Steel construction is dominated by a particular type of steel called mild steel. Mild steel is an extremely strong material. Consider a 1" / 25mm diameter circular steel bar. This bar could be attached securely to your ceiling to allow you to hang 20,000 kg (20 tons) or any other weight.
Buildings have great advantages due to their strength. Steel framing's flexibility is another important characteristic. Steel framing can bend without cracking which is another advantage. A engineered structural steel framing can also flex when it's pushed to one side, for example, by wind or earthquake.
Third, steel's plasticity or ductility is another characteristic. It is able to bend slowly and resist breaking under extreme pressure, unlike glass. This property allows steel buildings to bend out of shape, or deform, thus giving warning to inhabitants to escape. Failure in steel frames is not sudden – a steel structure rarely collapses. Steel in most cases performs far better in earthquakes than most other materials because of these properties.
Steel has one key property: it quickly loses strength when heated. Mild steel can lose nearly half of its strength at 500° celsius (930° F). This was what happened collapse of the World Trade Towers in 2001. Steel buildings should be protected against fire and high temperatures. This is done usually by covering it with boards or spray-on materials called fire protection.