Iron has two different crystal structures at atmospheric pressure: the body centered cubic (bcc) and the face centered cubic (fcc). In the ground state the bbc α-phase is stable, and at the temperature T=1671 F (A3 point), α-Fe transforms into fcc α-Fe, which is stable up to 2537 F K(A4 point).

The Body-Centered Cubic (BCC) unit cell can be imagined as a cube with an atom on each corner, and an atom in the cube’s center. It is one of the most common structures for metals. BCC has 2 atoms per unit cell

The Face-Centered Cubic (FCC) unit cell can be imagined as a cube with an atom on each corner, and an atom on each face. It is one of the most common structures for metals. FCC has 4 atoms per unit cell

This means, that as the jet fuel burns at 1, 517 F, and heat is contained (such as in a building), as the iron/steel beams are heated, they at first expand - due to thermal heating. But, when the temperature of the iron/steel beams exceeds 1,671 F; the iron SHRINKS BY HALF

This PULLS the building down, melting is unnecessary.