Due to the thermal elongations coupled with reductions in steel strength and stiffness that occur at elevated temperatures, even minor member end restraint, imperfections, crookedness, or force eccentricity can initiate visible local flange and/or web buckling, or overall member buckling, above about 600 °F (315 °C). With complete restrain from thermal expansion, these may occur at temperatures as low as 250 °F (120 °C). Buckling is very likely to occur at temperatures in the 1,200 to 1,400 °F (650 to 760 °C) range, when the strength and stiffness are less than 50 percent of their nominal ambient values. Past experience from flame curving and straightening indicates that local buckling often can occur quite suddenly at, and above, this temperature range.
In addition to these buckling distortions of the member, the steel will experience increasing end rotation and vertical deflections during the fire from the existing dead and live loads. Under fire conditions, both for uncontrolled natural exposures and in standard tests, the temperature-induced deflections of fire-resistive steel beam/concrete floor systems can be large. Actual fires have produced deflections ranging from several inches up to, in extreme cases, 3 to 4 ft, which are an order of magnitude greater than the normal serviceability limits that are anticipated for buildings. In this sense, it must be remembered that the intended structural outcome of fire safety design is to maintain building integrity and prevent (or delay) catastrophic collapse, despite suffering potentially extensive structural and nonstructural damage. Thus, even rated fire-resistive construction will often experience major structural and nonstructural damage during a severe fire.
