husbandry | Crossroads

Minnesota farm equipment is getting larger and heavier, causing strain on rural bridges. However, there are no nationally recognized specifications for what size and weight of tractors can safely travel over them.

Currently, bridge load limits are based off semi-trucks, not farm machinery, which have much different axle configurations and wheel dimensions.

“Their geometry is atypical; their length, widths are different; they have different suspension characteristics,” explains Brent Phares, director of the Bridge Engineering Center at Iowa State University.

A new pooled fund study led by the state of Iowa is attempting to determine how much stress heavy farm vehicles put on bridges. This data will be used by local agencies to develop weight restrictions specifically for farm equipment.

“It will help limit the confusion of current load posting signs for farmers,” said MnDOT bridge load rating engineer Moises Dimaculangan.

Wisconsin, Minnesota, Nebraska, Oklahoma, Illinois, Kansas and the United States Department of Agriculture are also participating in the study, which is examining three types of local bridge superstructures: those with steel girders and concrete decks; bridges with steel girders and timber decks; and timber bridges with timber decks.

Through physical testing and modeling, the study will determine how different types of farm machinery distribute their loads on the bridge superstructure.

About a half-dozen farm vehicles were tested on 20 different bridges which were representative of those tending to be the most problematic for farm equipment traffic on secondary road systems, Phares said.

Instrumentation measured the response of the structures to the vehicles. This data was then used as a baseline to calibrate analytical models, which could be applied to 250 different bridges and 121 different farm vehicles.

Researchers will develop a generic tractor profile, which represents the worst-case scenario, for use in determining load limits. With the information developed, signs might be able to be added to the bridges, which show a tractor and the weight limit.

“I get a number of pictures emailed to me of bridges that have failed with a tractor implement of husbandry on top,” Phares said. “That’s the problem that people are looking to avoid; the goal isn’t to restrict the size of farm vehicles, but to develop better tools for engineers to make sound and solid analyses for the bridges, so they can provide that information to the people who need to have it.”

Phares said a couple previous studies have also looked at farm machinery weight restrictions. One study, from around 2004, took a high level look at the impact of farm vehicles on bridges. A more recent pooled fund study analyzed the impact of machinery on pavements.

Related resources

Research in Progress: Study of the Impacts of Implements of Husbandry on Bridges

The Effects of Implements of Husbandry “Farm Equipment” on Pavement Performance

Soaring construction costs and a rapidly aging infrastructure will require states to revolutionize how they maintain their roadways — but without each other’s help, they won’t be successful.

That was a key message from pavement researchers last week at a MnDOT-hosted peer exchange event, where pavement experts from around North America shared their ideas and research experiences.

“You’ve got to partner with other states, the FHWA and industry,” said Research Engineer Steve Bower of the Michigan Department of Transportation. “We can’t go it alone anymore.”

Researchers at the event reviewed recent pooled-fund studies conducted at MnROAD, MnDOT’s innovative pavement testing center, to review successful implementation strategies, develop common practices to calculate benefits and help prioritize research topics for MnROAD’s core 2016 research and reconstruction.

The pavement engineers gathered for the event face similar problems in their home states, as demonstrated by the seven pooled fund projects that were discussed. These included developing a better understanding of pavement damage caused by oversized farm equipment, knowing when to chip seal a roadway, developing a test to predict asphalt cracking , creating a national design method for concrete overlays of asphalt roadways and improvements in diamond grinding of concrete pavements.

MnROAD leading the way

State research departments often lack the time or resources to focus on innovations that could reduce future maintenance costs. If not for Minnesota leading the effort on many of these topics and providing a top-notch research facility, the peer exchange attendees said much of this research just wouldn’t happen.

“We don’t have a closed-loop facility with all these different test sections that MnROAD has; no one does,” said Larry Wiser of the Federal Highway Administration’s Turner-Fairbank Highway Research Center.

Researchers came from Missouri, Maine, Texas, Illinois, Michigan, California, Ontario, Wisconsin, Indiana and Washington for the three-day workshop.

WisDOT Chief Materials Management Engineer Steven Krebs said the research done at MNROAD on the impact of modern farm implements on pavement was invaluable in drafting new state legislation. WisDOT was able to quantify the amount of damage done to the pavement and use the data to dispute mistruths and misinformation. The state is now working with counties on possible remedies and weight-limit enforcement techniques.

Whereas Minnesota has taken the lead on studying such issues, it is now asking fellow states to not only participate in future such studies, but to also partner in the operations at MnROAD. At the peer exchange, the response to this idea — especially from states closest to Minnesota — was positive, despite everyone’s lean budgets.

Peer exchange participants said more effort and funding is needed to implement research findings, which FHWA officials said costs significantly more than the research itself.

Past research also needs to be more accessible and there should be better sharing of information, particularly online, they said.

“This (peer exchange) gave us ideas to take back. Our research budget is getting tighter. It’s nice to be able to say, ‘You do a part of it and we’ll do a part of it,’ ” said California transportation researcher Joe Holland.