The media, and even occasionally highway agencies, have done a poor job of reporting exactly why the I-35W Bridge collapsed. This has led to additional misunderstanding about whether other truss bridges, whether or not similar to the I-35W Bridge, pose a collapse risk. Some explanations and thoughts about the I-35W Bridge collapse follow.
The I-35W Bridge collapsed because the gusset plates that formed the connections for the truss bridge members were not thick enough and as a result lacked the proper strength to bear the load the bridge was designed to bear. This problem was not detected when the bridge was first designed because the engineers failed to perform the calculations needed to check whether the gusset plates were the correct size.
The overall design of the I-35W Bridge was safe, reliable, and effective. Had the gusset plates been the correct thickness, the I-35W Bridge collapse would not have occurred. The gusset plates were the only problem with the bridge.
Sverdrup and Parcel, the designers of the I-35W Bridge, were experienced engineers who knew how to properly design a bridge and check the design calculations. While they failed to check the design calculations of the I-35W Bridge, they built many other truss bridges without making this mistake, and those bridges have proven to be safe and reliable bridges.
There is nothing about a "deck cantilever truss" type of bridge that is less safe or less reliable than other types of cantilever truss bridges.
Most truss bridges are considered "fracture critical" meaning that if one part of the bridge fails the entire bridge might collapse. While that sounds scary, the reality is that fracture critical bridges are safe and reliable as long as they are inspected routinely as required by law. A proper bridge inspection will detect problems with truss members long before they even come close to complete failure.
Bridge inspectors and highway agencies are now required to check truss bridges for the type of design errors that were present on the I-35W Bridge. As a result, truss bridges standing today and still open to traffic have been checked to make sure a similar problem does not exist.
The fact that a truss bridge is fracture critical should not be used to justify demolishing and replacing a truss bridge. If a truss bridge is to be replaced, additional reasoning should be present (deterioration, width of roadway, etc) to justify a replacement.
The fact that a bridge "looks like" the I-35W Bridge should not justify a demolition and replacement project. Unless the similar-looking bridge was found to have gusset plates that are too thin, such a bridge would not be at risk for an I-35W type of failure and subsequent collapse.
Official NTSB I-35W Bridge Report: http://www.historicbridges.org/info/bridgehunterfiles/i35wre...
NTSB Presentation Discussing The Cause of the Collapse: http://www.historicbridges.org/info/bridgehunterfiles/i35pre...