FRP thermal expansion calculator
How far a pultruded FRP member grows and shrinks between its fixings, how that compares with the steel, concrete or glass it is fixed to, and how wide a sealed joint must be.
- FRP lengthwise α
- 8 × 10⁻⁶/K
- Materials
- 8
- Sealant classes
- ISO 11600 / ASTM C920
Use the member temperature, not the air temperature. A dark profile in full sun runs well above the shade temperature; take the local design values from EN 1991-1-5, ASHRAE or the project climate data.
Hot: 1.08 mm more growth in the substrate. Cold: 0.84 mm more shrinkage in the substrate.
Slotted holes or sliding brackets at all but one fixing must take this relative movement, plus the bolt clearance.
Axial stress about 8.3 MPa compression when hot and 6.4 MPa tension when cold, a force of about 9.3 kN on the fixings.
Upper bound: E · α · ΔT with E = 23 GPa. Restraint also puts compression members at risk of buckling, so provide movement rather than rely on the section.
Minimum joint width at installation: 8.64 mm (larger of the opening and closing movement divided by the sealant class).
Add construction tolerance, and follow the sealant maker's width-to-depth ratio.
Coefficients used
Free movement is α · L · ΔT. The steel, aluminium, concrete and glass values are the ones given in the Eurocodes and EN 572-1. The FRP and PVC-U values are typical published ranges and vary with the glass content and resin, so a specification should quote the value declared for the supplied product (measured to ASTM E831 or ISO 11359-2).
| Material | α (10⁻⁶/K) | E (GPa) | Source |
|---|---|---|---|
| Pultruded GFRP profile, lengthwise | 8 | 23 | Typical E-glass pultrusion; manufacturer design manuals give about 6–11 × 10⁻⁶/K. Declare the supplied value |
| GFRP rebar, lengthwise | 8 | 50 | ACI 440.1R-15 Table 4.1 range 6.0–10.0 × 10⁻⁶/K |
| Carbon steel | 12 | 210 | EN 1993-1-1 §3.2.6 |
| Stainless steel 1.4301 / 1.4401 (304 / 316) | 16 | 200 | EN 1993-1-4 Table 2.3 |
| Aluminium alloy | 23 | 70 | EN 1999-1-1 §3.2.5 |
| Concrete | 10 | 33 | EN 1992-1-1 §3.1.3(5) |
| Soda-lime glass | 9 | 70 | EN 572-1 Table 1 |
| PVC-U window profile | 70 | 2.5 | Typical PVC-U value (about 60–80 × 10⁻⁶/K); declare the supplied value |
Low movement is one reason FRP window frames stay tight against the glass. See FRP window frames and the window U-value calculator. For spans and loads, use the FRP profile calculator.
Frequently Asked Questions
What is the thermal expansion coefficient of pultruded FRP?
Lengthwise, manufacturer design manuals give about 6 to 11 × 10⁻⁶ per K for E-glass pultruded profiles, close to glass and concrete and below carbon steel (12 × 10⁻⁶/K) and aluminium (23 × 10⁻⁶/K). Crosswise the value is several times higher because the resin controls it. The calculator uses 8 × 10⁻⁶/K lengthwise; replace it with the value declared for the supplied profile when your specification needs it.
Why does the fixing substrate matter?
A member fixed at both ends to a material that moves differently is strained by the difference, not by its own movement. An FRP rail on a steel platform moves about 4 × 10⁻⁶/K less than the steel, so over a 6 m run and an 80 K range the relative movement is only about 2 mm. An FRP frame against aluminium would see almost four times that.
How wide should a sealed joint be?
The sealant can only stretch or compress by its movement class, a percentage of the joint width at installation (ISO 11600 and ASTM C920 classes). The minimum width is the larger of the opening and closing movement divided by that percentage. Add construction tolerance and follow the sealant maker's width-to-depth ratio.
Which temperatures should I enter?
Enter the temperature of the member itself. Dark or sunlit surfaces run well above the shade air temperature, and members in water or indoors stay closer to it. Use the design values that apply to the project, for example EN 1991-1-5 in Europe, and the installation temperature expected on site.
Is the restrained stress a design check?
No. It is the upper bound E · α · ΔT for a member held rigidly at both ends, without creep relaxation or fixing slip. It shows whether movement has to be provided. A restrained compression member can also buckle, so the usual answer is slotted holes or sliding supports rather than a stronger section.