Design of FRP Bridges

Forest Service Manual 7720.04a requires approval by the regional engineer for designs of all "major and complex" trail bridges. All FRP bridges are considered to be complex. Each forest is responsible for its decision to use FRP materials. The bridge must be designed by a qualified engineer experienced in the design of trail bridges and the use of FRP materials. Other jurisdictions may have different requirements—know the requirements you need to meet.

Design Specifications for FRP Pedestrian Bridges

By early 2006, no design specifications for FRP pedestrian bridges had been approved in the United States. E.T. Techtonics, Inc., has submitted Guide Specifications for Design of FRP Pedestrian Bridges to the American Association of State Highway Transportation Officials (AASHTO) for approval. These guide specifications are in appendix B. Other professional organizations are addressing the recommended use and specifications of FRP materials and products using them, including the American Society of Civil Engineers (ASCE), the American Society of Testing and Materials (ASTM), and the FHWA.

Design and material specifications are now available only through manufacturers of FRP materials. In the absence of standard material and design specifications, manufacturers' specifications should be followed. There is no way to validate the information manufacturers supply other than by performance history or testing. Errors may exist. Different manufacturers use different resin-to-reinforcement formulas when constructing FRP members, so material properties will differ. The designer should be certain to use the manufacturer's design manual and specifications.

Design Concerns

With any new technology, methods must be developed to predict long-term material properties and to predict structural behavior based on those properties. This information is incorporated in specifications for design parameters, material composition and variance, size tolerances, and connections. Methods for inspection and repair also are derived from long-term testing and observation.

Although specification development and further testing is in progress, standard FHWA specifications and ASCE Load Resistance Factor Design (LRFD) procedures won't be available for the next 5 to 6 years, as reported by Dan Witcher of Strongwell and chairman of the Pultrusion Industry Council's Committee on LRFD Design Standards. Two leading manufacturers of FRP structural products, Strongwell, and Creative Pultrusions, Inc., have specifications and design safety factors listed on their design manual CDs. Appendix G has contact information for these manufacturers.

The designer should be aware that shear stresses add more deflection to loaded beams than the classic flexural deflection. Temperatures above 80 degrees Fahrenheit reduce allowable stresses and FRP materials may sag or elongate under sustained loading (time-dependent effects, called creep). A temperature of 125 degrees Fahrenheit decreases FRP strength by 30 percent and stiffness by 10 percent (Creative Pultrusions, Inc. 2004; Strongwell 2002). The design needs to consider the service temperature range. FRP members must be designed for lower allowable stresses (no more than 40 percent of the ultimate allowable stress) to minimize creep.

Lateral stability needs to be addressed for different types of bridge configurations. For spans of 30 feet or more, side-truss FRP bridges should have outriggers at all panel points (see figure 8) to provide lateral restraint for the compression flanges. FRP bridges longer than 60 feet that are used by pack trains should have a deck-truss design. That design places the trusses under the deck, increasing restraint on the compression flanges (see figure 7) and increasing the frequency characteristics of the bridge, an important consideration for the live loads generated by pack trains.

Attention to details can help reduce performance problems with FRP bridges:

• Avoid hollow tubes with walls less than ¼ inch thick.
  
  
• Fill at least 12 inches of each end of hollow tubes with solid material.
  
  
• Provide a drain hole at the bottom of the tube so trapped water can drain.

Bridges made with FRP materials perform differently than bridges made with steel, concrete, or wood. Take these differences into account when designing bridges with FRP materials.

Other Concerns

FRP bridges have many different design considerations. Pack trains may produce vibrations that match the fundamental frequency of the bridge, which may cause the bridge to fail. The natural frequency of the bridge and live loads should be taken into account when ordering the structure. Because of FRP's typically low modulus of elasticity, most designs will be controlled by deflection limitations and not strength requirements. Although the criterion for deflection is somewhat arbitrary, AASHTO guidelines for pedestrian bridges recommend that the deflection of members (in inches) be less than the length of the supporting span divided by 500 (L/500). FRP manufacturers and designers recommend L/400, which would allow more deflection.