With harvest right around the corner, local officials are asking growers to be aware of load limits on county bridges when they are moving equipment. Bridges without a load limit sign typically have a weight limit of 80,000 pounds or 40 tons.
“Just be careful what you drive your big equipment over. The last thing I ever want to do is have a conversation with a farmer who broke a bridge and inform him he needs pay for the bridge,” said Mark Storey, Whitman County Public Works director. “Farm equipment is getting so large and so heavy now that it cannot safely travel over bridges. Bridges, especially the short spans, are susceptible to those heavy loads, and it doesn’t matter whether it’s a low ground pressure piece of equipment or not.”
In particular, Storey points to equipment that is really meant to stay in the field and is not a legal, on-the-road vehicle, such as a bank-out wagon or a combine; both carry a lot of weight on short wheelbases. Instead of older equipment that spread 50,000 pounds over 50 feet, newer equipment can easily weigh 70,000 to 80,000 pounds spread over 20 or 30 feet. Shorter span bridges were not made to carry all of that weight at once.
Today’s combines present another issue besides just weight and how it is distributed; they are also wider, which puts more pressure on the outside of a bridge where there is often less reinforcement, especially on wooden bridges. A few years ago near Oakesdale, Wash., a short span wooden bridge collapsed as a combine with header went over it. Storey estimated that the single front axle had several thousand pounds more on it than a regular axle on a fully loaded semitruck did.
“When you put the header on the front of a combine, you are sort of doing a cantilever, and you are putting more load on the front tires and taking load off the back tires at the same time as your total weight goes up,” Storey said.
There are approximately 300 bridges (and structures treated like bridges) in Whitman County. Storey estimated that the average age of the county’s bridges is 40 years. The issue is much bigger than just Whitman County. Other Washington counties and counties across the nation are also seeing agricultural equipment testing bridge load limits. Scott Yaeger, an engineer for Adams County, has the same warning for growers in his area.
“Older structures were not designed to handle more than a single axle wheat truck with a legal loading of 28,000 pounds,” he said. “These structures are capable of handling a semitruck and trailer unit as these units usually allow for the spacing between axle sets to typically be 22 feet or more, and the loading of those axle sets doesn’t exceed 23,000 pounds legally.”
Yaeger also asked farmers to be aware of the width of their equipment to avoid damaging a bridge’s safety rail system.
Adams County has approximately 287 bridges and structures treated like bridges, such as large culverts. In both counties, bridges are inspected every other year. Funding for county bridge replacement typically comes from the federal government; tight funding means there isn’t enough money available to replace all of a county’s aging bridges. However, replacing a wooden bridge with a concrete or steel structure doesn’t resolve the issue, as federal money comes with regulations that require bridges to be built for trucks, not farm implements. Newer bridges are built with the same weight limit of 80,000 pounds or 40 tons.
“The very first thing I would do (when a bridge is damaged during use) is look at what they drove over it. I am compelled to look at how the bridge broke,” Storey said. “If the bridge broke because of some lack of maintenance, it wouldn’t be the farmer’s issue. If some vehicle like a combine or bank-out wagon exceeds those maximum axle loads, then it becomes the responsibility of the person that broke the bridge.”
Storey said it could cost more than $150,000 to replace a 25-foot wooden bridge.
Bridges without posted load limits should be safe up to 80,000 pounds. Bridges with lower limits will have signage posted. Several years ago, counties were required to do bridge assessments for heavy single trucks that have four to seven axles. Whitman County has posted signs (SU4 through SU7) on approximately 110 bridges that may be susceptible to heavy farm equipment.
“If you see that posting, you should get a little nervous about what you are putting on the bridge, because that posting means that the bridge is susceptible to heavy, concentrated loads. That’s the kind of sign you need to watch for. If you see that, then be careful,” Storey explained.
Both Storey and Yaeger said the best thing farmers can do to prevent damaging or collapsing a bridge is to avoid the structures all together, even if that means taking a longer route from field to field. If avoiding a bridge isn’t possible, farmers should empty bins and storage tanks and remove headers before driving over the bridge.
“Farmers need to know the weight and length of their tractors and implements along with the width of their implements and predrive their transport route to identify any roadway or structure restrictions,” Yaeger said. “To reduce the potential of overloading a structure, I would suggest transporting any tractors weighing more than the old single axle wheat truck of 28,000 pounds on a trailer that will disperse the weight to a larger area, which will help with the concentrated loading.”
Growers with questions should contact their county public works office. However, Storey warned that there is no easy way to calculate exactly how much load a bridge can take before it is damaged or collapses.
“I’m happy to visit with any farmer on the phone. What I can’t do is have a farmer call me up on harvest day and say, ‘I’m sitting at the bridge, and I’ve got a big old honking tractor I want to cross the bridge with. Can I?’” he said. “I can give them guidance, but I don’t have exact answers, and it might take days to figure out their exact tractor weight and what it might do to a bridge. We are trying to keep people down to the lightest loads they can on the bridges.”
