TECHBRIEF Strengthening Historic 

Covered Bridges to Carry 

Modern Traffic 

FHWA Publication No. of this TechBrief: 

FHWA-HRT-07-041 

NTIS Accession No. of the report covered in this 

TechBrief: PB2007-103714 

FHWA Contact: Sheila Duwadi, HRDI-07, 202-493-3106 

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unpublished Federal Highway Administration report, 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. 

Objective 

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. 

Introduction 

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. 

Research, Development, and 

Technology 

Turner-Fairbank Highway 

Research Center 

6300 Georgetown Pike 

McLean, VA 22101-2296 

www.tfhrc.gov
2

Figure 1. GFRP rebars embedded in a 

wood beam. 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 largescale, 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. 

Research 

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 

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. 

Table 1. Summary of experiments. 

Number of Type of Test Comments Experiments

Bonded in GFRP rebars; strength of tension 

splice and development length tested. Tension—small scale 9 

Bending—small scale 3 GFRP plates flat; strength and stiffness tested. 

Bending—large scale 6 GFRP plates flat; strength and stiffness tested. 

Bending—large scale 2 GFRP rebars; strength and stiffness tested. 

GFRP plates on edge; strength and stiffness 

tested. Shear—large scale 3 

Total 23 
3

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. 

Recommendations 

Future research or actions should: 

 Examine the feasibility of 

reinforcing wood at jointed 

connections with GFRP composite 

materials. 

 Conduct full-scale tests on trusses, 

beams, and tension members to 

eliminate or reduce the effects of 

the reduced scale. 

 Evaluate the long-term 

performance under varying 

environmental conditions of GFRP 

rebars bonded in wood members 

that are subjected to tension. 

 Conduct the tests in this research 

study under field conditions on 

existing structures. 
4

 Evaluate the long-term 

performance of GFRP reinforced 

bridge members exposed to wood 

preservative chemicals and 

pressures. 

Additional Information 

For additional information, contact Sheila 

Duwadi at 202-493-3106 or the following 

address:  Develop a stand-alone manual, or 

a separate chapter in a broaderscope manual, on step-by-step 

procedures for the preservation 

and strengthening of historic 

covered bridges using GFRP 

composite materials. 

Turner-Fairbank Highway Research 

Center, Office of Infrastructure Research 

and Development, 6300 Georgetown 

Pike, McLean, VA 22101-2296. 

References 

Federal Highway Administration. Covered Bridge Manual. Report No. FHWA-HRT-04-

098. April 2005. McLean, VA: Federal Highway Administration. 

Researchers—This study was performed by the Constructed Facilities Center and the 

Institute for the History of Technology and Industrial Archaeology, both of West Virginia 

University, Morgantown, WV. 

Distribution—The unpublished report covered in this TechBrief is being distributed through 

the National Technical Information Service, www.ntis.gov. 

Availability—The report will be available May 2007, and may be obtained from the National 

Technical Information Service, www.ntis.gov. 

Key Words—Glass fiber reinforced polymer, GFRP, composites, wooden superstructure 

components, historic covered bridges. 

Notice—This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government assumes no 

liability for the use of the information contained in this document. The U.S. Government 

does not endorse products or manufacturers. Trademarks or manufacturers’ names appear 

in this TechBrief only because they are considered essential to the objective of the 

document. 

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HRDI-07/05-07(500)E