The first thing an engineer accustomed to designing with steel notices about pultruded FRP structural profiles is that the shape looks familiar — wide-flange beams, channels, angles, square tubes — but the numbers behind the shape are fundamentally different. A steel W6×12 and an FRP I-beam of approximately the same depth have section properties that are in the same general range, but the material stiffness behind those section properties is an order of magnitude lower. That single fact reshapes the entire design approach.
Anisotropy: The Material Behaves Differently in Every Direction
Pultruded FRP is an orthotropic material. In the longitudinal direction — along the length of the profile, parallel to the pultrusion direction — the continuous glass fiber rovings provide a tensile modulus of 17 GPa to 28 GPa, depending on the fiber volume fraction and resin system. This is roughly one-tenth the modulus of structural steel (200 GPa). In the transverse direction — across the flange width, perpendicular to the pultrusion direction — the modulus drops to around 7 GPa to 10 GPa, because only the continuous strand mat and surfacing veil carry load in that direction. The shear modulus is similarly lower: approximately 3 GPa to 4 GPa, compared to 77 GPa for steel.
This anisotropy means that standard isotropic beam formulas — the Euler-Bernoulli bending equation, the elastic buckling formula — must be applied with FRP-specific material properties. Deflection calculations that use the longitudinal modulus EL are generally valid for bending about the major axis, but shear deformation is more significant in FRP beams than in steel beams of similar proportions, particularly for deep, short-span beams. A beam with a span-to-depth ratio below about 10 may require a shear deflection correction.
Deflection Governs, Not Strength
In steel beam design, the depth selected for a given span is often governed by bending strength — making sure the extreme fiber stress does not exceed the allowable value. In FRP beam design, the beam depth is almost always governed by deflection. The allowable bending stress for pultruded FRP in the longitudinal direction is typically in the range of 100 MPa to 150 MPa (for a safety factor of 2.5 to 3.0 on the ultimate strength), and at typical industrial walkway and platform loadings (4.8 kN/m² live load), the bending stress at the span where deflection reaches span/180 is well below the allowable stress. The material is stronger than it is stiff. This means an FRP beam selected to meet a deflection limit will be stronger than it needs to be.
The practical result is that spans for FRP beams tend to be shorter than for equivalent steel beams carrying the same load, unless a deeper FRP section is used. A pultruded I-beam with a 200 mm depth spanning 3 m might provide an acceptable deflection under pedestrian loading; a steel beam of the same depth could span twice that distance. The FRP beam will, however, weigh approximately 75% less, which often offsets the closer support spacing in the overall structural layout.
Section Properties and Standard Shapes
Pultruded structural shapes are available in standard dimensional series that mirror steel nomenclature — typically wide-flange beams from 100 mm to 450 mm depth, channels from 75 mm to 350 mm, angles with equal and unequal legs, and square and round tubes. The section properties — moment of inertia I, section modulus S, radius of gyration r — are published by manufacturers and are comparable to steel values for the same nominal depth, though the wall thicknesses in FRP shapes tend to be greater than in steel (8 mm to 15 mm flange thickness is common in FRP beams, versus 6 mm to 12 mm in steel).
The thicker walls are required because FRP's lower elastic modulus means the section needs more material distributed to achieve a given bending stiffness. This does, however, give FRP shapes excellent local buckling resistance — the width-to-thickness ratios for FRP flanges are generally well below the limits that would trigger local buckling in steel. Lateral-torsional buckling is a more relevant limit state for long, unbraced FRP beams, and the design methodology follows the same principles as steel, with FRP-specific lateral-torsional buckling constants.
For section property tables of our standard pultruded profiles, refer to the individual product pages under FRP Structural Profiles.