Dill¹ concludes that, while exposure to fire will almost certainly cause warping and twisting of members, it does not inevitably follow that the strength of the steel is reduced. It is almost certain that any steel that has been heated hot enough to undergo damaging grain coarsening or that has been cooled rapidly enough to harden it will be so badly distorted that it would have no consideration for re-use anyway. This leads to the general statement that steel that has been through a fire but that can be made dimensionally re-usable by straightening with the methods that are available may be continued in use with full expectation of performance in accordance with its specified mechanical properties. Essentially then, the question is one of economics: if the steel can be straightened for less money than fabricating and installing a new piece, then that should be done. 

Two possible exceptions to the above include quenched and tempered structural steels and high-strength fasteners. The mechanical properties of such heat-treated items may be affected by prolonged fire exposure and should be tested to determine the effects of the fire, if any. 

Another reference is Council on Tall Buildings and Urban Habitat². See also 11.2.4.

[1] Dill, F.H., 1960, "The Effects of Fire on Structural Steel," Proceedings of the 1960 AISC National Engineering Conference, AISC, Chicago, IL.

[2] Council on Tall Buildings and Urban Habitat, 1980, Monograph on Planning and Design of Tall Buildings, Volume CL, Tall Building Criteria and Loading, ASCE, Reston, VA.

Common structural steel grades exhibit similar deterioration of mechanical properties at elevated temperatures. Thus all structural grades perform in essentially the same way. Over the years, there have been efforts in several countries to introduce a “fire-resistant” steel grade into construction. This type of steel reportedly has somewhat improved properties at elevated temperatures. However, the use of this steel remains very limited in construction, mainly because improved mechanical properties of steel at elevated temperatures, in general, do not translate into significant increases in the fire resistance of respective building elements and systems. 

Elevated temperatures are likely to develop faster in members than in connections, making connections less critical for fire-protection design. The connections usually contain more material (additional plates, bolts, etc.) than the connected members. Also, connections often have less exposure to heat and higher capacity for heat dissipation because of their proximity to other members.

It should be kept in mind that steel is born in a melting process that is significantly hotter than any building fire. Significant residual stresses are therefore present in all newly manufactured steel members. A detailed discussion of post-fire steel assessment issues is provided in:

R. H. R. Tide, “Integrity of Structural Steel After Exposure to Fire”, Engineering Journal, First Quarter, 1998, pp. 26-38.

A general rule of thumb reads: “If it is still straight after exposure to fire – the steel is OK”. Straightening techniques are also available for steel members that have been misaligned after fire exposure. See also 11.2.1.

The strength of steel remains essentially unchanged until about 600°F. The steel retains about 50% of its strength at 1100°F. The steel loses all of its capacity when it melts at about 2700°F. However, for design purposes, it is usually assumed that all capacity is lost at about 2200°F.

Water or concrete inside tubular steel members act as “heat sinks,” therefore reducing temperature rise in the steel and significantly enhancing fire resistance. In the case of the concrete-filled tubular columns, the concrete will also contribute to the load-bearing capacity when the outside steel shell deteriorates under heat exposure.

Open web steel joists and castellated beams are proprietary system designs. For many of them, fire resistance ratings are listed in the UL fire resistance directory (and other similar directories).