So, why don't the burners on my stovetop buckle and sag and collapse?


There is an almost infinite combination of iron and other elements that can properly be called steel. People learned a long time ago the carbon will make iron stronger. That combination is called steel.

More recent metallurgists have added chromium, cobalt, copper, manganese, molybdenum, nickel, phosphorus, silicon, sulfur, tungsten, vanadium, and probably others . Each combination has different characteristics of hardness, flexibility, heat resistance, corrosion resistance, etc.  

The alloy used to make a stove grill is too hard and brittle to make large steel buildings. Buildings require girders to flex in response to wind (and possibly earthquakes). The steel used in the Trade Towers was perfect for the job, under normal circumstances.

Due to the extreme heat from the burning jet fuel and other combustible materials, the steel beams in the Trade Towers lost strength, and, when sufficiently weakened, were unable to carry the load they were given. The buildings collapsed. It is simple physics.