The Florida Department of Transportation has targeted 24 bridges on major roads in South Florida as 'structurally deficient,' the same condition inspectors had labeled the Interstate 35 bridge which collapsed in Minnesota August 1.
The Miami Herald said bridges from Palm Beach to Monroe counties in various states of deterioration will harness state, county and cities with maintenance and repair costs in the hundreds of millions of dollars. The American Society of Civil Engineers (ASCE) reported 70,000 bridges in the country are “structurally deficient.” The ASCE estimated repairing them all would cost $188 billion.
The Herald also said Gov. Charlie Crist wants to know the status of all11,000 bridges in the state. South Florida lawmakers want the same information from the U.S. Government. All bridges in the state are inspected every two years by Florida Department of Transportation engineers and contractors. The state also requires annual inspections on all mechanical bridges. When a bridge is rated “structurally deficient,” it is inspected once a year and placed on a schedule for repair or replacement. It also may require extensive maintenance, according to the Herald report.
In cases of greater deterioration, like the historic, aging Venetian Causeway bridges that connect mainland Miami with Miami Beach, authorities impose weight limits or restrict traffic until repairs are made. Last week, Miami-Dade County public works officials closed the Northwest 17th Avenue bridge to heavy vehicles. Later this year, the bridge will be closed for repairs.
The Herald said only one bridge in Florida has the identical truss-type, steel-frame design as the one that collapsed over the Mississippi River. The crossing in Florida, near Bahia Honda in the Keys, has been closed to traffic since 1999.
”One of the problems with a [truss] bridge is there is no redundancy,' said Herb Saffir, a veteran Coral Gables structural engineer who designed 50 local bridges. ``If a joint or beam fails, possibly by corrosion or losing rivets, the whole truss is going to collapse,” he said. Most South Florida bridges are built on piers of reinforced concrete and supporting pre-stressed concrete beams, he added. 'If one beam is knocked out, it doesn't affect the integrity of the bridge.”
In Florida, funds are appropriated to repair or replace all 57 of the structurally deficient bridges on the Florida highway system. One of the local bridges on the list -- the 14th Street Causeway in Pompano Beach -- underwent a $4.8 million repair last year. Another bridge in Broward, on Sunrise Boulevard over Fort Lauderdale's Middle River, is scheduled for repair in 2009 and replacement in 2013. FDOT also is pushing for repairs to a failing bridge over the C-111 canal along the 18-mile stretch of U.S. 1 leading to the Florida Keys.
Another hazard is ships striking bridge supports. In 1980, 35 people died when a vessel crashed into Tampa Bay's Sunshine Skyway Bridge, sending 1,200 feet of the bridge into Tampa Bay. That span was demolished and replaced by a cable-supported structure similar to San Francisco's Golden Gate Bridge. Ships have struck bridge supports along the Miami River, but none of those bridges collapsed. In 2002, the bridge control tower suspended off Miami's Flagler Street bridge collapsed because of a hidden design deficiency inspectors did not detect, according to the Herald.
The Associated Press reported that in 1990, the federal government rated the I-35W bridge "structurally deficient," citing corrosion in its bearings. That made it one of 77,000 bridges in the category nationwide and 1,160 in Minnesota. Subsequent inspections on the I-35W bridge determined fatigue cracks and corrosion in the steel around the bridge's joints. Those deficiencies were repaired. Starting in 1993 the bridge has been inspected annually, the AP reported.
A 2005 federal inspection again rated the bridge “structurally deficient,” giving it a 50 grade on structural stability scale of 100. A 2006 evaluation recommended monitoring of "fatigue cracking" on the bridge's girders. Fatigue cracking involves a breakdown of the molecular structure in the steel. Vibrations and stress from 140,000 vehicles a day can diminish the steel’s support capacity, according to engineers.
At the time of the bridge collapse, workers were repairing road pavement, not supporting steel. Sixty vehicles still were submerged in the river two days after the collapse, according to the National Transportation and Safety Board. At that time, five were reported dead and eight missing, according to the Minneapolis Star-Tribune. The bridge opened in 1967. It has one continuous span of steel resting atop concrete piers submerged into the riverbanks of the Mississippi River.
http://www.miamiherald.com/459/story/190977.html
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