Thursday, August 02, 2007
Film from a security camera captured the collapse of the I-35W bridge across the Mississippi River
Boy what a mess! I am a structural engineer of sorts and my unqualified opinion on the bridge collapse is due to weak construction, salt erosion of steel structure from 40 years of rugged Minnesota winters being pressed upon by even more weight being added from construction applying a layer of roadbed on top of a flimsy bridge.
As you can see the 458 foot span gave out at the end points where the most gravitational stress would be concentrated causing it to fold up like a cheap suit.
Minnesota winter create a need for salt spreading on the roadways which prevents ice formation down to approximately 20 degrees. The salt seeps into the cracks and joints of bridges through capillary action causing the steel to separate(flake) and freeze in real cold weather further expanding the metal connections.
The running joke in MN is there are two seasons, winter and road construction. Of course now it is no joke.