Some independent experts said the evidence of buckling indicates fatigue of the steel or a corrosion-related crack, which is common in older bridges and difficult to detect during inspections. "The bridge must have been near a state of collapse for some time, and the construction might have contributed to its failure," said Zdenek Bazant, a professor of civil and environmental engineering at Northwestern University. Bazant said he suspects there may have been a hairline crack or fatigue in the steel joints near bridge supports, leading to the buckling. He said bridge inspections are not nearly as complete or sophisticated as the inspections that aircraft undergo. Other engineering experts said that the 1960s-design of steel-arched bridges did not contain structural redundancies, meaning that if one component fails, the whole structure is in jeopardy because the weight does not shift to other points on the bridge. "We know that we would not build a bridge like this today," said Kent Harries, an assistant engineering professor at the University of Pittsburgh.