"The designers had specified a metal plate that was too thin to serve as a junction of several girders, investigators say."
To the credit of engineers, this sort of bungled effort is a rare occurrence in bridge design. Acts of God such as floods and earthquakes are the most frequent cause, as cited in the book Why Do Bridges Fail?. Author David Smith studied 143 bridge failures that occurred between 1847 to 1975 and concluded the following:
"...Bridges generally do not fail because of inaccurate stress-strain calculations. Sound design is achieved by the rational matching of safety factors to analytical techniques and erection procedures, and above all by wisdom and judgment.."
It may well be that there really is a simple answer of engineers should have "made it a bit thicker here." But I worry that In an era of tightened budgets and lack of political will for dealing with the prosaic elements of day to day life in our increasing high-tech culture, it will be just another excuse to push off worrying about looming problems with our crumbling infrastructure projects until the next big crisis.