Very few bridges built in '67 would meet current seismic
True, but I just watched a UC Berkeley engineering professor interviewed on TV, and he stated that there are no bridges in California of the same design as the collapsed Minneapolis bridge. I doubt that design would have met California seismic standards of 1967 either.
Not only high strength steels but also more advanced engineering analysis has enabled the design of structures with less overall material. This means less static weight, and further reductions in material may also have been made to take advantage of that. It's a law of diminishing returns - where the lighter construction means that load limits must be much more carefully observed. It seems like the 10th Street bridge was built to the "lighter, better" philosophy, which I think was also part of the '60's thinking for many products (for example, "Danish modern" furniture vs. much heavier overstuffed older but perhaps sturdier traditional designs).
There is also the disconnect between engineering and execution. I think a famous sports dome collapsed a few decades ago because the carefully planned design was misinterpreted and parts didn't fit right. The construction crew used brute force to make it all fit, and the eventual result was a disastrous collapse. Then there are the dishonest contractors and suppliers who cut corners and supply inferior key parts (like bolts and pins) that may fail and cause disaster. As I recall there was a famous collapse of a balcony inside an Hyatt Hotel atrium that resulted in many deaths, and it was traced to shoddy construction execution. Part of this problem may be that older construction bosses may not fully realize that newer designs rely upon top quality materials, and that what they were able to get away with in the past no longer works with newer designs.
Thinner steels surely do have less mass to resist rust, but they may also be more inherently rust resistant thanks to the presence of alloying elements. But the slightly increased rust resistance may be minor when compared to truly rust resistant alloys like 304 stainless. Impurities in the steel may result in a greater tendency to corrode; I've heard that many cheaper European cars of the 70's (think FIAT) were rust buckets in part to their use of inferior but cheap Soviet supplied sheet metal. Of course, proper metal prep, primer, and paint plays a very big role as well. The advent of zinc-rich primers in the 90's led to greatly extended rust-through warranties on new cars. The downside is that newer more eco-friendly water-based topcoats didn't stick to the zinc-rich primers very well, and often failed after a few years... leaving a formerly attracive car look like it has some horrible skin disease (topcoat paint peels off in sheets, leaving dull gray zinc primer behind).
