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Sabo bridge closure extended after more damage found

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Failed diaphragm plate No. 9
Failed diaphragm plate No. 9 from the Martin Olav Sabo Bicycle and Pedestrian Bridge in Minneapolis.
Courtesy City of Minneapolis

Minneapolis city bridge engineers said Tuesday they found more damage to the Martin Olav Sabo bicycle and pedestrian bridge than previously reported.  

Last week, a metal plate connecting a pair of cables to the bridge broke and fell.  The city closed the bridge after engineers found another damaged plate nearby. Now, two more plates have been found cracked and in need repair. 

The city posted a photo of the fallen fragment on its website.  City officials said the piece weighs about 60 pounds.  It has two big holes in it, and looks like a giant metal Lego piece that was stepped on by Godzilla.   

"This was found on the ground," said Heidi Hamilton, the deputy director of the city's public works department. Cotter pins holding it in place had failed.

The fragment, which appears to have rust along the broken edge, has been shipped off to Lehigh University in Pennsylvania for study.  The rest of the plate is still attached near the top of the steel pylon that rises more than 200 feet above Hiawatha Avenue. Eventually, that other part of the plate will also need to be removed and shipped away for analysis.  

A Chicago-based firm examined the remaining 17 plates last weekend and found more cracks.

"No 7 seven had a very minor defect, that they were able to take care of and it's not of concern.  No. 6, they did not find any defects.  The next one down, No. 5, they found two cracks beginning at the edges where the other cracks have been beginning," she said.

Before engineers reopened traffic under the bridge, they made sure that temporary shoring would keep the bridge from falling down if plate No. 5 failed.  Hamilton said the cables support most of the bridge, but not the section that passes over the Hiawatha Light Rail tracks.  However, engineers didn't want to reopen the tracks until they could detach a cable from one of the damaged plates at the top of the bridge. 

"Our concern was if that cable number 8 broke, we didn't know where it would go.  And we didn't know what exactly would happen to that suspension part of the bridge," Hamilton said. If it began to shift, "possibly you could affect the other part of the LRT bridge.  So that's why we chose to keep LRT closed until that number 8 cable was down."

The bridge was designed by URS Construction, which was also involved in the design of the former Interstate-35W bridge.  A study by the NTSB found that undersized gusset plates led to the 2007 collapse of that bridge into the Mississippi River.  But other experts said excess weight on the bridge was also a major factor.

City officials said that several days before the plates on the Sabo bridge failed, an ambulance and a police car drove onto the bridge in response to a 911 call.  However, Heidi Hamilton said the bridge is designed to handle the weight of a 20-ton vehicle -- far heavier than an ambulance and police car.  And she said it may be one to two months before the analysis on the failed diaphragm plates is complete.  

City officials say it will probably take longer than that before the bridge opens to bikes and pedestrians.