Documents obtained by the Star Tribune questioned how well cable supports could withstand tension.
Construction on the Martin Olav Sabo Bridge was underway in December 2006 when an engineer noticed that a plate anchoring the lowest set of cables on the east end was showing "high stresses."
An engineer with Ames Construction, which had contracted with Hennepin County to build the distinctive bicycle and pedestrian bridge over Hiawatha Avenue in Minneapolis, wrote a memo to the bridge designer, URS. An Ames consultant had done some calculations "to further look into this and they do have a few concerns," the engineer, Justin Gabrielson, wrote.
He added: "Could you please confirm with our designer the adequacy of this plate?"
Just over five years later, on Feb. 19, the bridge was abruptly closed after a steel anchor plate cracked and caused the tallest set of cables to fall 100 feet. The fracture of another plate led engineers to remove a second group of cables later that week.
The plates that fractured were on the other ends of the cables from the ones questioned by the Ames engineer nearly a year before the bridge opened. Still, documents obtained from Hennepin County by the Star Tribune reveal early concerns about how well the cable supports could withstand the tensions, as well as whether the bridge's design would make it difficult for inspectors to detect trouble.
Now, teams of engineers, consultants and inspectors are investigating the cause of a bridge failure that curtailed service at three Hiawatha light-rail stations, diverted traffic and likely will keep the well-traveled $5.1 million bridge shut down for weeks.
Minneapolis and Hennepin County have awarded Wiss, Janney, Elstner Associates Inc. a $100,000 contract to investigate the cause of anchor plate fractures. Officials said a determination could take four to eight weeks.
Roger McBride, a vice president at Ames, said Friday that the Burnsville-based company was reviewing internal documents to learn more about what happened.
He said that based on conversations with Gabrielson, the problem appeared to have been addressed. The engineer had been concerned with the plates that fastened the cables to the bridge deck. Those anchors had a different design than those on the pylon, or mast, where at least three of the plates were compromised.
"This was Ames doing its due diligence. ... He must have been satisfied with the outcome, or he would have stayed with it until it was resolved." McBride said.
A spokesman for URS said in a statement that the anchor plate questioned by Ames was part of a cable stay system manufactured by a different company. It was chosen by Ames for the project and was "not part of URS' design of the bridge," said spokesman Ronald Low.
"As public infrastructure projects move forward, it is common for contractors and owners responsible for reviewing the design plans to have questions for the design engineers," the URS spokesman said.
In a March 2006 document, before the bridge was built, URS responded to the Minnesota Department of Transportation's "serious concern" that internal parts of the pylon, including cable connections, would be inaccessible for an in-depth inspection.
After URS provided instructions detailing how to remove a screening panel to get a look inside the pylon, state officials were apparently satisfied.
Tom Styrbicki, a state bridge construction and maintenance engineer, said Friday that those instructions are now in the Sabo Bridge's inspection manual "and it looks like the concern was addressed."
After examining the bridge in October, city inspectors gave the cable anchors the highest rating for soundness.
Maya Rao • 612-673-4210