The Illinois Department of Transportation and the Illinois Center for Transportation have been investigating timber bridge pilings used to support bridges throughout the state, primarily on the local road system.
In 2009, IDOT and ICT launched a research project titled “Strength of Bridge Wood Pilings Retrofits for Moment Resistance (R27-082)” in an effort to find cost-effective ways to maintain Illinois bridges. In a follow-up to that study, called “Strengthening of Bridge Wood Pilings Retrofits for Moment Resistance, Phase II (R27-134),” investigators used piles aged both in the field and in the lab to determine how much load a deteriorated pile could sustain and to see whether a process using fiber reinforced polymers (FRP) would be an effective way to reduce the rate of future deterioration and prolong the life of the structure by reinforcing deteriorated piles. FRP retrofitting involves the installation of either fiber sheets or strips, which are then impregnated with an epoxy-like polymer substance.
Principal investigator Bassem Andrawes, professor in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign, believes this study is of prime importance. He states: “This project addresses one of our nation’s most pressing issues, namely, the deterioration and crumbling of our transportation infrastructure. The first spark triggering the interest in this project was the collapse of a bridge supported on timber piles back in 2008 in DeKalb County, Illinois [see ICT special project R27-SP12].”
Andrawes believes FRP retrofitting is an effective way to restore strength to piles damaged by weather and time. The first phase of this study examined IDOT’s load rating method for the capacity of timber piles located within bridge piers. Traditionally, ratings were based on the compression forces in the piles. However, in certain circumstances, a moment component is induced by the eccentric loading of the superstructure. Because this can have a substantial effect, Andrawes and his team considered methods to treat and strengthen the timber piles to prevent future structural failure. Phase I introduced the wrapping of the timber piles at the piers with FRP. The study found these wraps reinforce the piles and can restore them close to the same load-carrying capacity the pilings had at original installation.
The second phase of the project expanded the retrofit concept to include the abutment piles and investigate the long-term effectiveness of the treatments used in the first phase. Andrawes explains: “In Phase II we were able to look in depth at the impact of aging and harsh chemical and environmental conditions on bridge timber piles and how rehabilitating these piles with advanced composite materials (FRP) could mitigate this impact.”
This project used a few timber piles taken from the field and newer piles that the research team could age in the lab. “The primary challenge was to try to create a climate-like setup in our lab to test the piles,” says Andrawes. “This was done with several pieces of equipment that were designed to control the temperature and humidity inside the lab. The test specimens were subjected to a large number of thermal/humidity cycles that mimic real climate conditions in both winter and summer times.”
This cycling accelerated wear on the piles to something resembling what happens in the field. Once the piles were appropriately aged, researchers could then test different FRP retrofitting methods. In their final report for the project, Andrawes and his team made several key recommendations for IDOT to consider
According to the project’s TRP chair, Jeff Burke, with Phase II completed, IDOT has improved the load rating analysis process for timber piles at both the piers and abutments. They have introduced additional repair or strengthening methods to prolong the life of timber-supported structures. Bridge owners may be able to salvage timber piles rather than tearing down a structure because of the condition of the timber piles. The strengthening will allow bridge owners to maximize their funds to keep structures open without restrictions and minimize the effects on the traveling public. Over 1,000 bridges on the Illinois local system are potentially affected.
Burke, Senior Repair Plans Engineer of IDOT’s Bridge Investigation and Repairs Unit, adds that a training video will soon be available online to demonstrate the FRP construction method. Design guidelines, details, and policy still need to be developed based on the findings of this research in order to standardize and streamline the use of FRP on timber piles.
Looking forward, Andrawes sees more work that could be done on bridge pilings: “Future research in this area could focus on understanding the response of the deteriorated and rehabilitated timber piles under various man-made and natural hazards (earthquake, flood, etc.). This will help us design better retrofitting schemes that will ensure the safety of our bridges under such hazards.”