Pultruded FRP Structural Shapes: Design Data for Beams, Columns and Stairs
Reference design values for pultruded fiberglass reinforced plastic (FRP) structural members — allowable stresses, deflection limits, stock sizes, stair tread deflection, pedestrian load criteria and grating clip rules. These are industry-published reference values commonly used for preliminary engineering planning.
The design reference values below reflect criteria commonly published in the pultruded FRP structural shapes industry, provided for preliminary engineering planning. They are not a substitute for project-specific structural engineering. ZEALL supplies pultruded FRP profiles — angles, channels, I-beams and wide-flange beams — in the standard stock sizes listed. Confirm all final designs with our engineering team before construction.
1. Design Criteria for FRP Structural Beams
FRP structural members used as beams supporting live load plus the dead load of the FRP structure are typically designed using the following reference allowables. Higher safety factors and deflection ratios may be needed for other loading or environmental conditions.
| Parameter | Reference Value | Notes |
|---|---|---|
| Maximum deflection | L / 180 | Applies to beams under live + dead load |
| Allowable bending stress | 10,000 psi (F.S. = 3) | Assumes adequate lateral bracing of compression flange |
| Allowable shear stress | 1,500 psi (F.S. = 3) | |
| Allowable bearing stress | 10,000 psi (F.S. = 3) | For bolted connection bearing on FRP plate or web |
Applicability conditions: These allowables assume (a) adequate lateral bracing of the compression flange, and (b) a flange width-to-thickness ratio (b/t) not exceeding 12. For wider or thinner flanges, a column-failure / local-buckling check is required. See also our pultruded FRP profiles page for available section properties.
2. Preferred Stock Sizes of Pultruded FRP Structural Shapes
The following pultruded FRP structural profiles are commonly available from stock without mill-run lead times. Members are typically stocked in 20'-0" (6 m) lengths. Non-stock shapes can be obtained in mill-run quantities.
| Shape Category | Available Sizes (inches) |
|---|---|
| Equal Leg Angles | 3" x 1/4" · 3" x 3/8" · 4" x 1/4" · 4" x 3/8" · 4" x 1/2" |
| Channels | 6" x 1-5/8" x 1/4" · 8" x 2-3/16" x 3/8" · 10" x 2-3/4" x 1/2" |
| I-Sections | 8" x 4" x 3/8" · 12" x 6" x 1/2" |
| Wide Flange Sections | 4" x 4" x 1/4" · 6" x 6" x 1/4" · 6" x 6" x 3/8" · 8" x 8" x 3/8" |
| Standard stock length | 20'-0" (all shapes) |
These profiles are available in fire-retardant vinyl ester (beige), fire-retardant polyester (dark gray), and non-fire-retardant polyester (green) resin systems. For section properties and load tables, refer to our grating load table page or contact engineering.
3. Stair Tread Load / Deflection Reference (1-1/2" Deep Gritted Tread)
FRP stair treads made from 1-1/2" deep molded grating with an integral gritted nosing are commonly used in industrial stair systems. Tread deflections are typically limited to L/150 or less. The table below shows typical load-deflection performance for spans up to 42 inches, based on 250 lb and 500 lb point loads applied over a 4" wide x 6" deep load plate starting at the nosing edge.
| Load (lb) | 18" span | 24" span | 30" span | 36" span | 42" span |
|---|---|---|---|---|---|
| 250 | 0.03" | 0.05" | 0.09" | 0.16" | 0.25" |
| 500 | 0.06" | 0.10" | 0.19" | 0.32" | 0.50" |
Spans greater than 42" require a stiffened nosing or intermediate support to reduce deflections. For standard compliance reference, see our DIN 24537-3 standards page.
4. Pedestrian Walkway Loads, Cantilever & Clip Rules
The table below summarizes minimum load criteria for FRP grating pedestrian walkways, cantilever limits, hold-down clip spacing, and unreinforced opening rules. These are reference values commonly published for industrial FRP grating installations.
| Category | Item | Reference Value |
|---|---|---|
| Pedestrian loads | Uniform live load | 50 – 65 lb/ft² |
| Concentrated load | 250 lb | |
| Dead load (1-1/2" molded grating) | 3.75 lb/ft² | |
| Maximum deflection | L/120 or 3/8" (whichever is less) | |
| Cantilever limits | 1" deep grating | ≤ 6" from last support centerline |
| 1-1/2" deep grating | ≤ 12" from last support centerline | |
| 2" deep grating | ≤ 12" from last support centerline | |
| Clips & openings | M-hold-down clips: minimum per support beam | 2 clips per beam |
| M-hold-down clips: max spacing | ≤ 4'-0" on center per support beam | |
| M-hold-down clips: minimum per panel | 4 clips per panel | |
| F-type abutment clips: max spacing | ≤ 24" on center for abutting edges | |
| Unreinforced opening limit | ≤ 1/3 of panel width removed; no extra support needed |
Cantilevered gratings require a minimum of two supports and at least three hold-down clips at each interior support to prevent overturning.
