Ahead of the curve: Reducing girder rotation and increasing bridge life span

10/21/2019 McCall Macomber

Illinois has the third-largest number of bridges in the U.S., according to the American Road and Transportation Builders Association. This makes it important to offset upkeep costs by building longer-lasting bridges.

A key component for longer-lasting bridges? Quality bridge deck construction.

When bridge deck finishing machines are used to construct the deck, the extra load will rotate the exterior girder, which can cause undesirable torsion in the beam or a thin concrete deck. Previous Illinois Department of Transportation-specified temporary tie-bracing systems were not consistently effective in preventing this problem.

That’s why Illinois Center for Transportation and IDOT researchers investigated girder rotation and prevention systems in project “R27-140: Effectiveness of Exterior Beam Rotation Prevention Systems for Bridge Deck Construction.”

This initial project — led by Riyadh Hindi, principal investigator and Saint Louis University professor, and Mark Thomson, Technical Review Panel chair and IDOT bridge design and construction review unit chief — studied rotation-prevention systems for contractors’ use.

Here an exterior girder supports a bridge deck.

The duo decided to further investigate this challenge in project “R27-179: Effectiveness of Exterior-Beam Rotation-Prevention Systems for Bridge Deck Construction — Phase II,” joined by Doug Dirks, TRP co-chair and IDOT construction project review engineer.

Hindi evaluated four steel-girder bridges in Illinois, using a design that was developed in the initial project to limit girder rotation. This design utilizes a temporary bracing method consisting of transverse tie bars between exterior girders along with diagonal pipe bracing underneath.

After installation of the bracing system, Hindi monitored exterior-girder rotation during bridge deck concrete placement by placing sensors on the exterior girders to record real-time behavior.

Hindi also modeled the tie-pipe system for different bridge structure configurations and construction conditions. This also included an analysis of the effectiveness of the systems with different variables, such as girder depth, bracing spacing and bracing components.

In the project, researchers determine a maximum ratio between bridges’ girder depth and unbraced length.

Thanks to their efforts, Hindi, Dirks and Thomson developed an optimum bracing approach to control girder rotation, helping prevent problems for contractors in the field.

“The project was able to provide bridge designers with tools to predict the behavior of exterior girders during construction,” Hindi said. “It also provided contractors with an economical solution to reduce the rotation of exterior girders.”

Illinois drivers will feel an additional perk from the findings of the study: “smoother rides,” Hindi said. Smoother bridges are a key component for longer-lasting bridges.

Dirks and Thomson are excited about the results of the project, because they believe the research will advance the state of practice in bridge construction.