When unusual depths of snow fell in the Chicago area there came news of literally hundreds of commercial and industrial roof failures and thousands of home garage roof failures. At the General Motors plant at Fairfax, Kansas, 56,000 square feet of roof supported by 200-foot-long steel trusses collapsed, again credited to local weather fluctuations. The Rosemont (Illinois) Horizon Sports Stadium roof, a laminated wood girder design, collapsed as the end of construction neared, presumably due to wind loads. Remarkably, none of these failed roofs reported were concrete.
It may be time for both designer and building official to take a hard look at the safety implications of constructing an arena such as the Rosemont Horizon near the end of a runway of one of the world's busiest airports, or allowing a roof to hang from a few high-strength bolts, even if the design calculations indicate adequate safety factors. Have we become so entranced with the marvelously complex analyses now possible that we no longer take time to view the basic concept of the design from a life-safety perspective? Are we asking enough of the tough "what if" questions, such as: "Can loads find their way to ground via more than one path?" and "Will the entire structure collapse if an unusual unanticipated load occurs?" But for concrete, we should remember that our present design requirements have served the designer, the owner and the occupant of concrete structures very well indeed. If this were not so, many more of those major roof structures that have collapsed would have been concrete.
Concrete has a greater mass or self-weight than most other structural materials. And so it seems to have a greater capacity for actual live loads by borrowing from its safety factor for dead loads. It is this mass that provides the extra strength and protection for the tenant of a concrete structure and pays big dividends when unusual loads occur.