Steel + Concrete = Safe Steel only = Expensive Concrete only = Are you mad? Try this at home, take a long small piece of polystyrene sheet and compress it, not matter how much ever you compress it, piece will not crush but do the exact opposite and pull it. SNAP..! It breaks with no efforts. Why is it so? Well it is good in compression, that is all its inner particles are compressed with each other and they have no where to go. But because they are soft they cannot crush each other. In case of concrete, as concrete is made of cement, sand and stones or any coarse aggregates there are many chances that if you exceed its capacity it will crush. But in case of tension, the particles are not capable of pulling each other and this bond breaks at a very low strength making it weak under tension. Now do one thing, take a copper wire, insert it through polystyrene sheet with some glue so that once copper wire enters in, it sticks with the polystyrene sheet. Now pull the same piece, it will form some sort of cracks but it will not break completely and it will maintain its form. This shows the bonding between the steel and concrete and strength of steel in tension. What happens is, all the gravity loads transfer from top to bottom in the form of tension as well as compression forces. To carry tension we need steel and to carry compression we have both steel and concrete. for example, bending has tension in bottom side of beam and compression in top, so better provide steel in bottom and make our beam work. In case of earthquake things get out of hands. And so in order to resist those forces we need steel to take loads. Apart from just tension, steel has one more beautiful property and that is ductility. When considering earthquakes all the structural members go under heavy cyclic loading and to keep our structure standing we have to make sure that our members can release this energy from cyclic behavior and steel can easily do that because of its ductility. Also we use steel to make sure concrete does not break and its chunks do not fall of the member. It is called spalling of concrete. So we use stirrups to tie up the members so this do not happen and even in cyclic loading it remains in place. Picture below shows spalling of concrete. So it does not only carry tension force, but it maintains the building integrity after heavy impacts, vibrations, earthquakes and winds.