How to Choose Pultruded FRP Rectangular Tubes

(Primary keyword: how to choose pultruded FRP rectangular tubes)

In the workshop, there are often stacks of pultruded rectangular tubes. These vary in size and glass-fiber content. They may look similar in color, but their performance can differ so much it would affect an entire tunnel. Whenever a new project RFQ arrives, my biggest fear is hearing “quote the cheapest first.” Using cheap grades in load-bearing areas leads to costly rework. It’s often cheaper to buy higher-grade tubes initially. Here’s a quick summary of my experience for structural engineers and procurement teams. This way, you can avoid the same pitfalls I faced.

1. What makes pultruded rectangular tube (pultruded FRP rectangular tube) so good?

Specific strength outperforms carbon steel. For the same cross-section, it is about 65% lighter. This saves the cost of at least one crane for lifting on site.
Corrosion resistance without coating:
- Immune to acids, alkalis, salt spray, and electrolytic corrosion.
- Expected service life begins at 30 years.
Low thermal expansion (about ¼ of aluminum): Long-span cable trays and utility galleries experience less thermal stress. This often means one expansion joint can be removed.
Radio-transparent and non-magnetic: perfect for radomes and subway catenary supports. EMC testing passes easily.
High designability: You can adjust wall thickness, fiber angle, and resin system during production. One die can make sections with three different stiffnesses.

Also known as:

Pultruded fiberglass rectangular tube
Pultruded GRP rectangular tube
Pultruded FRP box section
FRP pultruded rectangular profile
Pultruded composite rectangular tube

2. Seven-step method to select the right tube

Confirm the load combinations:
- Dead load
- Live load
- Wind
- Seismic
Then, run an FEM deflection check with the limit of L/250.
After calculation, you may find that a 5 mm vs. 6 mm wall thickness differs by only 1 mm but reduces deflection by 18%.
Check glass-fiber content: 65 wt% is the watershed. Tubes with glass content under 60% lose strength faster than prices drop. So, you cannot use them in load-bearing areas.
Choose the resin system:
- General atmospheric exposure → orthophthalic polyester (or isophthalic for better corrosion resistance).
- Chemical wastewater or seawater exposure → vinyl ester.
- Fire-retardant needs → phenolic + ATH (aluminum trihydroxide); the oxygen index is easily above 32.
Surface finish & appearance:
- Grooved texture + quartz-sand-coated film for anti-slip walkways.
- Film-faced finish for visible profiles; surface roughness Ra ≤ 1.6 for aesthetic and UV resistance.
Deflection verification + safety factor: After calculating deflection, apply a 1.5× safety factor. FRP is brittle and lacks a clear yield point. So, limit-state methods are more reliable than allowable-stress methods.
Connection methods:
- Use stainless steel end plates and bolted sleeves for primary load nodes. This way, we do not need any on-site drilling.
- Ladders and guardrails can use blind rivets and angle brackets for fast setup. But don’t rely on them as main load connections.
Request accompanying test reports with each shipment:
- Tensile strength ≥ 450 MPa.
- Interlaminar shear strength ≥ 35 MPa.
- Barcol/hardness retention after 24-h water immersion is ≥ 90%. If any reports are missing, reject the delivery.

(Keywords inserted throughout: pultruded FRP rectangular tube, pultruded fiberglass rectangular tube, pultruded GRP rectangular tube.)

3. On-site case studies

Ningbo Petrochemical utility rack support Spec: 200 × 100 × 6 mm, span 4.5 m, design load 3.0 kN/m². Started in 2017, the biannual salt-spray inspections show no fiber exposure or cracks near the bolt holes.
Shenzhen Metro Line 14 catenary crossarm specs: 150 × 75 × 5 mm. It must be non-magnetic and flame retardant B1. Used vinyl ester resin, LOI = 34; on-site magnetic induction measured 0.5 mT, well below the 1 mT limit.
Hainan offshore wind inspection walkway specifications:
- Grating beam size: 250 × 50 × 8 mm
- Resists Typhoon Class 12.
In 2021, Typhoon “Yuan-gui” passed close by. The largest deflection recorded was 9 mm. After unloading, the beam returned to zero residual deformation.
Beijing Winter Olympics temporary alpine grandstand Spec: 100 × 50 × 4 mm assembled truss. Low-temperature impact kept 85% performance at −25 °C. It was assembled and taken apart three times with no delamination.

4. Details that are often overlooked (field tips).

Seal cut ends: After sawing on-site, use vinyl-ester putty to seal the ends. This stops water from getting in and prevents delamination.
Drill hole allowance: Leave a +2 mm allowance on bolt holes. The drill bit friction heats and softens the resin, which may cause the hole to spring back about 0.2 mm. This pre-allowance stops bolts from getting too tight.
Avoid point-contact stacking. Tube ends can reach 80 °C under sun exposure. Point contact leads to white blotches. Use three timber bearers to spread support.
When using carbon steel, add EPDM isolation pads. This helps to prevent galvanic and electrochemical corrosion.
**Winter Handling:** Toughness decreases in the cold. So, avoid dropping tubes on the ground during transport. For sub-zero installations, prefer phenolic systems for lower brittleness.

5. Final note

When choosing pultruded FRP rectangular tubes, don’t focus solely on cross-section dimensions. Resin system, glass-fiber content, and node details are the keys to longevity. Add the safety factor to the drawings. Keep the headaches in the workshop. This way, the site won’t surprise you. Hongwo’s factory has 22 pultrusion lines. All resin tanks record temperature with PLCs. Each tube features a QR code for traceability. Share your load conditions, and I’ll calculate the best wall thickness in ten minutes. Save money and gain thirty more years of service.

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