Published by pultrx, the export trading company of Henan Zhongsheng. This is a buyer planning guide; the manufacturer confirms project specifications in writing.
Our factory pultrudes fiberglass I beams 3 to 8 in deep
How to read a fiberglass I-beam span table
Each row is a span in feet; each column is a limit. A manufacturer's table usually gives four kinds of column, and the allowable load for your beam is the smallest one that applies.
| Column | What it limits | When it applies |
|---|---|---|
| Laterally unsupported | Lateral-torsional buckling of the compression flange | When nothing braces the top flange along the span |
| Stress (flexure or shear) | Bending stress or web shear divided by the safety factor | Always; the upper bound for a braced beam |
| Deflection L/100 to L/360 | Mid-span sag as a fraction of the span | Pick the one your project specification sets |
| Section data | Weight per foot, moment of inertia, section modulus | To check the beam yourself or add other loads |
All three manuals cited here assume a single simple span with a uniform load. A beam continuous over supports, a point load, a cantilever or a beam loaded on its weak axis needs its own calculation.
Published allowable loads for four common sizes
The table quotes the L/180 column, laterally supported, from two manufacturers' tables for the four I-beam sizes we pultrude. They are short excerpts to show the order of magnitude, not a design table for our beams; open the source tables for every span and deflection limit.
| I-beam (depth × flange × thickness, in) | Span (ft) | Strongwell | Bedford |
|---|---|---|---|
| 3 × 1-1/2 × 1/4 | 4 | 176 | 197 |
| 3 × 1-1/2 × 1/4 | 6 | 57 | 62 |
| 3 × 1-1/2 × 1/4 | 8 | 25 | 26 |
| 4 × 2 × 1/4 | 4 | 391 | 452 |
| 4 × 2 × 1/4 | 6 | 136 | 150 |
| 4 × 2 × 1/4 | 8 | 61 | 66 |
| 4 × 2 × 1/4 | 10 | 32 | 34 |
| 6 × 3 × 1/4 | 6 | 404 | 479 |
| 6 × 3 × 1/4 | 8 | 193 | 221 |
| 6 × 3 × 1/4 | 10 | 105 | 118 |
| 6 × 3 × 1/4 | 12 | 63 | 69 |
| 8 × 4 × 3/8 | 8 | 595 | 707 |
| 8 × 4 × 3/8 | 10 | 336 | 390 |
| 8 × 4 × 3/8 | 12 | 206 | 234 |
| 8 × 4 × 3/8 | 14 | 135 | 150 |
| 8 × 4 × 3/8 | 16 | 92 | 101 |
Why deflection, not strength, sets the load
Pultruded FRP is strong along its length but has roughly one ninth of the modulus of steel, so a fiberglass beam sags long before it is overstressed. Strongwell's 4 × 2 × 1/4 in beam over 8 ft shows it plainly: the stress limit is 275 lb/ft, the L/180 limit is 61 lb/ft and the L/360 limit is 30 lb/ft. Choosing the deflection limit is therefore the biggest decision in sizing, and it belongs in the project specification, not in the supplier's catalogue.
| Limit | Allowable load (lb/ft) |
|---|---|
| Laterally unsupported | 21 |
| Stress, laterally supported | 275 |
| Deflection L/100 | 110 |
| Deflection L/180 | 61 |
| Deflection L/240 | 46 |
| Deflection L/360 | 30 |
The PulTex manual computes deflection as 5wL⁴/384EI plus a shear term, wL²/8A′G, because the low shear modulus of FRP adds noticeable deflection on short, deep beams. A steel span formula that leaves out shear will understate an FRP beam's sag.
Brace the compression flange
In the same 8 ft example the laterally unsupported load is 21 lb/ft, a third of the L/180 figure, because a narrow-flanged FRP beam twists sideways under load if nothing holds its top flange. Grating, decking or purlins fastened to the top flange, or bracing at intervals, give the laterally supported values. State on the RFQ how the top flange is restrained.
Fiberglass I-beam vs steel
A fiberglass I-beam weighs about a quarter as much as a steel beam of the same section and does not corrode or conduct electricity, but at equal size it deflects many times more. Where a steel beam was chosen by strength, an FRP replacement often needs a deeper section or closer supports to meet the same deflection limit. Use FRP where corrosion, electrical insulation or handling weight justify the deeper section.
For the property comparison behind this, read FRP vs steel
What to send for a beam quotation
- Span, support conditions and whether the beam is continuous over supports
- Loads: uniform, point loads and their positions, and whether the beam's own weight is included
- Deflection limit from the project specification (L/180, L/240, L/360 or other)
- How the top flange is braced, and at what spacing
- Service environment and temperature, which decide the resin
- Quantities by cut length, and holes or connection details
Our four inch sections with their calculated lb/ft are in the size chart: I-beam sizes and weights
Sources and further reading
- Strongwell: EXTREN Design Manual, Section 8, Flexural Members (Beams), Rev.0924
I-shape load tables, pages 8-18 to 8-22: simple span, uniform load in the plane of the web, safety factor 2.5 for flexural and 3.0 for shear stress, loads reduced when laterally unsupported. Values quoted for EXTREN 500 and 525 series.
- Bedford Reinforced Plastics: FRP Structural Shapes Design Guide
I-beam allowable uniform load tables, pages 4-15 to 4-21 (dated 8/2021): simply supported, uniform load, part weight deducted, safety factor 3.0 for flexural and shear stress and 2.5 for buckling.
- Creative Composites Group: The PulTex Pultrusion Design Manual, Volume 5, Revision 15 (imperial)
Load tables exclude beam weight; deflection includes a shear term; its SuperStructural I-sections use a higher modulus than the other two manuals, so its loads are not comparable with them.
Questions buyers ask
- How far can a fiberglass I-beam span?
- It depends on the load and the deflection limit. In Strongwell's published table a 6 × 3 × 1/4 in beam carries about 105 lb/ft over 10 ft at L/180; Bedford's table gives 118 lb/ft. For a different load or limit, read the matching row and column, or have the span calculated.
- How much weight can a fiberglass I-beam hold?
- Read it from the deflection column your project uses, not the stress column. A 4 × 2 × 1/4 in beam over 8 ft carries about 61 to 66 lb/ft at L/180 in the two manufacturers' tables quoted here, against about 230 to 275 lb/ft for strength alone.
- Are the loads in a span table total or net of the beam's weight?
- It depends on the manual. Bedford deducts the beam's weight; the PulTex manual does not include it; check the notes of the table you use and add or subtract the self-weight accordingly.
- Can I use these tables for pultrx beams?
- No. They describe each maker's own beams and design basis. Ask for the section properties and test data of the beam you order, and have the project engineer check the span with them.
- Why is the laterally unsupported load so low?
- A narrow FRP flange twists sideways under bending if it is not braced. Fasten grating or decking to the top flange, or add bracing, to use the laterally supported values.
Ready to move from reading to a scoped quotation?
Send span and load for a beam quotation