A New Standard for FRP Bridges

By Corey Sechler, Contributing Author

Fiber Reinforced Polymer (FRP) has become a viable alternative to conventional construction materials. Evolving industry needs — like accelerated construction, product resilience and minimizing traffic disruption — have fostered wider adoption of FRP.

The American Association of State Highway and Transportation Officials (AASHTO) recently published the second edition of the Load and Resistance Factor Design Guide Specifications for the Design of FRP Pedestrian Bridges. It provides structural engineers, transportation officials and manufacturers a with a road map for using FRP.

The new standard updates AASHTO’s 2008 guide, and it further standardizes safe design criteria for FRP truss bridges bolted with pultruded elements.

The Need to Standardize

Modular and prefabricated assembly has become the go-to method for FRP bridges and bridge decks in particular. This manufacturing approach produces lightweight panels that can be transported to the work site for rapid installation — a capability crucial for projects with tight lane-closure windows.

Corrosion resistance, design flexibility and low to zero maintenance costs also make FRP attractive to state transportation agencies and local counties, cities and towns.

FRP has moved from a niche material to an established option as more companies have their FRP composite bridges and bridge decks tested and validated.

In addition to AASHTO, industry collaboration with the American Society for Testing and Materials (ASTM), Underwriters Laboratories (UL) and the American National Standards Institute (ANSI) helps establish codes for FRP to ensure products meet requirements such as H-5 loading, a common light-truck AASHTO standard required for structures with clear deck widths between seven and 10 feet.

Efforts to develop standardized designs in depths of 3.5, 4 and 5 inches that align with typical stringer spacing and industry regulations have expanded the line of composite products for the bridge industry.

Updating the Standard

Coastline Composites, a technical marketing and consulting firm, spearheaded the industry-led effort to draft the second edition for AASHTO on behalf of the American Composites Manufacturers Association (ACMA).

“The new guide specification was needed because the first edition focused entirely on FRP truss-style bridges and did not address contemporary bridge projects like composite decking and vacuum infusion molded structures,” said Gregg Blaszak, president of Coastline Composites and a former bridge engineer. “The updated guide specification will support the composite industry’s future growth by giving practitioners the confidence to design and specify FRP for pedestrian bridges and other applications.”

Aside from expanding advice for FRP truss-style bridges, the new standard provides guidance for FRP decking, such as pultruded planks or molded deck panels, supported by steel or concrete superstructures.

Blaszak worked with the AASHTO Specialized Structures Committee to ensure committee members’ feedback was incorporated into the document before it was sent to state DOTs for review and approved at AASHTO’s annual meeting of the Committee on Bridges and Structures.

“The second edition was largely drafted by a group of FRP subject matter experts and practitioners. In addition to covering FRP materials and design, the guide specification includes practical information on issues like non-slip overlays, coatings and joints,” Blaszak said.

Unlike the 2008 edition’s singular focus on FRP truss bridges, the new guide incorporates modern, performance-based design with broader direction for a variety of FRP materials including: decking, vacuum-infused panels and hybrids.

The new guide’s expanded scope also applies to FRP projects such as rail platforms and cantilever sidewalks on vehicle bridges, elevated roadways and rail trails. It also places responsibility on the manufacturer to design and detail connections to meet specific performance requirements set by the owner, rather than having the owner provide a fully engineered design.

The second edition aligns with modern Load and Resistance Factor Design (LRFD) principles, updated load combinations and refined methodologies for material behavior. The latest standards for non-slip overlays, improved guidance on vibration limits and clarification of pedestrian loads are also included.

Meeting Standards

Engineers who are new to FRP but want to start the design process for a bridge or bridge deck application should begin with the design requirements.

It’s also good practice to talk with the manufacturer about their requirements and capabilities. The OEM prefers to be included upfront in the process. If they can review design details at the start of a project, the design can be evaluated for manufacturability. If there is an issue during the design/development phase, it can be flagged for the design engineer who can propose an alternative approach.

For the design engineer, FRP panel dimensions are typically governed by freight restrictions, superstructure layout and erection requirements. Prefabricated to fit existing structures, depth and cutouts, FRP panels can be manufactured up to 40 feet long and up to 12 feet wide. Strength-to-weight and weight-to-safety ratios along with design flexibility are other important factors to consider (FRP panels are 80% lighter than concrete, an ideal alternative for older structures and existing bridges with weight limitations).

When designing for an FRP rail platform, some design criteria engineers must consider are:

• Structural load capacity.
• Corrosion resistance against deicing chemicals.
• Compliance with safety standards.
• Fire resistance (ASTM E-84 Class 1).
• Surface safety.

FRP panels are lightweight and prefabricated, enabling overnight installation with minimal to no disruption to train schedules. This “fast-track” capability has become the preferred choice for rehabilitating aging infrastructure where closing platforms for extended periods to accommodate concrete work is not feasible.

FRP Meets High Standards

FRP is no longer new technology reserved for one-off demonstration projects. This structural composite material is in-demand for a wide range of applications where light weight and corrosion resistance are important factors.

As market interest continues to grow, updated standards provide rigorous guidelines for new products, including pultruded decking and vacuum infusion molded panels, which were not covered in earlier, more limited standards.

From an eight page first edition to a highly detailed 75-page second edition, the new guide gives engineers the standards information they need to take their project from design to specification.

Corey Sechler is the vice president of sales for civil infrastructure, Creative Composites Group