Study Improves Safety of Bridge Timber Piles and Expands Useful Lifespans

Damaged timber piles are the number one cause of posting and closing of bridges in the state of Illinois. Despite their cost-effectiveness, wood pilings can decay, significantly reducing the capacity of the piles. The collapse of a bridge in DeKalb County, Illinois, on August 19, 2008, is a clear example that shows the vulnerability of damaged wood pilings to eccentric loading. Many local and rural bridges are supported by timber piles, and it is essential that these bridges are safe. Additionally, it is necessary to determine the most cost-effective way to maintain timber piles.

Evaluation of Load Rating Procedure

This study first evaluated the Illinois Department of Transportation’s (IDOT’s) current load rating procedure for timber piles supporting multiple-span, simply supported bridges. Under current IDOT specifications, piles are rated under concentric loads, and the effect of bending in the piles is neglected. But as this study’s principal investigator, Bassem Andrawes of the University of Illinois at Urbana-Champaign notes, recent research indicates that under extreme circumstances, the effect of bending in the piles is great and could have an impact of their load rating. Therefore, an alternative rating method was used to test the piles. The proposed load rating method produced much lower ratings for piles with moderate to high levels of deterioration when compared with ratings using the conventional approach.

Fiber Reinforced Polymer Retrofitting

This study also examined the effect of using the fiber reinforced polymer (FRP) retrofitting method for timber piles subjected to bending. Overall, tests found piles retrofitted with glass FRP were stronger than unretrofitted piles. On average, specimens retrofitted with glass FRPs showed a 10% greater strength than piles that had not been retrofitted. FRP retrofitting is significantly less expensive than replacing deteriorated piles.

Plans are under way to use the modified load rating procedure and to begin employing the glass FRP retrofitting method. Daniel Tobias, the Technical Review Panel chair for this project, explains, “Effective retrofitting for these typically aging bridges lengthens their useful life span, which can save considerable money in a constrictive financial atmosphere.”

This project was conducted by the Illinois Center for Transportation, a research partnership between IDOT and the University of Illinois at Urbana-Champaign. For more information on this or other projects, visit the ICT website. The full report for this project can be found here.

Above: This bridge in DeKalb County, Illinois, collapsed in 2008 due to wood piling decay.

– April 2013