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Publication Number: FHWA-HRT-07-041
Date: May 2007
Strengthening Historic Covered Bridges to Carry Modern Traffic
FHWA Contact: Sheila Duwadi,
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This document is a technical summary of the unpublished Federal Highway Administration report, Strengthening Historic Bridges to Carry Modern Traffic (FHWA Contract No. DTFH61-00-C-00081), available only through the National Technical Information Service, www.ntis.gov.
This TechBrief describes research on the use of glass fiber reinforced polymer (GFRP) composites to strengthen wooden superstructure components of historic covered bridges. The research was conducted during the years 2000 to 2004.
At one time, the United States reportedly had as many as 14,000 covered bridges. Fewer than 900 now survive.(1) Under the National Historic Covered Bridge Preservation Program, the Federal Highway Administration provides funds for the rehabilitation, restoration, and preservation of covered bridges. If the goal for a particular bridge is to strengthen it sufficiently to support today's vehicular traffic, a major engineering challenge arises. The research described in this TechBrief could help meet that challenge.
Figure 1. GFRP rebars embedded in a wood beam.
Laboratory experiments were conducted on the use of GFRP composite materials—plates and rebars—to reinforce timber components of covered bridges. The use of adhesive to affix the GFRP materials in place also was included in the experiments. The GFRP plates were designed to increase the bending and shear capacities of flexural floor members. Tension and bending tests were conducted to establish the bond strength of GFRP rebars embedded in wood (figure 1), and to establish the bending strength and stiffness of floor beams reinforced with GFRP plates and rebars. In addition, increasing the shear capacity of floor beams by bonding GFRP plates on edge in narrow slots in the beams was investigated using large-scale, floor-beam specimens. GFRP rebars were tested as axial reinforcement for truss members.
Because maintaining the historic appearance of covered bridges is very important to preservationists, several methods of concealing the reinforcements also were examined. In one of the more successful methods, a GFRP plate was bonded inside a wide, shallow slot on the bottom face of a wooden member and hidden with an integrated veil matching the grain and color of the wood.
The major experiments conducted are summarized in table 1. Other experiments were conducted to determine the best combination of adhesives and the methods, such as pressure injection, to apply that combination.
Results and Conclusions
Results of and conclusions from the research include the following points.
The adhesive used in the research performed very well in bonding the GFRP materials to the wooden structural members. It required less stringent preparation than epoxy.
In the tension tests, sand-coated GFRP rebars embedded in the wooden members performed well in terms of pullout force and bond strength. The rebars were 1.27 centimeters (cm) in diameter and approximately 10.16 cm in length.
In small-scale bending tests, the strength and stiffness of wooden members was improved by bonding a GFRP plate to the tension face of the members; however, this method had limitations. To provide a suitable bonding area, the wood needed to be prepared so that the surface was plane. If the bond was not adequate, the GFRP plate would peel away from the member. Also, the surface had to be degreased and freed of loose material.
The results from the large scale bending tests indicated that the strength and stiffness of wooden members can be improved significantly.
Bending tests with GFRP rebars at the top and bottom of the test specimens did not achieve the desired levels of performance; however, this method should be very useful with compression members in a truss.
A transformed section analysis based on strain compatibility and internal moment equilibrium accurately predicted the moment capacity of full-scale members and could be used for design purposes.
Moisture content and temperature of the test specimens were held as constants for the research project. All specimens were tested at an indoor ambient temperature that had very little variance. The question of degradation of the adhesive in a moist environment should be studied in the future.
Shear tests were performed on two GFRP flitch beams, which are created by inserting a GFRP plate on edge in a deep slot. Although the shear capacity was expected to improve significantly, it actually decreased slightly. The flitch beams tested were severely checked, which degraded their shear strength as compared to the solid control specimen. Also, the modular ratio of GFRP to wood was not very high.
Future research or actions should:
For additional information, contact Sheila Duwadi at 202-493-3106 or the following address:
Turner-Fairbank Highway Research Center, Office of Infrastructure Research and Development, 6300 Georgetown Pike, McLean, VA 22101-2296.
Federal Highway Administration. Covered Bridge Manual. Report No. FHWA-HRT-04-098. April 2005. McLean, VA: Federal Highway Administration.