I have been working on the behaviors of steel structures at evaluated temperatures. We all know that heat can be easily transferred through steel. So, in real cases, will there be a large temperature gradient in the cross-section of a steel member under fire?
You may find attached document quite interesting and informative on this subject.
Firstly, some highly alloyed steels have good heat transfer resistance (unexpectedly!). Secondly, everything depends on the heat source intensity and localization, steel section sizes and boundary conditions, e.g. one side is out of the building and another is inside. Finally, the dependence of a steel stiffness from temperature will play a significant role in the steel structure deformation - elastic and then plastic. This is a non-linear system above the yield stress point, which will gradually move down under rising up the temperature.
the temperature gradient in steel is varied steel to steel.if you want to know about it melting condition then take a particular steel grade, and find out its notes
The gradient will tipically vary from ambient, say 20 deg-C or 68 deg-F, to an agreed maximum tolerable by the commodities being supported, say 200 deg-C. The latter temperature will depend on assurances from the fire protection engineer regarding available means of suppressing the highest heat generated by the fire, for a period of one to two hours, depending on building or project requirements. Otherwise, according to standards on both sides of the Atlantic, the highest fire temperature that can be expected could reach 1,000 deg-C (2,000 deg-F).
A full/detailed answer to your question depends on a number of factors that mainly fall under geometric features, material properties, thermal boundary conditions etc. However, qualitatively, and assuming you are dealing with a "typical" section (say W16) & "uninsualted" steel member, then I doubt that a worthy-of-mention thermal gradient will develop; whether at moderate or high temperatures (regardless of the temp.-time curve).
There is a reason why we insulate steel sections, and it is primarily due to the fact that steel is very conductive -> temp. rises rapidly & uniformly within a steel cross section -> leading to rapid temp.-induced material losses throughout the whole section -> failure! I hope this helps a bit.
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