Fiberglass Thickness Standard for Bridge Frames

The minimum fiberglass thickness for FRP bridge frames varies by width and structural application, typically ranging from 1.0 mm to 5.0 mm depending on the component size and type.

National Thickness Standards

According to the Chinese standard NB/T10287-2019 for composite fiberglass cable bridges, the minimum allowable thickness for fiberglass components is specified as follows :

• Composite fiberglass cable trays:
  • Width < 150 mm: 3.0 mm
  • Width 150–300 mm: 3.5 mm
  • Width 300–500 mm: 4.0 mm
  • Width 500–800 mm: 4.5 mm
  • Width > 800 mm: 5.0 mm
• FRP-lined steel frame sheets:
  • Width ≤ 150 mm: 1.0 mm
  • Width 150–300 mm: 1.2 mm
  • Width 300–600 mm: 1.5 mm These values provide a baseline for structural integrity and are intended to ensure sufficient strength and durability under expected loads.

Manufacturer and Design Guidelines

For pultruded FRP bridge components, manufacturers such as Bedford Reinforced Plastics and Arete Structures recommend using continuous fiberglass rovings, strand mats, and surfacing veils in sufficient quantities to meet the required mechanical properties . The exact thickness of each member is determined based on:

• Span and load type: Truss spans over 30 ft may require thicker top and bottom chords, while I-beams for shorter spans can be thinner .
• Allowable stress design (ASD): Safety factors for tension, bending, compression, shear, and connections are typically 3.0, influencing the required laminate thickness .
• Deflection and vibration criteria: Live load deflection limits (L/240) and fundamental frequencies (≥5 Hz vertical, ≥3 Hz horizontal) affect the structural thickness needed to maintain serviceability .
• Environmental protection: UV inhibitors and resin-rich surfacing veils are used to prevent degradation, which may require slightly thicker laminates to maintain long-term performance .

Practical Considerations

• Component width and load: Wider or heavily loaded members require thicker fiberglass laminates.
• Creep and long-term performance: Sustained loads and environmental exposure may necessitate additional thickness to prevent excessive deflection or stress relaxation.
• Compliance with standards: Designers should reference both national standards (e.g., NB/T10287-2019) and international guidelines (e.g., CIRIA C779, Eurocodes) for FRP bridges . In summary, fiberglass thickness for bridge frames is not a single fixed value but depends on the width, type of structural member, load conditions, and environmental factors. Minimum thicknesses range from 1.0 mm for narrow FRP-lined steel sheets to 5.0 mm for wide fiberglass trays, with additional design considerations applied for structural safety and serviceability.