A. Buildings: Since we began constructing tall buildings using concrete + steel (skyscrapers), 1 - we have learned that it is necessary to design/execute building which flex when exposed to horizontal dynamic forces (such as strong winds/gales/hurricanes, or earth tremors during earthquakes); 2 - that the ability of the structures to flex instead of fracturing or shattering was critical to the design; 3 - and the specification of "flex steel" and the inclusion "expansion joints" into the design became the industry standard. B. Bridges: Since we began constructing long spanning single-structure bridges (suspension bridges), 1 - we have learned that it is necessary to allow the structure to flex + "swing" when exposed to dynamic forces (such as strong winds, earth tremors, or more commonly the creation of interference waves by traffic: the rhythmic cadence of marching soldiers keeping time on rigid bridges are known cause rigid structures, including bridges, to rapidly and traumatically fail); 2 - that the ability of the bridges to flex instead of fracturing or shattering was critical to the design;3 - and the specification of "flex steel" and the inclusion "expansion joints" into the design became the industry standard. C. Steel Rods and Girders: Even in grade school we learned that "hardness equals fragility" ... 1 - many of us learned that we may convert "strong"  objects into "highly fragile" objects simply by increasing their rigidity (i.e. "hardness"); 2 - by simply freezing objects in order to condense their molecular structure (in fact making them "harder"); 3 - hence the "hard" steel rod became a fragile, glass-like object: after submersal into liquid nitrogen, the steel rod would shatter into fragments with a light rap.