1988 Northeast snowstorm building collapses

Storm of the century, through Princeton NJ and eastern Pennsylvania. Prefabricated buildings with computer-optimized roof joists (designed to 1.67 minimum rather than the historical 2.0+ rule-of-thumb) collapsed under snow loads. Cause: optimization had eaten the implicit safety margin that had previously made 1.67 effectively higher.

Everything was fine until the big snowstorm of 1988 I think it was. They had a huge snowstorm, it was the storm of the century, and it came right up through Princeton, New Jersey and stuff. Buildings were collapsing all over New Jersey and eastern Pennsylvania, because they had bigger snow loads than they were used to. Snow loads had been built up from historical experience: one little township might have a 28 pound per square foot snow load, another one might have 35. It depends on how much snow they got in their local region. Those standards are almost by township. People start getting in arguments about, well, we exceeded the snow load, oh no we didn't. But what really turned out in a lot of cases is these things had been designed to 1.67, which is what the code said. But the code had been built up at a time when you couldn't do all this optimization, and the real standard was actually significantly greater than 1.67. So today with computer technology we've shaved the safety factors down to the bare minimum — the bare minimum that was built up where it was just a guideline before, and we always exceeded it by twenty, thirty percent. We've been shaving the guideline all along.