File:Sea of electrons.jpg

Summary
Steel is sometimes described as a sea of electrons. Protons are virtually surrounded by electrons. It is easy to see how the addition of heat first causes expansion and then softening, to the point of liquification. That is how steel is manufactured and that is how it acts as a structural element in a building fire. Proper fireproofing mitigates this. Still, care must be taken to ensure that expansion of structural elements does not damage wall and floor assemblies required to have a fire-resistance rating. Of particular concern are any penetrants in a firewalls and ferrous cable trays in organic firestops.