FRP Span Guide
Span and load guidance for FRP (fiber reinforced plastic) grating products.
How to read an FRP span guide
FRP (Fiber Reinforced Plastic) grating behaves very differently from steel. Steel is stiff and heavy, and bending stress usually governs. FRP is lightweight (about 1/4 the density of steel) but has low stiffness — deflection almost always governs the span selection. Instead of “allowable load at span,” FRP tables typically show the maximum safe span for a given uniform load at a chosen deflection limit.
Design basis
- Product: molded FRP grating, isophthalic polyester resin, fiberglass roving reinforcement (standard Wiberg WM-M series).
- Top surface: concave or grit-top for slip resistance (ADA compliant).
- Support: simple span, two ends, clamp-mounted on steel or FRP stringer.
- Design deflection limit: L/100 for maintenance platforms, L/200 for production walkways, L/240 for office / public access.
- Typical pattern: 1-1/2" × 1-1/2" (38 mm) square mesh.
Maximum safe simple span (L/200 deflection) — molded FRP
| Panel thickness | 100 psf (4.8 kPa) | 250 psf (12 kPa) | 500 psf (24 kPa) | 1,000 psf (48 kPa) |
|---|---|---|---|---|
| 1" (25 mm) | 36" (915) | 24" (610) | 18" (460) | — |
| 1-1/4" (32 mm) | 45" (1145) | 30" (760) | 22" (560) | 16" (405) |
| 1-1/2" (38 mm) | 54" (1370) | 36" (915) | 28" (710) | 20" (510) |
| 2" (51 mm) | 72" (1830) | 48" (1220) | 40" (1015) | 28" (710) |
Spans are in inches (mm). All values assume L/200 midspan deflection on molded grating with 1-1/2" square mesh, simple-span, uniformly loaded. For L/240 reduce span by ~10%; for L/100 increase span by ~15%.
Pultruded FRP — high-strength / long-span option
Pultruded FRP grating (I-bar profile) is 2–3× stiffer in the primary span direction than molded. Typical Wiberg WM-P series achieves the spans below at 100 psf, L/200:
| I-bar depth | 100 psf span | 250 psf span | 500 psf span |
|---|---|---|---|
| 1" (25 mm) | 48" (1220) | 36" (915) | 24" (610) |
| 1-1/2" (38 mm) | 72" (1830) | 54" (1370) | 36" (915) |
| 2" (51 mm) | 96" (2440) | 72" (1830) | 48" (1220) |
| 3" (76 mm) | 144" (3660) | 108" (2745) | 72" (1830) |
When to choose FRP over steel
- Corrosive environments: chlorine tanks, water/wastewater, food processing, marine splash zone.
- Electrical/non-magnetic zones: substations, MRI rooms, radar / telecom shelters.
- Weight-sensitive applications: offshore topsides, retrofit where load-bearing structure is marginal.
- Corrosion-ordered life: >25 years design life in chemical exposure where hot-dip galvanized steel would need replacement at 8–12 years.
FAQ — FRP grating
- Why does FRP span less than steel?
- FRP's modulus of elasticity is about 1/10 that of steel (E ≈ 2,700 ksi vs 29,000 ksi). Although FRP is very strong, it deflects more, and deflection, not stress, controls the selection. The tables above already account for this.
- Can FRP grating take forklift wheel loads?
- Generally no for standard molded panels. Pultruded 1-1/2" I-bar at a 24" clear span can safely support pedestrian dollies and small electric pallet trucks (up to ~500 lb wheel). Full forklift traffic should use heavy-duty steel grating.
- Is molded FRP fire rated?
- Standard isophthalic polyester has ASTM E84 Class II (Flame Spread < 25). A vinyl-ester or phenolic resin upgrade is available for ASTM E84 Class I with self-extinguishing behavior and reduced smoke.
- Does FRP meet OSHA slip resistance?
- Yes. Grit-top finish delivers a static coefficient of friction > 0.8 (wet and dry), exceeding OSHA and ADA requirements. The concave top is a slip-resistant alternative for lighter-duty walkways.
- How long does FRP grating last outdoors?
- With a UV-inhibited resin and surface veil, design life is typically 25–30 years. Color fade is cosmetic and does not reduce structural capacity. Phenolic or vinyl-ester resins are recommended for aggressive chemical service.
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