White Paper · Structural Design
FRP grating is a composite, orthotropic material with a much lower elastic modulus than steel. Designing it correctly is less about ultimate strength and more about controlling deflection, applying the right safety factors, and orienting the bearing bars along the span. This paper sets out the load cases, beam theory, section properties and a fully worked walkway example.
- Deflection usually governs: because E is roughly 1/20th that of steel, the FRP section that passes strength often fails the deflection limit.
- Safety factor 4:1–5:1 on ultimate strength is the ZeAllgrate-consistent practice for molded grating.
- Bearing bars must run across the span; orienting them parallel to the span is the single most common design error.
What the Grating Must Carry
| Load Category | Typical Design Value | Notes |
|---|---|---|
| Uniform dead load (grating self-weight) | ~0.5–0.8 kN/m² | Molded 25–50 mm; light vs. steel |
| Pedestrian live load | 2.4–3.1 kN/m² (50–65 psf) | OSHA/ASCE walkway minimums |
| Industrial live load | 4.8–14.4 kN/m² (100–300 psf) | Equipment decks, plant walkways |
| Concentrated / line load | Wheel or equipment foot, e.g. 1.0–13.5 kN | Check patch area and distribution |
| Impact / dynamic | Reduce static value by ~50% | Forklift, truck traffic |
| Wind uplift / suction | Per project wind map | Often governs connections, not bars |
Pedestrian live load 50–65 psf and industrial 100–300 psf are consistent with OSHA 29 CFR 1910.23 walking-working surfaces and ASCE/SEI 7 practice; industry-standard manufacturer load tables tabulate max-recommended vs. ultimate.
Treating an Orthotropic Panel as a Beam
Molded grating is bi-directional but is normally designed in the bearing-bar direction (the deeper bar) as a series of simply supported "beams" of unit width (1 m or 1 ft). Pultruded grating behaves as a set of parallel I/T bearing bars tied by cross-rods. For a uniformly loaded simply supported span:
Mmax = w L² / 8 (maximum bending moment, N·m per m width) Z_req = Mmax / σ_allow (section modulus required, m³/m) δ = 5 w L⁴ / (384 E I) (midspan deflection, m) σ_allow = σ_ult / FOS (ZeAllgrate molded: FOS 4:1 – 5:1)
Here w is load per unit length (N/m) on a 1 m-wide strip, L is the clear span, E the flexural modulus in the bearing-bar direction, and I the moment of inertia per meter width. Because molded grating is bi-directional, an "effective width" distributes concentrated loads; for uniform loading the per-meter strip approach is conservative and standard.
Why Deflection Governs for FRP
Steel designs are often strength-limited because E ≈ 200 GPa makes deflection small. FRP E is only ~7–17 GPa, so a section that is comfortably strong may visibly sag. Excessive deflection causes a "bouncy" or unsupported feel to pedestrians and can overstress connections. Common limits:
| Application | Deflection Limit | Rationale |
|---|---|---|
| Sensitive / crowded / public areas | L/200 | Stiff, reassuring feel; crowd comfort |
| Standard pedestrian walkway | L/150 | General industrial/commercial practice |
| Heavy industrial / equipment | L/100 | Rigid deck under machinery |
ZeAllgrate Guidelines for the Engineer/Designer and manufacturer catalogs recommend deflection limits (commonly the lesser of L/120 or 3/8 in for molded pedestrian service); L/150 is adopted for the worked example below.
Indicative Per-Metre Values
I = moment of inertia, Z = section modulus, per metre width. Values indicative; verify against ZeAllgrate load tables for the exact resin/mesh.
| Grating Type | Weight (kg/m²) | I per m (cm⁴/m) | Z per m (cm³/m) | Typ. max span at L/200 |
|---|---|---|---|---|
| Molded 25 mm (1 in) | ~9.5 | ~45 | ~4.2 | ~0.75 m (L/200, 5 kN/m²) |
| Molded 30 mm | ~12.0 | ~90 | ~6.0 | ~0.95 m |
| Molded 38 mm (1.5 in) | ~15.5 | ~180 | ~9.5 | ~1.20 m |
| Molded 50 mm (2 in) | ~20.0 | ~380 | ~16.0 | ~1.65 m |
| Pultruded I-4010 | ~11.0 | ~150 | ~8.5 | ~1.10 m |
| Pultruded I-6010 | ~13.5 | ~260 | ~13.0 | ~1.35 m |
| Pultruded T-5020 | ~16.0 | ~340 | ~16.0 | ~1.55 m |
Indicative ranges aligned with industry molded and pultruded (I/T-bar) load tables. Exact I/Z vary with mesh and glass content; use manufacturer data for final design.
Design a 1.2 m-Span Walkway for 5 kN/m², L/150
Given: clear span L = 1.2 m; uniform live load q = 5 kN/m²; deflection limit L/150; molded vinyl-ester grating, E = 10 GPa = 1.0×10⁴ N/mm²; ultimate flexural stress ≈ 150 MPa, FOS = 5 → σ_allow = 30 MPa = 30 N/mm². Design a 1 m-wide strip.
