Strength of materials

I remember first seeing something that was touted as being stronger than steel in a tire commercial. They showed how they were using Kevlar® in their tire tread and that, pound for pound, it was five times stronger than steel. After that, it seemed every few months someone was developing this or that material that was, pound for pound, stronger than steel. Even natural materials were being mentioned. "Spider silk is, pound for pound, stronger than steel."  I came up with a rule (trying to jump on the bandwagon started by Murphy) by stating,

It seems that anything, pound for pound, is stronger than steel.

What, then, was the use of steel? Why were we still building bridges, car frames and girders for buildings out of steel? Why not just create building materials out of Kevlar or spider silk? You can't build bridges or building from Kevlar for a few reasons, of course. One of them is bulk. The amount of material you would need to reach the strength of steel would be too large. Also, the cost would be prohibitive. Making steel is a fairly cheap process - pound for pound!

Another interesting aspect to this story is that Kevlar was discovered at the Du Pont Company by Stephanie Kwolek in 1965 (and patented in 1966). Yes, a woman developed this amazing, stronger than steel material. It's used in many places including body armor, drum heads (as in the musical instrument)  and in sports equipment. It was also used to make the retractable covering in the design for Olympic Stadium in Montreal for the 1976 Olympics but it was delivered ten years late and needed to be replaced only ten years after that. So, it seems, steel's place in construction may be safe for now.