Recommendations

More FRP trail bridges are being constructed on national forest lands. The pros and cons of FRP bridges need to be considered when deciding the type of bridge that best suits the needs.

Selection Considerations

When deciding whether to use FRP materials for a trail bridge, consider:

• How does the overall durability of the material compare to concrete, steel, or timber?
  
  
• How does the cost of the FRP structure compare to a similar structure of concrete, steel, or timber?
  
  
• How difficult is site access and construction?
  
  
• Will the temperatures be above 100 degrees Fahrenheit during peak load periods? If so, FRP bridges should be avoided because they lose strength and become more flexible at high temperatures.
  
  
• What is the likelihood of impacts from flood debris or collisions?
  
  
• How would a collision compromise the structure?
  
  
• Could the structure be repaired easily?
  
  
• How much would repairs cost and how would the repairs affect the overall strength of the member?
  
  
• Does the appearance of FRP trail bridges concern wilderness land managers?

Materials, Testing, Specifications, and Standardization

Researchers and developers in the bridge-building industry seem to be focusing on material testing. Because of the unfamiliarity of FRP composites in this industry, a great deal of materials testing needs to be done and standards need to be established. Methods need to be developed so material properties can be predicted over the long term. Analytical methods that can predict structural behavior also are needed.

A database needs to be developed recording the long-term performance of existing bridges. The performance data can be used to develop much needed material specifications, leading to new and improved design methods and procedures.

Other barriers to the widespread use of FRP materials include:

• The high initial cost of FRP materials compared to timber
• The lack of design codes, standards, and guidelines
• The lack of proven inspection methods for FRP composites
• The lack of proven inservice durability data

Establishing guidelines and minimum performance requirements is essential before FRP can become a common material for Forest Service trail bridges.

In some ways, manufacturers make it more difficult to overcome these barriers. FRP composites are engineered materials, meaning that the composition of the material is adjusted to produce particular performance characteristics. Each manufacturer sells different products. These products are proprietary and manufacturers have been unwilling to make their specific fiber architecture (precise material proportions and fiber orientation) available. This makes it difficult to produce standard tests, general design procedures, and specifications. The proprietary nature of the materials also makes it difficult to assure quality control during their manufacture. The industry may have to loosen its hold on information about the materials if it wishes to develop a broad market in the bridge industry.

The results of the initial testing suggest that the methods used to model the load-carrying capacity of the 44-foot bridge tested at the Forest Products Laboratory were very accurate. When the actual performance of the tested bridges is considered as well, the design procedures described in appendix H appear to provide a good basis for a thorough, reliable design of an FRP composite truss bridge. However, these procedures represent only a beginning and will need to be adapted as materials and our understanding of their behavior advance.

FRP composite bridges are not yet a practical solution for bridges designed to meet AASHTO and similar codes. Further study and testing are needed to better understand the material and its uses. However, FRP materials have the potential to meet an important need for lightweight, strong, low-maintenance, attractive trail bridges in remote locations.