Step 1 Allowable deflection
δ_allow = L/150 = 1200/150 = 8 mm
Step 2 Required I from deflection δ = 5 w L⁴ / (384 E I)
w = 5 kN/m² × 1 m = 5 N/mm (per mm width? use SI)
w = 5 N/mm ; L = 1200 mm ; E = 10000 N/mm²
I_req = 5 w L⁴ / (384 E δ)
= 5 × 5 × 1200⁴ / (384 × 10000 × 8)
= 5.184e13 / 3.072e7
= 1.69e6 mm⁴ per m width ≈ 169 cm⁴/m
Step 3 Select section
38 mm molded grating provides I ≈ 180 cm⁴/m = 1.80e6 mm⁴/m
180 > 169 → deflection OK (governing)
Step 4 Verify actual deflection
δ = 5 × 5 × 1200⁴ / (384 × 10000 × 1.80e6)
= 5.184e13 / 6.912e12 = 7.5 mm < 8 mm ✓
Step 5 Check strength
Mmax = w L² / 8 = 5 × 1200² / 8 = 900,000 N·mm per m
Z_req = Mmax / σ_allow = 900000 / 30 = 30,000 mm³ = 30 cm³/m
38 mm molded provides Z ≈ 95 cm³/m >> 30 ✓ (strength not governing)Result: a 38 mm molded vinyl-ester grating spans 1.2 m at 5 kN/m². Deflection (7.5 mm vs. 8 mm allowable) governs; strength is comfortably below allowable. For a stiffer "crowd" requirement (L/200), δ_allow falls to 6 mm and the section must be upsized toward 50 mm.
Detail Rules That Keep the Design Valid
- Support bearing length: end bearings should provide at least 50 mm (2 in) of bearing on steel or FRP supports to avoid local crushing/punch-shear.
- Clip spacing: at least one clip per bar per support; typical spacing ≤ 300 mm along supports, with a clip at each panel corner.
- Edge condition: unrestrained panel edges should be limited in length; long free edges sag independently and need edge framing.
- Thermal expansion: FRP expands ~2–3× more than steel; use slotted holes or expansion clips on long runs to avoid buckling.
Five Mistakes to Avoid
| Error | Consequence | Correction |
|---|---|---|
| Bearing bars parallel to span | Panel carries load on the weak direction; large deflection/failure | Orient bars across the span |
| Ignoring deflection, checking strength only | Bouncy deck, connection overload | Apply L/100–L/200 limit |
| Insufficient bearing support | Local punch-shear at ends | ≥ 50 mm bearing, edge framing |
| No thermal-expansion allowance | Panel buckles in summer | Slotted holes / expansion clips |
| Concentrated load not checked | Wheel/foot point-load fails bars | Check line/concentrated load tables |
Conclusions
Design FRP grating as a stiffness-limited composite beam, not as a thin steel plate. Orient bearing bars across the span, apply ZeAllgrate-consistent safety factors of 4:1–5:1 on ultimate, and let deflection (L/150 standard, L/200 sensitive) select the thickness first — then confirm strength. Detail the supports, clips and thermal-expansion joints as carefully as the bars themselves.
References
- Industry molded grating technical catalog (manufacturer data) — load/span and section data. Manufacturer technical data available upon request.
- Industry pultruded grating technical brochure (manufacturer data) — metric section properties, ASTM D7290 design-value method. Manufacturer technical data available upon request.
- ZeAllgrate — Guidelines for the Engineer/Designer (ACMA) — safety factors, deflection limits, connection practice.
- Industrial grating load-span tables (manufacturer data) — pultruded I/T-bar properties and wheel-load spans. Manufacturer technical data available upon request.
- ASTM D638 (tensile), D790 (flexural), D695 (compressive), D732 (punch shear); OSHA 29 CFR 1910.23; ISO 14122 (permanent machinery access).
Load Combinations and the Governing Case
Design starts from the load set, not from the table: dead load (the panel itself), live load (pedestrians, equipment, maintenance tools), and occasional cases (wind on exposed decks, seismic where applicable, dropped loads). The uniform live load is checked against the uniform table; any point load — a wheel, pump base, ladder footprint — is checked against the concentrated table, and the governing case is the one that demands the larger section. In practice the concentrated case governs most single-span walkways, which is why the load/span tables present both columns and why skipping the concentrated check is the most common design error.
Why Deflection Controls FRP Design
FRP's elastic modulus is roughly one-twentieth of steel's, so the section that passes a strength check will typically fail the deflection limit first. The design consequence is that FRP is selected on stiffness: the section is chosen so the sag at the design load stays inside the chosen limit (L/200 for walkways, L/150 for infrequently walked platforms), and the strength check becomes a confirmation rather than the driver. Load/span tables are therefore deflection-limited values, and the limit used is stated on the table — always read the table at your project's limit.
Bearing, Supports and Edge Conditions
- Bearing bar orientation — bearing bars run along the span, across the supports; orient the panel accordingly on the drawing.
- Support condition — tables assume simple supports with the panel bearing fully on each support; verify seating width for the specified span.
- Bearing length — provide full bar seating at supports; a partial seat reduces capacity and concentrates stress.
- Openings — cut-outs interrupt bearing bars; verify the remaining section separately and reinforce edges per the cutting guide.
- Fasteners — fixings transfer load without clamping the panel rigid; clips with clearance permit the thermal movement the table assumes.
- Bearing bar orientation — bearing bars run along the span, across the supports; orient the panel accordingly on the drawing.
- Support condition — tables assume simple supports with the panel bearing fully on each support; verify seating width for the specified span.
- Bearing length — provide full bar seating at supports; a partial seat reduces capacity and concentrates stress.
- Openings — cut-outs interrupt bearing bars; verify the remaining section separately and reinforce edges per the cutting guide.
- Fasteners — fixings transfer load without clamping the panel rigid; clips with clearance permit the thermal movement the table assumes.
Send Us Your Load Case
Share span, live load, deflection limit and bearing-bar orientation — ZeAllgrate will return a load-verified section selection with COA data.