There is a rating system for bridges used nationwide. Bridges are rated on a scale from 1 to 100 (one is the worst). Bridges receive a rating that is noted as structurally deficient or functionally obsolete. A number of variables are used to produce this rating but the main issues we look at are capacity and condition.
With regard to capacity, we look at whether the bridge can safely accommodate the amount of traffic. Also, we look at the load capacity -is it able to safely carry heavy vehicles. Condition issues include the condition of the bridge parts and materials. Is there corrosion on the concrete and steel? Is there girder corrosion? Has the bridge been damaged internally due to exposure to snow removal products and water? Is there fatigue (cracking that grows over time due to flexing of steel members)?
To give you a comparison, the bridge in Minneapolis received a structurally deficient rating of 50 (as reported in the media). When CDOT replaced the I-25 bridge over Broadway in Denver, it was rated a 2. Just because a bridge receives a low rating does not mean it's unstable.
If CDOT were to experience a bridge failure like the one in Minneapolis, we'd start our investigation by looking at external damage. Was the bridge impacted, damaged by the water source or did it receive construction damage? We'd also look at the internal bridge parts-is there corrosion, rusting of steel and rebar, and fatigue?
We can't begin to speculate as to what caused this horrible event in Minneapolis as there are so many factors that come into play.
CDOT spends approximately $30 million a year on bridge replacement and major rehabilitation projects. We also try to replace bridges with other funding sources as combined with other projects (like interchange improvements).
This level of funding does not allow us to replace high-dollar bridges like elevated I-70 near the Denver Coliseum, estimated to cost between $200-$500 million.
CDOT is responsible for 3,757 bridges. Of those, 110 are in need of replacement, and 375 are in need of rehabilitation. Rehabilitation means that we need to repair and replace damaged bridge parts, not the entire bridge. This can include such items as the bridge deck or other steel parts.
CDOT believes it has a very thorough bridge inspection program but will certainly pay close attention to the investigation in Minneapolis to learn what caused such a horrendous event. We are always looking to see what lessons can be learned and how we can improve to be sure we're doing everything possible to keep Colorado travelers safe on our highways. Our hearts go out to the victims and those involved in this tragedy.