Gov. Rod Blagojevich on Thursday ordered state inspectors to examine all bridges considered to be critical as Illinois officials sought to assess the safety of its bridge network in the wake of the deadly collapse in Minneapolis.
But even as the governor took aggressive measures, structural engineers and bridge safety experts cautioned that there was no cause for panic -- Illinois bridges are inspected regularly and tragedies such as the one in Minnesota are isolated incidents, often involving many factors.
"American bridges simply don't collapse of their own volition," said David Schulz, director of the Infrastructure Technology Institute at Northwestern University. "It's almost always a chain of low-probability events coming together. It takes a lot to bring these down.
"I don't worry about bridges," he said. "When we look at spectacular bridge failures, I think what you're going to find is that somebody did something that affected the bridge and that it wasn't the structure or the bridge itself."
Blagojevich acknowledged that Illinois regularly and vigorously inspects its bridges and that he undertook this measure as a precaution. He was among a slew of governors across the country who ordered new inspections. By Thursday evening, the federal government had alerted states to inspect spans similar to the Minneapolis bridge.
"While we have a rigorous inspection system that ensures the safety of our bridges in Illinois, a tragedy like this demands that we step up our efforts and do everything in our power to guarantee the safety of our bridge network," the governor said in a written statement.
Nearly 2,450, or 9.4 percent, of 26,000 bridges rated in Illinois were considered "structurally deficient," according to the Federal Highway Administration's National Bridge Inventory. Nationally, 12.4 percent of rated bridges fall in that category.
Illinois compiles a list of priority bridges based on a rating scale that uses various measures to assess safety and deterioration. The 1-to-100 scale is used to prioritize replacement or repair of bridges. The higher the rating, the better the condition. A rating of 80 or less means the bridge probably needs some rehabilitation. A rating of 50 or less means the bridge may need to be replaced.
At least three spans on the Skyway were rated structurally deficient, with ratings in the low 70s.
But engineers stress that when a bridge is structurally deficient, that does not mean it is unsafe for travel. It simply is no longer able to carry the load size for which it was designed. In response to a structurally deficient bridge, engineers will assess the damage and then determine new weight limits.
"A structurally deficient bridge is still perfectly safe to use for years and years and years, provided the limits are not overly stressed," Schulz said.
Inspectors from the Illinois Department of Transportation and the tollway will examine bridges that are similar to the truss-style Minnesota bridge, that are under construction or that carry high volumes over waterways, state officials said.
Those critical spans would be visually inspected for general structure alignment and anything out of the ordinary, officials said. Inspectors could pore over a truss-designed bridge for as long as a week, officials said.
Most of the dozen truss-style bridges are Downstate, including spans along Interstate Highway 74 near Peoria and several in the Metro East area, said Maria Kollias, an IDOT spokeswoman.But engineers also are looking at a highly trafficked bridge on the Bishop Ford Expressway near 130th Street because it has been deemed structurally deficient in past inspections, Kollias said.
Slowdowns through the Dan Ryan Expressway construction zone have put additional vehicles on the Bishop Ford bridge recently, she added.
But overall, "there is not a reason for people to be worried about driving in their vehicles," Kollias said. "We have done our inspections."
Bridges are designed to carry twice their stated weight capacity, said Richard Hughes, an engineering consultant based in Pennsylvania. Another way bridges deteriorate is through wear and tear from the weather, he said.
Given the vast fluctuations in weather in places such as Minnesota and Illinois, thermal expansion can prove detrimental to bridges. From below-zero to the mid-90s, a bridge can expand by as much as 3 feet as it heats up, Hughes said.
During the winter, sand and salt get into areas that allow the bridge to stretch, and the resulting rust can lock them into position. When the bridges heat up there is insufficient flexibility, which can lead to a collapse, Hughes said.
Weather-related damage can be difficult to detect, he added.
"You could be looking at the [area] on that bridge and think, 'It looks all right,' but if it's locked up somehow, how would you know?" Hughes asked.
Though visual inspections of bridges are necessary and important, they are not adequate for all structures, said Farhad Ansari, professor and head of civil engineering at the University of Illinois at Chicago.
More complicated structures, such as the truss-style bridge in Minneapolis, older bridges and very heavily traveled ones should also be equipped with electronic sensors and computers to accurately monitor movement and vibration, said Ansari, who made that point in a 2003 report for IDOT.
The cost of installing sensors can range from $50,000 to $200,000 per bridge, and states have done little to install such monitoring equipment, mainly because of limited infrastructure funding, Ansari said.
In Springfield, the Minnesota bridge collapse sparked outraged legislators from both parties to call for quick action on long-stalled construction projects in need of funding throughout the state, including potential bridge repairs.
Blagojevich has been unable to win approval of a capital project plan because legislators have not trusted the governor to be impartial in the way he distributes the money.
Further complicating the matter is regional politics that are always in flux. Illinois is a state where Downstaters compete for transportation dollars to keep community infrastructures and long miles of open highways up to date.
It was uncertain Thursday whether there would be additional costs from the immediate inspections. But officials said inspectors will take their new mission seriously.
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IN THE WEB EDITION Find a list of the 2,449 Illinois bridges rated "structurally deficient" at chicagotribune.com/bridgelist
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