This graph is from NIST NCSTAR 1-6C
And this graph is from the Engineering Toolbox.
As you can see, neither the NIST yield or tensile curves match with the ET structural steel curve. Why? And which one is correct?
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$\begingroup$ It's not clear if the ET graph is for materials, and the NIST graph is for Elements. At failure, elements will neck (and weaken) even as the material gets tougher. The effect is strongly temperature dependent. $\endgroup$– davidCommented Mar 6 at 9:38
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$\begingroup$ Yield strength of a steel should go down with the temperature when you start at the room temperature. $\endgroup$– Tomáš LétalCommented Mar 6 at 17:03
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I think the NIST graph isn't actually a tensile strength curve. I think it's a curve for design factors. Just because your steel beam is 30% stronger at 250C and your application operates at 250C doesn't mean you want to undersize your steel by 30% because then what happens when it reaches room temperature? It's pretty difficult to ensure your structure will always be at 250C and never fall below that.
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$\begingroup$ But here it is being used to define properties for FEA analysis of WTC collapse. Surely you would not use such an approximation here. $\endgroup$ Commented Mar 6 at 2:08