Since 2007, bridges are designed to have a life expectancy of at least 75 years. This is an increase of more than 25 years from the previous planned design life standard. That longer horizon raises expectations for every component on the structure, including the drainage system.

Bridge drains, the gravity-fed piping that carries stormwater, snowmelt and spills off elevated decks, are easy to overlook until they fail. Corrosion, clogs, damaged components and hard-to-reach locations can turn a modest system into a recurring maintenance problem.

Material choice is one factor in how well a drain system holds up. In a poll of attendees at a recent Roads & Bridges webinar, more than two-thirds of engineers said they most often specify PVC or steel bridge drains, and fewer than one in four said they were familiar with fiberglass-reinforced polymer (FRP) systems. Below are answers to questions raised during the webinar about where FRP fits, what it offers and its limitations.

Standards and Approvals

Many state departments of transportation (DOTs), including New York, New Jersey and South Carolina, accept fiberglass bridge drains for new construction and rehabilitation, typically alongside ductile iron and steel pipe. Approvals vary by state and by manufacturer, so check your state’s specifications and approved products list.

When evaluating a system, look for: 

• Pipe manufactured to ASTM D2996, Standard Specification for Filament-Wound “Fiberglass” Pipe
• Fittings manufactured to ASTM D3840 or an equivalent recognized standard
• A reinforced thermosetting resin pipe (RTRP) classification, which identifies the resin type, stiffness and other properties that define the pipe’s performance
• Independent third-party test data, such as short-term hoop tensile strength, rather than manufacturer claims alone
• Some manufacturers also provide structural load calculations stamped by a professional engineer. 

How Fiberglass Compares

Corrosion. Glass-fiber composites don’t corrode. Because they are nonconductive, they can be connected to metal pipe without creating the galvanic corrosion that occurs between dissimilar metals.

Temperature. Fiberglass is a thermoset. Unlike PVC, a thermoplastic, it does not soften and sag permanently between hangers in high heat or become brittle in extreme cold. It expands and contracts more than steel but considerably less than PVC, so designers still need to account for thermal movement. Manufacturers add UV inhibitors to the resin, but buyers might ask whether exposed runs need a protective coating over the long term.

Strength and weight. Fiberglass drainpipe is typically filament wound: continuous glass fibers are wrapped around a mandrel at a set angle and bonded with resin. This produces high hoop strength with a relatively thin wall. PVC, by contrast, is extruded through a die. The result is a pipe that is much lighter than steel or ductile iron, which makes handling and installation easier and can reduce the number of hangers and anchors required. However, fiberglass is less stiff than steel, so confirm hanger spacing against the manufacturer’s data and the design loads, including a full pipe and ice.

Flow. The smooth interior wall resists buildup and supports efficient hydraulic flow.

Making Connections

To metal pipe. Flanges can be bonded to fiberglass pipe ends and drilled to the specified bolt pattern, such as ANSI Class 150. Some manufacturers also offer grooved-end adapters for mechanical couplings. Systems sized to the Iron Pipe Size (IPS) standard simplify transitions to existing metal piping.

To PVC. Fiberglass and PVC can be joined with a compatible epoxy adhesive, following the manufacturer’s instructions.

To concrete. There is no single method. The appropriate hanger and anchorage approach depends on the concrete’s type and strength, whether the bridge is new or a retrofit, and whether inserts can be cast in or anchors must be post-installed. Work this out early with the structural engineer.

Limitations and Trade-Offs

Fiberglass is not the right answer for every project. Engineers should weigh:

• Fittings. Suppliers that only produce filament-wound pipe may not offer specially shaped transition fittings. Those may need to come from manufacturers with molding or pultrusion capabilities, or be custom fabricated, which can affect cost and lead time.
• Concrete connections. As noted above, anchorage requires project-specific design.
• Cost. Compare initial material and installed costs with PVC and steel and ask for lifecycle cost data to support any long-term savings claims.
• Fire performance. Ask how the system performs if a vehicle fire occurs on or below the bridge.
• Impact and repair. Understand how the pipe handles impact damage and how field repairs are made.
• Familiarity. Contractors and maintenance crews may have less experience installing and repairing fiberglass systems.

Questions to Ask Suppliers

• Which state DOT approvals does the system hold, and to which standards is it manufactured?
• What independent test data is available?
• Can the system be prefabricated to reduce installation time and time spent working at height?
• Are custom elbows and transition fittings available, and at what lead time?
• Is the pipe sized to IPS or another known standard?
• Can it be painted, or is color available throughout the material?
• What hanger spacing is recommended for the design loads?
• Can the supplier help with scupper sizing?

Bridge owners are asking every component to last longer. For drainage systems, that makes material selection worth a closer look, and fiberglass deserves a place in that evaluation alongside conventional options.

Chad Moore is a vice president of sales for United Fiberglass Creative Composites Group.