Fiberglass-reinforced polymer is not one material with one set of properties. It is a family of composites whose characteristics shift with resin chemistry, glass type, fiber volume fraction, and fiber architecture. That said, there is a well-established envelope of values that covers the vast majority of industrial FRP products — molded grating, pultruded structural shapes, and wound pipe. What follows are the numbers an engineer reaches for when performing a first-pass feasibility check on an FRP application.
Physical Properties
Density. The density of FRP ranges from approximately 1.5 g/cm³ to 2.1 g/cm³, determined primarily by the glass content by weight. A typical pultruded structural shape with 55–65% glass by weight has a density around 1.7–1.9 g/cm³. Molded grating, with a glass content closer to 30–40%, sits at the lower end, around 1.5–1.7 g/cm³. By comparison, structural steel is 7.85 g/cm³, and aluminum is 2.7 g/cm³. An FRP beam weighs roughly one-quarter of the steel beam with the same external dimensions — though the FRP beam will be deeper for the same stiffness, so the weight comparison on an equal-stiffness basis is more nuanced.
Fiber volume fraction. This is the proportion of the composite cross-section occupied by glass fiber, expressed as a percentage. Pultruded profiles achieve 50–65% fiber volume fraction because the continuous pulling process compacts the fibers before curing. Molded grating typically achieves 25–40%. The fiber volume fraction is the primary driver of tensile strength and modulus in the fiber direction — higher glass content means higher strength and stiffness, but also higher weight and material cost.
Mechanical Properties
Tensile strength (longitudinal). For pultruded shapes with the load applied parallel to the fiber direction, the ultimate tensile strength typically falls in the range of 200–400 MPa, with 250–350 MPa being the most common range for commercial profiles. Molded grating, with its bi-directional fiber architecture, has a lower tensile strength in any single direction — typically 70–100 MPa — because the glass is distributed in two perpendicular directions rather than concentrated in one.
Tensile modulus (longitudinal). The modulus of elasticity in the fiber direction is 17–28 GPa for E-glass pultruded profiles, with 20–23 GPa being typical. This is roughly one-tenth of structural steel (200 GPa). The transverse modulus — across the fiber direction — is substantially lower, in the range of 7–10 GPa, because it is governed by the resin and the continuous strand mat rather than the main fiber rovings. This orthotropy is the single most important mechanical characteristic of FRP and the one that most often surprises engineers accustomed to isotropic materials.
Flexural strength and modulus. Under bending, a pultruded I-beam with the load applied about its major axis will exhibit a flexural strength in the range of 200–350 MPa and a flexural modulus close to the tensile modulus — 17–28 GPa. The limiting factor in bending is typically the compressive strength of the flange (lower than the tensile strength by 20–30%) and the shear strength at the web-flange junction.
Shear strength. Interlaminar shear strength — the strength through the thickness of the laminate — is governed by the resin and the fiber-matrix bond. Typical values from short-beam shear testing (ASTM D2344) are 25–35 MPa. This is the critical property for bolted connections, where the bolt bears against the hole wall and the load is transferred into the laminate through shear.
Thermal Properties
Coefficient of thermal expansion (CTE). In the longitudinal fiber direction, the CTE of pultruded FRP is 8–12 × 10⁻⁶ /°C — remarkably close to steel (12 × 10⁻⁶ /°C) and concrete (10–14 × 10⁻⁶ /°C). This is one of the happy coincidences that makes FRP reinforcement in concrete and FRP-to-steel connections thermally compatible. In the transverse direction, the CTE is higher — 20–30 × 10⁻⁶ /°C — because the resin dominates the expansion. Design details that restrain FRP in the transverse direction must allow for this differential movement.
Service temperature. The glass transition temperature (Tg) of the resin determines the upper continuous service temperature. Isophthalic polyester Tg is typically 80–100 °C; vinyl ester Tg is 100–120 °C; phenolic Tg can exceed 150 °C. Above Tg, the resin softens and the mechanical properties decline rapidly. For continuous service, a margin of 20–30 °C below Tg is standard practice.
Electrical Properties
FRP is electrically non-conductive. The dielectric strength of pultruded FRP exceeds 7.9 kV/mm (200 V/mil) for typical profiles. This property is the basis for FRP cable tray applications in substations and for FRP handrail and platform use near energized equipment. It also means FRP structures do not require grounding or bonding.
For section properties and load data incorporating these material values, see our FRP Beam Span Tables and FRP Grating Load Tables.