Thermal Expansion & Movement
FRP Moves More Than Steel — Plan for It
Glass fibers suppress expansion along their length, but the resin-rich directions expand more than mild steel. If panels are rigidly clamped tight, thermal movement buckles or cracks the panel; if they are left free, they rattle. The fix is controlled gaps plus sliding hold-down clips.
Coefficient of Thermal Expansion (Typical)
| Material / Direction | CTE (×10⁻⁶ /°C) | Note |
|---|---|---|
| Molded grating — longitudinal / transverse | ~15–30 | Quasi-isotropic; roughly balanced in both directions |
| Pultruded — along bearing bars (longitudinal) | ~5–9 | Glass-dominated, close to steel |
| Pultruded — transverse to bars | ~20–35 | Resin-dominated, moves the most |
| Mild steel (reference) | ~11.7 | Design baseline |
| Aluminium (reference) | ~23 | Useful for mixed-material frames |
Indicative values; actual CTE depends on resin, glass content and direction. Use panel test data for final design.
Calculating the Required Gap
Thermal movement ΔL = α × L × ΔT. Leave an installation gap that absorbs the maximum seasonal movement between extremes of installation and service temperature.
| Step | Formula / Rule |
|---|---|
| 1 · Movement | ΔL (mm) = CTE (mm/mm·°C) × panel length L (mm) × ΔT (°C) |
| 2 · Temperature range | Use installation-to-summer / installation-to-winter extremes, e.g. ΔT ≈ 50–60 °C |
| 3 · End gaps | Reserve ΔL at each end; leave 6–10 mm minimum nominal gap at panel ends |
| 4 · Around obstructions | Add clearance around fixed columns, walls and penetrations |
| 5 · Fasteners | Use oversized holes / slotted holes and sliding clips — do not rigidly fix every corner |
Installation-Joint Best Practice
Slip-Clamp One End
Anchor one end of a panel rigidly; let the other end slide under clips. This absorbs movement without stress.
Never Over-Tighten
Hold-down clips are locators, not clamps. Over-torquing defeats sliding and invites cracking at holes.
Long Runs
On long continuous runs, plan expansion joints every few metres; do not butt panels tightly end-to-end.
Worked Expansion Calculation
For a 3.66 m pultruded panel in isophthalic polyester, longitudinal CTE ≈ 15 × 10⁻⁶/°C, seasonal range 50 °C: movement = 3.66 m × 15 × 10⁻⁶ × 50 ≈ 2.7 mm. Across a 20 m continuous run of adjoining panels this accumulates to roughly 15 mm — enough to push clips or buckle edges if no gap is allowed. The design rule is to provide ~10 mm nominal gaps at panel ends and around fixed members, drill fastener clearance rather than forcing clips, and slip-clamp one end of each run while fixing the other, so movement goes into the gaps instead of into the structure.
Installation-Joint Practice
- Allow a nominal 10 mm gap between panels and at abutments; do not butt panels hard.
- Slip-clamp one end of a run; fix the other end — never rigidly fix both ends of a long deck.
- Use clips with clearance that permit sliding; never over-tighten to the point of crushing the bar.
- For long runs, plan a deliberate expansion joint at a chosen location and detail it on the drawing.
- Allow a nominal 10 mm gap between panels and at abutments; do not butt panels hard.
- Slip-clamp one end of a run; fix the other end — never rigidly fix both ends of a long deck.
- Use clips with clearance that permit sliding; never over-tighten to the point of crushing the bar.
- For long runs, plan a deliberate expansion joint at a chosen location and detail it on the drawing.
Calculating Expansion for Your Site?
Tell us panel length, resin system and your min/max service temperature — we return gap recommendations.
Source: ZeAllgrate (Fiberglass Grating Manufacturers Council / ACMA) design guidelines; ASTM International; ISO 14122; OSHA 29 CFR 1910. Indicative data for specification guidance only — final design verified by ZeAllgrate engineering against certified load/span charts.