Designing a structural frame in FRP is not a material substitution exercise. You cannot take a steel frame design, swap the steel sections for FRP sections of the same depth, and expect it to work. The design methodology for FRP starts from a different place — not from strength, but from stiffness — and follows a logic that reflects the orthotropic nature of pultruded composites, the governing role of deflection, and the particular behavior of bolted connections in a material that does not yield. What follows is the design philosophy, not a step-by-step procedure.
Orthotropy: Three Directions, Three Sets of Properties
FRP is not isotropic. The longitudinal direction — along the pultrusion axis, parallel to the main glass rovings — has the highest strength and stiffness. The transverse direction — across the flange width — has a modulus roughly one-third of the longitudinal value. The through-thickness direction has no continuous fiber reinforcement and is governed entirely by the resin. This orthotropy means that every design calculation — bending stress, shear stress, bearing stress, buckling — must use the property appropriate to the direction of the load. The longitudinal modulus (EL, 17–28 GPa) is used for bending deflection and axial stress. The transverse modulus (ET, 7–10 GPa) is used for flange bending and web crippling. The shear modulus (GLT, 3–4 GPa) governs shear deformation, which is proportionally larger in FRP beams than in steel and may require a shear correction to the bending deflection for deep, short spans.
Deflection-Governed Design
In steel design, a beam is often sized for bending strength, and the deflection is checked afterward. In FRP design, the sequence is reversed. The allowable bending stress for pultruded FRP — typically 70–140 MPa after applying a safety factor of 2.5–3.0 to the ultimate strength — is high enough that, for typical walkway and platform spans, the beam reaches its deflection limit (commonly span/180 for pedestrian loading) long before it reaches its bending strength limit. An FRP beam sized for deflection will be stronger than it needs to be. The designer therefore starts with a deflection target, selects a section depth and span that meet it, and then verifies strength as a secondary check.
Bolted Connections: Bearing, Not Yielding
FRP connections are bolted, not welded. The bolt bears against the hole wall in the FRP, and the load is transferred into the laminate through bearing stress. The bearing strength of FRP (approximately 140–210 MPa for pultruded material loaded parallel to the fibers) is lower than that of steel, so bolt diameters and edge distances must be larger relative to the load. The edge distance — the distance from the bolt hole center to the edge of the FRP member — should be at least 2.5 times the bolt diameter in the direction of load and 2.0 times perpendicular to the load. These values are higher than for steel because FRP does not yield around the bolt hole to redistribute bearing stress; it remains elastic until it fails by crushing or shear-out. All structural connections in FRP should be designed as bearing-type connections with a safety factor of at least 3.0 on the ultimate bearing capacity.
Environmental Reduction Factors
The mechanical properties of FRP are measured under laboratory conditions — room temperature, dry, short-term load. Real service conditions differ. Sustained load in a humid, chemically aggressive environment can reduce the long-term strength of FRP through creep and environmental degradation. The design methodology accounts for this by applying environmental reduction factors to the short-term properties. For outdoor industrial exposure with isophthalic polyester resin, a reduction factor of 0.7–0.8 on tensile strength is common. Vinyl ester, with its superior chemical resistance, may allow a factor of 0.8–0.9. The exact factor depends on the specific chemical environment and the intended design life, and should be selected with reference to the resin manufacturer's long-term aging data.
Key Differences from Steel Design
The methodology parallels steel design in its overall framework — load paths, equilibrium, compatibility, limit states — but the material-specific differences are pervasive: orthotropic properties replace isotropic ones; deflection governs over strength; bolted connections are bearing-critical rather than slip-critical or weld-designed; buckling is a more prominent limit state for compression members because FRP's lower modulus reduces the Euler buckling load for a given slenderness; and environmental degradation must be explicitly accounted for in the allowable stress, not assumed away by a protective coating. An engineer comfortable with steel design can become proficient in FRP design, but the transition requires unlearning the assumption that stiffness and strength travel together.
For design data, see our FRP Beam Span Tables and for application-specific guidance, see FRP Profile Design Guide.