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Culvert research may ease the workload of MoDOT engineers

Missouri Department of Transportation crews assist Mizzou Engineering researchers collect data on the bridges by driving loaded trucks onto and across the bridges. Researchers collected data both moving and static truck data.

Missouri Department of Transportation crews assist Mizzou Engineering researchers collect data on the bridges by driving loaded trucks onto and across the bridges. Researchers collected data both moving and static truck data.

The Federal Highway Administration (FHWA) requires bridges within certain criteria to be inspected every two years to be compliant with National Bridge Inspection Standards and every bridge must be load rated. With more than 10,000 bridges in the Missouri Department of Transportation’s (MoDOT) system, load rating the culverts underneath the roadways becomes a time-consuming and costly task for MoDOT. Department researchers wondered whether there was a way to save time and money load rating those bridges and funded a project with Sarah Orton, an assistant professor in the Civil and Environmental Engineering Department, and other researchers from the University of Missouri College of Engineering to determine if there was a way to show that load rating is not needed for every culvert.

Culverts are structures that allow water to pass under a roadway, railroad or similar obstruction and those examined by Orton’s group were reinforced box concrete. Culverts spanning more than 20 feet along the roadway’s centerline are designated as bridges and require load ratings. The fill around the culvert may be soil, rock or other materials on top or on the sides of the structure. The FHWA requires bridge culverts with fill depths less than the span of the culvert, which can reach lengths as far as 45 feet, to be load rated for the effects of live loads on those structures.

This graphic shows the vertical stress distribution in the soil around a two of the analyzed bridges. The top is of a bridge in Callaway County, Mo., which has two, 12-foot cells in the culverts. This bridge has a fill-depth of 13.25 feet and shows the more evenly distributed stresses. The bottom image is of a bridge in Audrain County, which has three, 14-foot cells underneath. This bridge has a fill depth of about 2.5 feet, and shows the pressure less evenly distributed across the culverts.

This graphic shows the vertical stress distribution in the soil around a two of the analyzed bridges. The top is of a bridge in Callaway County, Mo., which has two, 12-foot cells in the culverts. This bridge has a fill-depth of 13.25 feet and shows the more evenly distributed stresses. The bottom image is of a bridge in Audrain County, which has three, 14-foot cells underneath. This bridge has a fill depth of about 2.5 feet, and shows the pressure less evenly distributed across the culverts.

“The idea is that reinforced box concrete culverts are already really strong, and to do a load rating on each of these culverts would cost a lot of money, so this may be a place where MoDOT can save money,” said Orton, who served as principle investigator for the project. The team, composed of Orton, civil engineering Associate Professor Erik Loehr, Research Engineer Andrew Boeckmann and graduate student Garrett Havens, investigated whether there was a point at which the live load effects on reinforced box concrete culverts are insignificant when compared to the dead load.

Live loads are temporary or moving loads on structures such as large, cargo-filled trucks driving across bridges. Load rating represents the amount of live load that may safely cross a structure. Dead loads are constant, including the weight of the structure itself, and in the case of bridge culverts, the soil fill around the culverts.

“We looked at, ‘what fill-depth does the culvert still respond to the live load?’” Orton said.

Beginning in August 2012, researchers tested 10 different active MoDOT culverts, most of which were located in mid-Missouri.

“We wanted a variety of depths, but we also wanted ones without any signs of deterioration,” Boeckmann said. All of the culverts were in good condition and had fill depths ranging from 2.5 feet to 13.5 feet.

Examining the culverts onsite are, from left to right, Missouri Department of Transportation Senior Research Analyst Andy Hanks, Mizzou Engineering Assistant Professor Sarah Orton, Research Engineer Andy Boeckmann and graduate student Garrett Havens.

Examining the culverts onsite are, from left to right, Missouri Department of Transportation Senior Research Analyst Andy Hanks, Mizzou Engineering Assistant Professor Sarah Orton, Research Engineer Andy Boeckmann and graduate student Garrett Havens.

After instrumentation was gathered, researchers worked with MoDOT crews — who drove loaded trucks over the bridge length culverts — and looked for deflections and strains in the culverts. They conducted the tests in two phases — five in the first group and five in the second. The research team looked for at which fill depths resulted in a live load that was less than 10 percent of the dead load. That mark was found to be at about six feet, Orton said, meaning fill depths greater than six feet will results in a live load reduced enough and the culverts may not need to be load rated.

The American Association of State Highway and Transportation Officials (AASHTO) Research Advisory Committee (RAC) for Region 3, which includes nine states, recently honored the research. Andy Hanks, a senior research analyst for MoDOT and the project’s MoDOT liaison, submitted the research for recognition as one of the organization’s 2014 High Value Research projects from the region. The research tied for second place with a project from the Iowa Department of Transportation. The 16 top projects among four regions, will be presented at the annual AASHTO RAC meeting in Madison, Wis., in July.

Orton said an article written from the research, “Live Load Effect in Reinforced Concrete Box Culverts Under Soil Fill,” has been submitted for review to the Journal of Bridge Engineering.