building may be required
to be protected with more expensive insulating materials, as discussed in Art. 7.51.
Standard fire tests for determining fire-endurance ratings of exterior steel members
are not available. But from many tests, data have been obtained that provide a basis
for analytical, thermodynamic methods for fire-safe design. (See for example, ‘‘Fire-
Safe Structural Steel—A Design Guide,’’ American Iron and Steel Institute, 1101
17th St., N.W., Washington, DC 20036.)
The tests indicate that an exterior steel spandrel beam with its interior side
protected by fire-resistant construction need only have its flanges fire protected.
This may be simply done by application of fireproofing, such as sprayed-on mineral
fibers, to the upper surface of the top flange and the under surface of the bottom
flange. In addition, incombustible flame-impingement shields should enclose the
flanges to deflect flames that may be emitted through windows. The shields, for
example, may be made of 1 ⁄4-in-thick weathering steel. This construction prevents
the temperature of the spandrel beam from reaching a critical level.
Exposed-steel columns on the outside of a building may be made fire safe by
placement at adequate distances from the windows. Such columns may also be
located closer to the building when placed on the side of windows at such distances