Iowa's 24,799 bridges keep officials busy with maintenance, renovations
Engineers keep tabs on bridge industry innovations
Charged with maintaining and replacing many more bridges than most states, highway officials in Iowa try to keep up with new developments.
Take Mehaffey Bridge, for example.
Initial bids last March on a new span carrying Mehaffey Road over Coralville Lake between North Liberty and Solon came in some 50 percent over the project's $6 million cost estimate, Johnson County Engineer Greg Parker recalled.
"Our consultant went back to the drawing board and talked to the contractors who bid on it," Parker said.
Design tweaks resulted in an alternative Parker hopes will go to bid in a few weeks.
"We’re hoping we can bid both the original design and the revised design and see if there was a cost savings," he said.
Parkers's counterparts across the state can identify. The streams and rivers lacing Iowa give the state the fifth-highest number of bridges in the nation, according to the Federal Highway Administration — more than even California.
The top 10 states are:
The Federal Highway Administration's National Bridge Inventory counts 24,799 Iowa spans. Of the total, 5,358 are considered structurally deficient, and another 1,320 functionally obsolete.
That doesn't mean those bridges are an imminent danger, but they see more frequent inspections.
The standards are based on a detailed points system for condition of supports, decks and other structural elements. But "structurally deficient refers to bridges needing significant maintenance attention, rehabilitation or replacement," according to the Iowa Department of Transportation.
"Functionally obsolete" means a bridge's deck, load-carrying capacity, vertical clearance or approaches "no longer meet the criteria for the system of which the bridge is a part."
Bridges must be inspected at least once every two years, with the results forwarded to the bridge inventory.
"We have bridges we inspect every year because of conditions we want to keep an eye on," said Scott Neubauer, head of bridge maintenance and inspection for IDOT. "An older bridge may get inspected more often than a newer one, but it’s always going to be 24 months or less."
The state has budgeted $30.6 million to replace non-interstate bridges in the current budget year. That's projected to rise to $43.2 million in fiscal 2015, then settle back to $40 million the following year.
Bridges should last 50 years "as a rule of thumb," Neubauer said. "We try to maintain them for as long as we can. The more sophisticated ones are the ones we try to maintain for as long as possible because of the cost of replacing them."
Designing a more durable bridge runs into practical considerations.
"You can always make a bridge huskier than you would today and say that’s going to last longer, but that’s more expensive," he said. "You start to pay for it today — you don’t get to pay for it the 100 years the bridge is there."
Mehaffey Bridge dates to 1955, when Coralville Lake was created by the Iowa River's damming.
"When you hit 60 years, you do start making choices," said Parker.
Linn County has no major bridge projects planned, County Engineer Steve Gannon said. The county will replace two timber spans a year and continue to replace smaller bridges with cement culverts.
None of Buchanan County's bridges is on the order of Mehaffey. But County Engineer Brian Keierlaber is responsible for 257 bridges, many of them relatively short spans on country and farm-to-market roads.
With less than $300,000 a year to replace and maintain those bridges, Keierlaber has become Eastern Iowa's go-to guy for innovative techniques — he's won federal grants to explore cost-saving practices.
"We tend to push the technologies a little bit here," he said.
One promising design: the geosynthetic-reinforced soil (GRS) system using a fabric-reinforced fill for bridge support and approaches.
Buchanan County has built three GRS bridges over the past two years. Keierleber estimated they save 30 to 40 percent over conventional designs.
"It gives counties with limited equipment the ability to build a bridge abutment," Keierleber said. "You do not need a crane."
Keierleber is also experimenting with folding steel plate into a bridge's load-bearing structure instead using of more expensive rolled-steel beams. A Muscatine company supplies the folded plate frame members, although savings have yet to materialize on low volume.
"As the technology expands, so will the abilities," said Keierleber. "It has the potential. The demand drives the marketplace, too."
On a somewhat grander scale, the new Highway 65/Oak Street bridge over the Iowa River in Iowa Falls may be the state's, if not the nation's, first "smart" bridge. The span is loaded with sensors and gauges to monitor traffic, corrosion, strain, and structural movements over time.
"We’ve got a pretty steady stream of data coming in," said Brent Phares, interim director of the Bridge Engineering Center at Iowa State University's Institute for Transportation. The center set up the sensors and their supporting software under a $300,000 IDOT grant.
The sensors send data in real time to engineers monitoring the bridge's condition. But Phares sees a day when at least some of the instruments are linked to smart-phone apps or in-car navigation systems.
"Is the deck freezing or not?" Phares said. "We can see if there’s congestion on the bridge — what’s the volume of traffic, what’s the speed of traffic? You could maybe see in advance through social media where the problems are."
The instrument package came to just over 2 percent of the bridge's $12.8 million total. Phares said that's a reasonable share of project cost.
"That’s sort of where we see that owners wouldn’t protest a little bit of extra cost," said Phares, who figures alerting truckers to Interstate traffic jams would pay off.
"I-80 and I-35 are so vital to the movement of people and goods that any disruption on these routes has a big ripple effect, not only in the state of Iowa but nationwide," said Phares.
The 2004 installation of an automatic anti-icing system on the Highway 30 bridge over the Cedar River south of Cedar Rapids failed due to software issues, said Jeff Tjaden, IDOT district operations manager. The $150,000 system was shut down before the June 2008 flood, he added.
"The flood did the final tear-out on it, but up until then we were having pr0blems," Tjaden said. "The software side of it wasn’t giving us what we needed."
The system applied magnesium chloride solution onto the bridge deck when sensors detected icing conditions, but the software had problems determining when those conditions existed, Tjaden said.
"We just lost (software vendor) support on it," said Tjaden, but if the bugs were removed "it’s always something to look at in the future."
At Coralville Lake, the new Mehaffey bridge will have just two piers instead of the present three, to "open the lake a little" and minimize boating hazards, Parker said. He hopes construction could begin as early as late summer 2013, with completion in two years. The present bridge will remain in place while the new one is built."It’s actually an exciting structure," Parker said. "It’s going to be wide enough to handle trail traffic on one side of the bridge, but those alternates are all going to come down to the money."