White Paper · Pultruded Design
Pultruded FRP grating carries load through a set of continuous, parallel I- or T-shaped bearing bars tied together by cross-rods. Because the bars are aligned, the section behaves as a beam — not as a bi-directional panel — and structural design becomes a matter of computing exact section properties, controlling deflection and detailing the connections that hold the bars to the support steel. This paper sets out the I/T bar section properties, the span-to-depth rule, the L/200 deflection criterion and the connection design rules that make a pultruded deck predictable.
- Uni-directional stiffness: pultruded bars run at ~70% glass content; E in the bar direction is roughly twice that of molded grating, so long spans become feasible.
- Deflection governs: with E ≈ 10–20 GPa, the bar that passes ultimate strength usually fails the L/200 serviceability limit first.
- Orientation is critical: bearing bars must run across the short span; a panel installed with bars parallel to the span is the most common and most dangerous field error.
How a Pultruded Bearing Bar Is Built
A pultruded bearing bar is a constant-section beam pulled through a heated die. The cross-section is an I-shape (wide top and bottom flanges, thin web) or a T-shape (one wide top flange, lower stem). Continuous glass rovings run along the length in the web and flanges; continuous filament mat and surfacing veil provide transverse strength and a resin-rich surface. Cross-rods (steel or FRP) are threaded through holes in the bars during pultrusion at 30–100 mm spacing, tying the bars into a panel.
| Dimension | I-Bar (indicative) | T-Bar (indicative) | Design Role |
|---|---|---|---|
| Overall depth h | 40–75 mm | 50–75 mm | Primary lever arm for bending |
| Flange width b | 20–25 mm | 25–40 mm | Compression/tension flange |
| Web thickness t_w | 3–5 mm | 4–6 mm | Shear transfer; local buckling |
| Flange thickness t_f | 3–6 mm | 4–8 mm | Bearing and local crushing |
| Bar spacing c/c | 25–50 mm | 30–50 mm | Sets number of bars per metre |
| Cross-rod diameter | Ø6–Ø10 mm | Ø6–Ø10 mm | Panel integrity / load sharing |
Dimensions indicative; exact I/Z per bar are tabulated in the industry pultruded grating metric design manual (manufacturer data). I-bar is the efficient deep beam; T-bar offers a wider walking flange.
Per-Metre Strip Calculation
Design a 1 m-wide strip of panel as a series of parallel simply supported bars. For a uniformly loaded span L:
n_bars = 1000 / c_cross (bars per metre width) I_panel = n_bars × I_bar (mm⁴ per m) Z_panel = n_bars × Z_bar (mm³ per m) Mmax = w L² / 8 (N·mm per m width) δ = 5 w L⁴ / (384 E I) (mm midspan deflection) σ_bend = Mmax / Z_panel (bending stress, N/mm²) σ_allow = σ_ult / FOS (FOS = 5:1 pultruded, per ZeAllgrate practice)
Here w is load per unit length on the 1 m strip (N/mm), L the clear span (mm), E the flexural modulus in the bearing-bar direction (N/mm², typically 10,000–20,000), I_bar the second moment of area of one bar (mm⁴) and Z_bar its section modulus (mm³). The cross-rods distribute concentrated loads across a few bars; for uniform loading the per-metre strip method is conservative and standard.
Per-Metre Values by Bar Depth
I = second moment of area, Z = section modulus, per metre width. Values indicative; verify against the ZeAllgrate pultruded load table for the exact bar and glass content.
| Pultruded Bar | Weight (kg/m²) | I per m (cm⁴/m) | Z per m (cm³/m) | Typ. max span at L/200, 5 kN/m² |
|---|---|---|---|---|
| I-40 (40 mm deep) | ~11 | ~150 | ~8.5 | ~1.15 m |
| I-50 (50 mm deep) | ~13 | ~230 | ~12.0 | ~1.45 m |
| I-60 (60 mm deep) | ~15 | ~340 | ~16.5 | ~1.75 m |
| I-75 (75 mm deep) | ~18 | ~560 | ~25.0 | ~2.10 m |
| T-50 (50 mm deep) | ~14 | ~260 | ~13.0 | ~1.50 m |
| T-65 (65 mm deep) | ~17 | ~420 | ~19.5 | ~1.90 m |
Indicative ranges aligned with the industry pultruded grating metric design manual. Exact values vary with flange width, web thickness and glass fraction; use manufacturer data for final design.
A Quick Rule of Thumb
For preliminary layout, experienced pultruded-grating designers use a span-to-depth limit of roughly L/h ≈ 20–25 for pedestrian walkways at L/200. That is, a 50 mm bar spans about 1.0–1.25 m; a 75 mm bar spans about 1.5–1.9 m. For industrial live loads or crowd deflection (L/240), reduce the ratio toward 18–20. This is a layout rule — the final design must always be confirmed by the deflection and strength calculation above.
| Service | Span/Depth (L/h) target | Deflection limit |
|---|---|---|
| Light pedestrian / equipment access | 22–25 | L/200 |
| Standard industrial walkway | 20–22 | L/200 |
| Crowd / sensitive / occupied platform | 18–20 | L/240 |
| Heavy industrial / wheel load | 16–18 | L/150 |
Rule-of-thumb ratios; ZeAllgrate Guidelines and manufacturer load tables govern final selection.
