A moderate rise in temperature of structural steel, say up to 500F, is beneficial in that the strength is about 10% greater than the normal value. Above 500F, strength falls off, until at 700F it is nearly equal to the normal temperature strength. At a temperature of 1000F, 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 in2. 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 in2, the resistance is 30 min, and where the area is 64 in2, the resistance is 1 hr.
The average coefficient of expansion for structural steel between the temperatures of 100 and 1200F 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 100F, 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 900F. Then, it drops at an increasing rate at higher temperatures.