A Bridge Tooooo Far?

I don't want to belabor the issue of the Minneapolis bridge collapse, but this picture caused ol' Pecozbill to scratch his head and wonder. So I'm gonna scratch that itch by posing my consternation here.I sure ain't no bridge building engineer or rocket scientist - but this particular picture sure poses some questions. Like how come these two bridges built side by side aren't equally stable? Look again at the picture. The remaining, solid bridge is built on arched trestles with a support in the middle of the river and the bridge. The section of the 1900 foot bridge that failed was (is) a spanned truss section some 470 feet long with no support in the middle.... because they didn't want to interfere with the river traffic at the time of construction.
Long spanned bridges are buildable with arched spans and suspension cabling. But the length of simple beamed or truss bridges is limited.
With all of the traffic growth in numbers and weights over the past forty years, it stands to reason that the middle (most stressed and weakest) part of the span that constantly rises and sags would eventually fatigue and fail! And that's apparently what happened.
If I had a little more ambition and time, I'd look back through the records to see if the fingerprints of some greenie organization weren't all over the decision to build without center support. When all the politicians get done pointing fingers at each other, I expect that somebody is going to recognise this and start to ask these questions.... just remember, you heard it here first.... (interested folks might check bridge building 101 at: http://www.faculty.fairfield.edu/jmac/rs/bridges.htm )
-See also Bridge postings below.....