Someone once told me that jet fuel can’t melt steel and you expect me to believe some burning trees did /sarcasm
It doesn't have to actually melt it, but it can get it hot enough to lose its strength. Years ago I saw a set of steel frame supported wood bleacher stadium and roof after it had burned at many of the steel beams looked like they had become wet spaghetti. To go to the horse's mouth for some information, from the AISC (American Institute of Steel Construction) web site,
"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."
. . . .
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."
From Fire Engineering: "
At approximately 1,100 °F, steel will start to fail. At this temperature, the steel begins to expand and twist."
Another site says, "The lack of deformation in a loaded condition is a visual clue that the steel was not heated to a temperature sufficient to result in the formation of martensite during the fire, and as a result, the steel's mechanical properties were probably not altered by the fire."
. . . .
And yes, forest fires can get hot enough to reach temperatures that can result in deformation of the steel, in other words above 1100F. Yes, there are inconsistencies in answers between sources. The twisted deck beams shown in the picture are really possible. The still straight column might or might not still be safe, but I would not trust them without cutting out samples for testing.