Lateral Support Requirements for Full Bending Capacity
To develop the full bending capacity of pultruded FRP beams, the compression flange must be laterally braced at intervals short enough to prevent lateral-torsional buckling. The maximum unbraced length depends on the shape and size. As a general reference:
- Channels (C): 48" (C6x1/4"), 60" (C8x3/8" and C10x1/2")
- I-sections (I): 24" (I4x1/4"), 36" (I6x1/4"), 48" (I8x3/8"), 60" (I10x3/8"), 84" (I12x1/2")
- Wide-flange (W): 60" (W4x1/4"), 84" (W6x1/4"), 96" (W6x3/8"), 108" (W8x3/8"), 156" (W10x3/8"), 168" (W12x1/2")
If your beam span exceeds the listed unbraced length, add intermediate lateral bracing (e.g., cross-bracing, floor deck attachment, or knee braces) to the compression flange. Without adequate bracing, the allowable bending stress must be reduced. For section properties and full beam selection tables, contact our engineering team.
Resin System Options
Pultruded FRP structural shapes are available in three standard resin formulations, each suited to different environmental conditions:
- Fire-retardant vinyl ester (beige) — best for harsh chemical exposure and fire-rated interiors; excellent corrosion resistance against acids and alkalis.
- Fire-retardant polyester (dark gray) — general-purpose industrial FRP with good fire performance; suitable for most walkway and platform applications.
- Non-fire-retardant polyester (green) — economical choice for mild environments where fire rating is not a specification requirement.
Resin selection affects both mechanical properties and long-term durability. Vinyl ester offers the highest corrosion resistance and is recommended for chemical plants, wastewater treatment, and offshore/marine environments. Polyester resins are adequate for general industrial walkways and structural supports in less aggressive atmospheres. For temperatures above 140°F (60°C), derating factors apply and should be confirmed with engineering.
Connection Design Notes
Pultruded FRP structural members are typically connected using stainless steel bolts (316 SS recommended for exterior or corrosive environments). Because FRP is anisotropic (stronger along the pultrusion axis than across it), bearing and shear at connections often control the design rather than bending stress in the member itself.
Key connection rules of thumb: edge distance from bolt centerline to the member end should be at least 3 bolt diameters, and to the side edge at least 2.5 diameters. Bolt pitch (center-to-center) should be at least 5 diameters. Always verify that the connection bearing on the FRP plate or web does not exceed the allowable bearing stress before assuming the member can develop its full bending capacity.
Typical Applications for Pultruded FRP Structural Shapes
Pultruded FRP angles, channels, I-beams and wide-flange sections are used across industries where steel would corrode or where weight reduction and electrical non-conductivity matter. Common structural applications include:
- Industrial platforms and walkways — support beams under molded or pultruded grating decks in chemical plants, wastewater treatment facilities, and pulp and paper mills where steel rusts within months.
- Stair stringers and handrail posts — channels (typically C8 or C10) serve as stair stringers; square tubes serve as guardrail posts. FRP stairs do not require painting and resist salt and chemical atmospheres.
- Equipment support frames and pipe racks — wide-flange beams and columns support ductwork, piping, and process equipment in corrosive environments. The non-conductive nature of FRP makes it ideal near substations and high-voltage equipment.
- Marine and offshore structures — FRP structural shapes do not rust in salt spray, making them suitable for dock walkways, offshore platform access, and shipboard retrofit structures.
- Cleanrooms and food processing — polyester and vinyl ester FRP do not leach into food-grade environments and can be washed down without corrosion.
Compared to steel, pultruded FRP weighs roughly one-quarter as much, which reduces installation labor and dead-load on supporting structures. However, FRP has a lower modulus of elasticity (typically 2.5–3.8 million psi versus 29 million psi for steel), which means deflection — not stress — often controls the design. This is why the L/180 deflection limit is the primary sizing criterion for most FRP beam applications, rather than the 10,000 psi bending stress allowable.
Understanding the Three Deflection Limits: L/180, L/150, L/120
Three different deflection ratios appear in FRP structural design, and they apply to different components:
- L/180 — applies to primary structural beams (I-beams, channels, wide-flange columns) under combined live + dead load. This is the stricter, more conservative limit because these members carry the full structural load.
- L/150 — applies specifically to stair treads. Treads can tolerate slightly more deflection than primary beams because they are local walking surfaces, not main load-carrying members.
- L/120 (or 3/8", whichever is less) — applies to FRP grating pedestrian walkway panels themselves (the grating deck, not the supporting beams). Grating panels are secondary surfaces and can tolerate more deflection, but the absolute 3/8" cap prevents perceptible bounce underfoot.
The b/t ≤ 12 limit and the lateral bracing requirement are prerequisites for the bending stress allowable of 10,000 psi. If the compression flange is not laterally supported, or if the flange width-to-thickness ratio exceeds 12, the member may buckle locally or laterally-torsionally before reaching the nominal bending stress, and the full allowable cannot be used. In those cases, a column-type buckling analysis is required.
These numerical values are reference values commonly used for pultruded FRP structural shapes, as published in referenced engineering guidelines. They are provided for preliminary planning only. Actual project designs must account for specific loading conditions, span lengths, bracing arrangements, resin system, temperature exposure, and environmental corrosion. Verify all designs with our engineering team before construction.