Investigators trying to figure out why the 35W bridge collapsed will likely be on-scene until November. The National Transportation Safety Board provided an update on the investigation this week but did not indicate whether they were any closer to figuring out why the bridge fell. Investigators did say that there were more than 280 tons of construction materials and equipment on the bridge when it fell. They're also looking at a de-icing system and the potential corrosiveness of the chemicals it used. Dr. Jon Hanson is a distinguished professor emeritus of Structural Engineering and Mechanics at North Carolina State University. He has worked on bridge failure investigations in the past and comments on the bridge collapse.