editorials

Minnesotans are closer today to having an answer to the state's most burning question since Aug. 1: Why did the Interstate 35W bridge collapse? After Tuesday's release of preliminary investigative findings by the National Transportation Safety Board, "gusset plates" will forever be part of the state's lexicon, synonymous with a built-in, fatal flaw that's undetectable until it's too late.

Or was it? Was there no opportunity for the design's carrying capacity to be reassessed -- perhaps when the bridge was renovated in 1977 and 1998, or in this decade, as its traffic load rose far beyond original levels?

That question lingers, despite NTSB Chairman Mark Rosenker's explanation that the gusset plate error had "little chance of being discovered after construction." He directed the finger of blame away from MnDOT's bridge inspectors and pointed it backward 40 years, saying that a "breakdown in the design review procedures" at the time of the bridge's construction allowed undersized gusset plates to be installed.

Clearly, that's where the error originated. But in 1993, state inspectors noted that one gusset plate was so corroded that it had lost half of its original thickness. Should that discovery, on what was by then the most heavily used bridge in the state, have triggered a recalculation of the bridge's overall strength?

Before the I-35W investigation, Rosenker said, inspectors operated under the assumption that gusset plates could not be undersized in the design process, and they focused on what they considered more vulnerable bridge elements. That assumption now must be questioned.

Rosenker also said there's no evidence to suggest that weak gusset plates are a problem on any other bridge in America. But that's not saying much, coming from someone who had just explained that inspections typically don't discover such problems. Better reassurance would come from the establishment of a protocol that allows design mistakes to be discovered before they cause a collapse.

The NTSB's advisory yesterday to the owners and maintainers of this nation's bridges appears to be a start toward the creation of such a system. It urges a recalculation of the strength of the structural components, including the gusset plates, of a bridge of similar type, "whenever planned modifications or operational changes may significantly increase stresses."

To his credit, Gov. Tim Pawlenty went one step further yesterday. He ordered the recalculation of load capacity for all 23 trunk highway bridges that are similar in design to the I-35W bridge, regardless of whether they are due for load-increasing changes. The high cost of bridge failure, which Minnesotans know painfully well, justifies that extra step.

Rosenker stressed that yesterday's word is not the final one on the I-35W bridge collapse. It's still not clear what made the weak gusset plates give way when they did. But the preliminary findings announced yesterday, and the advice issued with them, deserve immediate heed by the nation's bridge operators. Minnesota's experience demonstrates that watching for rust and cracks on aging, overused bridges is not enough to keep them from falling down.