How to Read an FRP Pultruded Profile Data Sheet: The Numbers That Actually Matter
FRP structural profile data sheets contain a lot of numbers. Tensile strength, flexural modulus, compressive strength, shear strength, density, thermal expansion coefficient, water absorption — the list runs long and the units vary. For someone who’s spent their career working with steel, where a grade designation captures most of what you need to know, an FRP data sheet can look like more information than is useful.
In practice, only a subset of those properties are relevant for any given structural application, and knowing which ones to focus on — and what the numbers actually mean for design — makes the data sheet much more useful than it appears on first reading.
Tensile strength: which direction?
FRP pultruded profiles have different strength properties in the longitudinal direction (along the length of the profile) and the transverse direction (across the width). The longitudinal properties are much higher because the pultrusion process aligns most of the fiber reinforcement in the lengthwise direction.
Data sheets typically list both. The longitudinal tensile strength of a standard glass/polyester pultruded profile is typically 200–300 MPa — roughly comparable to mild steel. The transverse tensile strength is much lower, typically 30–70 MPa, because there’s less fiber reinforcement running in that direction.
When you’re using a pultruded profile as a beam, the tensile strength that matters for bending is the longitudinal value — the fibers at the tension flange are pulling in the lengthwise direction. When you’re designing a connection where the bolt load pulls perpendicular to the profile axis, the transverse properties become relevant and the lower values govern.
The data sheet number without the direction qualifier is incomplete. If a data sheet only lists one tensile strength value, ask which direction it represents.
Flexural modulus: stiffness in practice
For most structural applications of FRP profiles, deflection — not strength — controls the design. The material is strong enough; the question is whether the structure will be stiff enough to keep deflections within acceptable limits.
The flexural modulus (also called the bending modulus or modulus of elasticity in bending) is the property that drives deflection calculations. For standard glass/polyester pultruded profiles, the longitudinal flexural modulus is typically 17–25 GPa — roughly one-eighth to one-twelfth of steel’s modulus of 200 GPa.
This lower stiffness relative to steel is the primary reason FRP structural designs tend to use larger sections than equivalent steel designs. An FRP I-beam may have the same bending strength as a steel I-beam but much greater deflection under the same load. To keep deflections equivalent, the FRP section needs a larger moment of inertia — achieved through greater depth, wider flanges, or both.
Compressive strength and the column behavior question
Compressive strength in the longitudinal direction is typically similar to tensile strength for pultruded FRP — around 200–300 MPa for standard profiles. This number, however, doesn’t tell the full story for column design.
Slender FRP columns fail by buckling (Euler or local buckling) before they reach material compressive strength, because the lower modulus of elasticity means that buckling occurs at lower load levels relative to material capacity than it would for steel. A column design that’s governed by buckling capacity uses the flexural modulus, not the compressive strength, as the controlling property.
Data sheets that list compressive strength without flagging the buckling behavior for slender columns are omitting something important. For column design, checking both material compressive strength and buckling capacity is necessary, and for slender ratios typical of structural columns, buckling usually governs.
Shear strength: the web failure mode
For I-beams and channel sections, web shear capacity is a beam design consideration alongside bending capacity. The inter-laminar shear strength — the shear strength through the depth of the profile — is typically 20–40 MPa for pultruded profiles, significantly lower than the in-plane shear strength.
This matters for short spans where shear forces are high relative to bending moments. In deep beams with short spans, the shear stress in the web can exceed the inter-laminar shear strength before the bending stress at the flange reaches its limit. Checking both bending and shear for each span is necessary; relying only on bending capacity checks can produce understrength designs for short spans.
Water absorption and what it means for property retention
Water absorption is listed on FRP data sheets as a percentage weight gain after specified immersion conditions (typically 24 hours in water at 23°C under ASTM D570). Standard polyester profiles absorb 0.5–1.5% by weight under these conditions; vinyl ester profiles typically absorb 0.2–0.5%.
The number itself is less important than its implication: absorbed water reduces the resin matrix properties (modulus, strength, glass transition temperature) and therefore affects the wet-condition structural properties of the profile. Data sheets that list properties only for dry conditions are presenting optimistic values for wet service applications.
For applications involving water immersion, high humidity, or direct water contact, ask whether the listed properties are dry or wet, and if dry, ask for wet-condition knockdown factors. The difference between dry and wet flexural modulus for isophthalic polyester can be 15–25% — significant enough to affect whether a design meets deflection limits.
The numbers that indicate manufacturing quality
Fiber volume fraction — the percentage of the profile volume that is glass fiber — is a quality indicator that doesn’t always appear on data sheets but is worth requesting. Higher fiber volume fraction correlates with better structural properties. Standard pultruded profiles have fiber volume fractions of 40–60%; products at the lower end of that range have proportionally lower strength and stiffness.
Some FRP pultruded profiles manufacturers publish fiber content by weight rather than by volume, which gives a different number (fiber is denser than resin, so weight fraction is higher than volume fraction for the same product). Confirming which unit the data sheet uses prevents confusion when comparing across suppliers.
Void content — the percentage of the profile volume that is air inclusions rather than fiber or resin — is another quality indicator. Higher void content reduces structural properties and increases moisture absorption. Void contents below 2% indicate good manufacturing quality; void contents above 5% indicate quality control issues that affect performance.
Using the data sheet for actual design
A practical approach to using an FRP data sheet for structural design: identify the failure mode that governs for the specific application (bending, shear, column buckling, connection bearing), find the property that controls that failure mode (flexural modulus for deflection-governed bending, inter-laminar shear for web shear, compressive modulus for buckling), confirm whether the listed value is for dry or wet conditions as appropriate, and apply the relevant reduction factors for sustained loading, temperature, and safety factors as specified in the applicable design guide.
That process extracts the actionable design information from the data sheet and filters out the properties that don’t govern the specific application. It also makes clear when important information is missing — when the data sheet doesn’t have wet-condition values, or when direction-specific properties aren’t listed — and those gaps are worth resolving with the supplier before finalizing the design.
