Young's Modulus of Structural Steel

Stiffness and strength come from different physics, and steel is the cleanest demonstration. Young's modulus measures the stiffness of the iron-iron atomic bond, so it is the same 200 GPa for A36, A992, A572 Grade 65 and a quenched-and-tempered alloy: heat treatment and alloying rearrange dislocations and grain structure, which governs where the material yields, but they barely touch interatomic spacing. Yield strength varies by a factor of five across those grades; E varies by about 2 %. FHWA's steel bridge handbook prints one number, 29 000 ksi, and uses it for every bridge steel.

The practical consequence is that a stronger steel does not give a stiffer beam. Deflection under service load depends on E and I only, so switching a floor beam from A36 to A992 buys strength and buys nothing at all in deflection: if the beam is deflection-governed, and long-span floor beams usually are, the only fix is more depth or more moment of inertia. The value falls with temperature (about 10 % by 300 °C, and roughly half by 600 °C), which is why fire design uses reduced-modulus curves rather than this figure.