> But engineers gradually recognized that the structure had so few crucial supports that if even one of them failed, an entire section could collapse.
> “There is no robustness, or the possibility of redistribution of the forces,” said Massimo Majowiecki, an architect and engineer in Bologna, northern Italy.
> That lack of redundancy, as it is now often called, “is not necessarily inconsistent with how bridges were designed in the 1960s,” said Donald Dusenberry, a structural engineer with Simpson Gumpertz & Heger in Boston.
The design was such that as long as all of the parts worked, it remained standing. But it was also such that a very small number of failures would push the remaining supports beyond their capacity to withstand the loads, leading to collapse.
The SF-OAK bridge likely had a much higher level of what this article calls "redundancy" or more generally of simply "over design" (design for loads some multiple of normal loading) such that collapse of the top deck did not push the lower deck beyond its load holding capacity. I'm surprised that "bridge design in the '60s" (as stated in the article) did not take these details into account.