MINNEAPOLIS (WCCO) — The City of Minneapolis has released a report of preliminary findings in the structural failure of the pedestrian bridge over Hiawatha Avenue.

According to public works staffers, the engineering firm hired to investigate the Feb. 19 incident, in which two suspension cables snapped on the bridge, put primary blame for the failure on wind-induced cable vibrations.

Steve Kotke, director of public works in Minneapolis, says according to the engineering firm’s findings, winds were reported to be at 6-13 mph for several hours before the cables gave way.

The engineering firm Wiss, Janney, Elstner Associates, Inc. said the vibration of the cables appears to have resulted in fatigue cracks in the connecting diaphragm plates. The result was that the two cables fractured.

WEB EXTRA: Full Report On Bridge Findings (.PDF)

The failure led to the bridge’s closure to pedestrian traffic and the temporary closure of Hiawatha Avenue to traffic for a few weeks.

Traffic was allowed to continue under the bridge once a series of support structures were placed underneath, however the engineering firm reported that the bridge’s design — which included structural redundancies — proved to work by supporting the bridge even after the cables broke loose.

The review says the stay cable vibrations (wind-induced) were not included in the original design calculations, but Wiss, Janney, Elstner Associates has come up with a way to fix the problem.

“Basically upgrade the connection to make it less acceptable to fatigue loading,” said Brian Santosuosso, project manager for Wiss, Janney, Elstner Associates.

But the cost of an upgrade is high.

“All costs right now are probably right around $1 million,” Kotke said.

A permanent fix could cost another million dollars, and it would also impact commuters, as the bridge will have to be closed for repairs, he said.

Wiss, Janney, Elstner Associates will work with the city and county as well as the original designer of the bridge, URS, to fix the problem.

URS worked on both the Sabo Bridge and the failed 35W bridge.

The full, final report will be released on June 28.


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