Large loads compromise our bridges

By Brian Keierleber

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The harvest that is wrapping up is taking its toll on Iowa's rural roads and bridges.

In a recent two-week period, Buchanan County experienced piling failures on three bridges. Two of them had load limits posted, while the third was OK for legal loads. In one case, the neighbor told me who to visit with and the response we received was typical: “We have crossed that bridge with our combine every year since I was a little tyke.”

The problem isn't who is at fault as much as it is the reality that the bridges are getting older while the agricultural equipment and loads are getting bigger and heavier. This farmer acknowledged he had his combine's corn header on but there was supposedly very little corn in the hopper. His momentum got him across even though a portion of the deck broke away and fell into the creek. Had the combine gone down, he could have been injured or killed.

In the other two cases, the bridges did not fall in. They just developed funny sags and the grain wagons in use were stopped. That prevented immediate failure but left a risk of later collapse under another vehicle.

One of our local farmers acknowledged that his two 750-bushel grain wagons filled weighed just less than 100,000 pounds without the weight of the tractor. Bridges are designed for 80,000-pound vehicles despite any governor's proclamation.

There is a strong tendency in all of us to follow the same path we always have regardless of what sign sits at the end of the bridge. What we fail to recognize is it is not just our life that we are putting at risk but the lives of those who follow.

When the harvest is finished, the liquid manure tanks come. Does anyone ever consider what a 10,000-gallon liquid manure tank weighs? I am told the loaded tanks with the weight of the tractor are near 140,000 pounds. They shouldn't be crossing the bridges.

The load limits that counties post on the bridges are not arbitrary. They state how much load a structure can safely carry. It is possible to unsafely cross such a bridge with heavier loads but doing so weakens the bridge with each passage, leading to premature failure and possible injury. Laws of physics do apply.

We can't replace all the bridges right away. We are funded to replace about one bridge per year with federal money. At that rate, we could get to all the bridges in Buchanan County in 257 years.

We often are told: “You always use such a large safety factor that there is never any problem crossing the posted bridge.” There are three closed bridges in our county to dispute that statement.

Brian Keierleber is Buchanan County engineer. Comments: bkeierleber@co.buchanan.ia.us