Design a 1.6 m-Span Pultruded Walkway, 5 kN/m², L/200
Given: clear span L = 1.6 m = 1600 mm; uniform live load q = 5 kN/m²; deflection limit L/200; pultruded I-60 bars at 40 mm c/c, E = 15,000 N/mm²; ultimate flexural stress ≈ 250 MPa, FOS = 5 → σ_allow = 50 N/mm². From the table, I-60 provides I_panel ≈ 340 cm⁴/m = 3.40×10⁶ mm⁴/m and Z_panel ≈ 16.5 cm³/m = 16,500 mm³/m.
Step 1 Allowable deflection
δ_allow = L/200 = 1600/200 = 8.0 mm
Step 2 Required I from deflection δ = 5 w L⁴ / (384 E I)
w = 5 kN/m² × 1 m = 5 N/mm
I_req = 5 × w × L⁴ / (384 × E × δ_allow)
= 5 × 5 × 1600⁴ / (384 × 15000 × 8)
= 3.277e14 / 4.608e7
= 7.11e6 mm⁴ per m ≈ 711 cm⁴/m
Step 3 Select section
I-60 provides I = 340 cm⁴/m = 3.40e6 mm⁴/m < 711 → deflection FAILS
Move to I-75: I = 560 cm⁴/m = 5.60e6 mm⁴/m < 711 → still short
For 1.6 m at L/200, upsize bar depth or reduce span:
I-75 at L = 1.45 m (per table) satisfies; for 1.6 m use
a deeper custom bar or closer supports.
Step 4 Verify strength (governing section chosen)
Mmax = w L² / 8 = 5 × 1600² / 8 = 1.60e6 N·mm per m
Z_req = Mmax / σ_allow = 1.60e6 / 50 = 32,000 mm³ = 32 cm³/m
I-75 provides Z ≈ 25 cm³/m → strength also needs upsize
Conclusion: at 1.6 m span, L/200 deflection governs; select a
deeper pultruded section or reduce clear span to ~1.45 m.Result: the I-60 bar that looked adequate for a 1.6 m span fails both deflection and strength at L/200. The pultruded metric manual recommends an I-75 (or a closer support spacing of ~1.45 m). This is the central lesson: read the load table at your exact span and deflection limit — never interpolate a neighboring thickness.
How the Bars Are Held to the Supports
- Bearing length: each bar end must bear at least 50 mm (2 in) on steel angle or FRP support to prevent local web crushing and punch-out at the cross-rod hole.
- Clip placement: one G-clip or wedge clip per bearing bar at every support; a clip at each panel corner is mandatory. Typical clip spacing ≤ 300 mm along the support.
- Bar-end gap: leave a 6–10 mm expansion gap between panel ends to accommodate thermal movement (FRP expands ~2–3× more than steel).
- Edge support: free (un-supported) panel edges should be framed with FRP angle bar; long unsupported edges deflect independently and overstress the first row of clips.
- Fastener material: stainless A2/A4 (304/316) clips and bolts; galvanized hardware is acceptable in dry service but corrodes faster than the FRP itself in corrosive service.
Wheels, Feet and Equipment Bases
Uniform load rarely governs pultruded decks — a forklift wheel, a pump foot or a drum base does. A concentrated load on one bar is distributed to neighboring bars by the cross-rods over a short patch width. For a patch of width b (mm) over the bar spacing c:
n_share = round(b / c) + 1 (bars sharing the load, typically 3–5) P_bar = P / n_share (kN per shared bar) Check P_bar against the concentrated-load table for that bar at span L. If the patch sits over a cross-rod, sharing improves; if between cross-rods, assume the load goes to the nearest bar (conservative).
For forklift or pallet-truck traffic, reduce the static value by ~50% for impact and select the bar from the wheel-load row, not the uniform row. Where wheel loads are high, specify a closer support spacing or a deeper bar rather than relying on the uniform-load table.
When the Long Bar Is the Right Answer
| Criterion | Pultruded | Molded |
|---|---|---|
| Glass content / stiffness | ~70% / ~2× stiffer | ~33% / standard |
| Typical span | 1.5–3.0 m | 0.6–1.2 m |
| Strength direction | Uni-directional (bars) | Bi-directional |
| Impact resistance | Good | Excellent |
| Best for | Long spans, heavy loads, traffic | Moderate loads, wet/corrosive, impact |
Use pultruded where span or load drives the section; use molded where impact, bi-directional routing or cost at short spans dominates. Mixed projects often use both — molded in process areas, pultruded on main walkways.
Conclusions
Design pultruded grating as a set of parallel I/T beams, not as a plate. Compute I/Z per bar, multiply by bars per metre, and let the L/200 deflection limit select the bar depth — strength rarely governs once deflection is satisfied. Respect the 50 mm bearing length, clip every bar on every support, frame free edges and leave thermal gaps. Read the manufacturer load table at your exact span; the worked example shows how a bar that looks adequate at first glance can fail both criteria.
Sources Cited in This Paper
- Industry pultruded grating metric design manual (manufacturer PDF) — bar section properties, load/span and ASTM D7290 design-value method. Manufacturer technical data available upon request.
- Industry molded grating engineering guide (manufacturer PDF) — bi-directional panel design and safety factors. Manufacturer technical data available upon request.
- ZeAllgrate — Guidelines for the Engineer/Designer (ACMA) — pultruded bar safety factors, deflection limits and connection practice.
- Industry load-span tables (manufacturer data) — pultruded I/T-bar uniform and concentrated load spans. Manufacturer technical data available upon request.
- ASTM D790 (flexural), D638 (tensile), D7290 (evaluating pultruded composites); OSHA 29 CFR 1910.23; ISO 14122.
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