The American Road & Transportation Builders Association uses federal data each year to issue an annual bridge report. The 2016 version showed Iowa with more structurally deficient bridges than any other state.
There are more than 5,000 structurally deficient bridges statewide. Slightly more than one in five bridges fits into the category. Iowa ranked second behind Pennsylvania for the previous two years, but they swapped places in the 2016 report.
Iowa held steady at third by another measure, the number of deficient bridges as a percentage of all bridges. Rhode Island topped that ranking.
Most area counties have a higher percentage of structurally deficient bridges than the state's 20.7 percent. More than 43 percent of Davis County's bridges are structurally deficient, the highest percentage in the area. In Wapello County the figure was 27 percent. Only Mahaska County, where 20.4 percent of the bridges are structurally deficient, beat the state's figure.
Jim Armstrong, the area's district engineer for the Iowa Department of Transportation, said a designation of structural deficiency involves assessment of three bridge components. There's the deck, where cars drive, and the superstructure, which supports the deck. The substructure includes foundations and bridge piers that ensure the bridge stays above the obstacle it crosses.
"When any one of those three elements ... due to age there's deterioration, we rate those and that's what counts as structurally deficient," he said. "We work very hard to repair, replace those bridges."
The bridge on Highway 34 over Bear Creek is an example of what happens as bridges age. The deck had problems requiring frequent repairs. "We were patching the deck a lot," Armstrong said.
Replacement work began in 2015. A temporary bridge is in place just south of the highway, though it's not yet completed. Traffic will be shunted along the temporary bridge while a new span is constructed.
Armstrong said the state planned for the work and knew the bridge was getting close to needing a replacement. Federal law requires agencies with responsibility for bridges to keep a close eye on them.
"We are required to inspect them every two years, and depending on the condition of the bridge we may step that up to every year," he said. "There might be one particular issue we're watching."
So structurally deficient involves safety issues. But there's a second figure that can come into play: functional obsolescence. Those bridges are considered deficient but not because of damage or wear.
Imagine a stone bridge built 100 years ago, when most of the traffic was composed of Model T cars and the occasional truck. The requirements were different then. Narrower lanes, or even one-lane bridges, were much more common.
That same bridge might be perfectly fine today. You could take the family car over it without concern, but you wouldn't want to try to drive a semi over it. The bridge is obsolete, but not dangerous.
That issue came into play with the Bear Creek as well. The bridge was built in 1963, but it was considered functionally obsolete because of the narrow width.
Despite falling into both categories of deficiency, the bridge did not pose undue risk for drivers. That's the case for the vast majority of deficient bridges and nearly all of the functionally obsolete ones. If you're allowed to drive over a bridge, officials say you can do so confidently.
"All of them are safe," Armstrong said.
Comparatively few of the deficient bridges belong to the federal and state highway system. Far more are part of county or city networks. That, said Wapello County Engineer Brian Moore, is largely a function of scale.
It's easy to underestimate just how big city and county road networks are. Of the 203 bridges the federal government lists in Wapello County, 120 are the county's responsibility. Another 19 or so bridges belong to Ottumwa.
Moore's office covers 565 miles of "granular surfaced" (rock or gravel) roads. County crews are responsible for another 138 miles of paved roads. How much is that? By comparison, Iowa's section of I-80 is only 306 miles long.
Local bridges follow the same, two-year inspection requirement as those under state care. Moore said structural deficiencies are most often addressed by placing weight restrictions on the spans.
The county's website indicates restrictions are posted when the bridges' capacities fall below 40 tons. That's far more than what people drive unless they're in construction or agriculture. So long as those larger vehicles respect the weight limits, normal vehicles are perfectly safe.
"Just because they're structurally obsolete or structurally deficient, that doesn't mean they're not safe," Moore said. "Structurally deficient has just reached the point where we have to put a load rating on it."
The county's road network is considerably larger than Ottumwa's city roads. But Ottumwa still has 170 miles of streets and another 60 miles' worth of alleys. City Engineer Dwight Dohlman said he generally agrees with Moore's assessment that there is a difference between deficient and dangerous.
The most recent major project, replacement of the Market Street Bridge, means several things to the city. It eliminates concerns about a bridge with obvious wear, which cuts maintenance costs. But even when the new bridge needs work it should be cheaper than the old version.
Advances in technology and technique mean newer bridges can be built to facilitate maintenance. That doesn't matter when you already have older bridges built, but can make a big difference after replacement.
"New is always better," said Dohlman. "Take the Market Street Bridge for example. It's a new bridge and it's going to be easier to maintain."
Unless some version of the Jetson's flying car goes into common use within the next couple of decades, bridges will remain a key element of transportation networks. And, as they age, maintenance costs will be considerable.
Both Dohlman and Moore said funding is a major part of the challenge for maintaining transportation networks. Steps like Iowa's hike in the state's gas tax help, but that falls far short of solving the problem.
Most governments spend a very large percentage of their budgets on public safety. That's necessary. But it means the remaining departments, including transportation, have to figure out how to divide a much smaller piece of the pie chart.
"Funding would be the obvious answer," Dohlman said. "But the city receives enormous pressures on where the funding comes from. It's going to continue to be a challenge unless more funding becomes available."
One partial solution might be new materials and construction techniques. Wapello County has experimented with that approach. Back in 2012 the new bridge over Little Cedar Creek won an award for its innovations. The bridge doesn't absorb water, a major factor in deterioration.
"Concrete is a porous material. It has voids in it that allow water to get into it," Moore said. "You have the natural freeze and thaw. And then in winter, what's used to clear snow and ice? Salt."
Salt clears the bridge's deck of snow and ice, making it safer for drivers. But it creates a new problem.
"Typically, bridges are very heavily salted, so they go through more freeze-thaw cycles," said Dohlman.
Salty water freezes, just at lower temperatures than fresh water. So it's entirely possible to have the water in the bridge freeze when a storm starts, thaw when it's salted, the re-freeze overnight before starting the entire cycle over again the next day.
When water freezes, the ice pushes out in all directions. That can break up the concrete. If the Little Cedar bridge doesn't absorb water, that process should be interrupted, preventing damage.
"It's too early to tell at this point," said Moore. "But the technologies that we used are being expanded across the state and across the nation."
In other words, we'll have to cross that bridge when we get to it.
©2016 Ottumwa Courier (Ottumwa, Iowa) Distributed by Tribune Content Agency, LLC.