Load Capacity and Weight: Fiberglass vs Steel Cable Tray Side by Side

Fiberglass (FRP/GRP) cable tray carries roughly 40–75 kg per metre at a 3 m support span, while a comparable hot-dip galvanized steel tray carries about 75–150 kg per metre — yet the fiberglass tray weighs around 70–75% less, and that weight difference is often the number that actually decides the specification.

That single sentence is the honest answer to the headline. Steel wins on raw load capacity and stiffness. Fiberglass wins on mass, corrosion resistance and installation practicality. The rest of this article puts numbers behind both statements so you can check them against your own structural drawings.

The comparison below uses published load/span behaviour for pultruded and hand-lay-up FRP trays against hot-dip galvanized steel ladder and perforated trays of equal side-rail height (100 mm) and width (300 mm). These are industry ranges, not one supplier's data sheet — always verify against the tray manufacturer's tested load table before you commit a route to drawing.

The Answer in One Table

Property	Fiberglass (FRP/GRP) tray	Hot-dip galvanized steel tray
Density	1.8–2.0 g/cm³	7.85 g/cm³
Tensile strength (ASTM D638)	60–200 MPa depending on laminate	~400 MPa
Flexural modulus (ASTM D790)	10–20 GPa	~200 GPa
Typical support span	1.5–3 m	3–6 m
UDL at 3 m span, 100 × 300 mm ladder	40–75 kg/m	75–150 kg/m
Weight per metre, same size	2.5–4 kg/m	8–12 kg/m
Thermal expansion	20–30 × 10⁻⁶ /°C	12 × 10⁻⁶ /°C
Corrosion in chloride or acid	Not affected	Coating-dependent (ASTM A123)
Governing product standards	IEC 61537, NEMA VE 1	IEC 61537, NEMA VE 1

Key Takeaways

• FRP is roughly four times lighter per metre at equal tray size; steel offers roughly double the load at equal span.
• Deflection, not breaking strength, limits almost every FRP tray design — expect shorter support spacing and a slightly denser hanger layout.
• Steel trays carry heavier cable bundles but need a dry, coated environment or an added corrosion allowance.
• Pick FRP for chemical plants, brine, coastal air, washdown bays and underground ducts; pick steel for long spans and dry indoor runs.
• A load rating means nothing until it is tied to a span and a deflection limit. Demand both numbers.

What "Load Capacity" Actually Means for Cable Tray

Before pitting the two materials against each other, pin down the vocabulary. Most tray arguments start because two people are quoting different definitions.

• Uniformly distributed load (UDL) — kilograms per metre of cable, cleats and accessories spread across the span. This is the number in most load tables.
• Concentrated load — a maintenance worker standing on the tray, or a cable puller leaning on the side rail. Test protocols such as NEMA VE 1 include a mid-span concentrated load case for exactly this reason.
• Span — the distance between supports. Halving the span roughly doubles the load a given section can carry, which is why span and load are never quoted separately.
• Deflection limit — specifiers commonly cap mid-span sag at L/180 or L/200. This limit, not rupture, usually governs FRP.
• Safety factor — NEMA VE 1 requires published ratings to include a minimum safety factor of 1.5, so the tray will not fail at the number printed on the sheet.
• Load combination — cable weight plus ice, wind, seismic and maintenance activity, combined per the project's structural code.

Why FRP Loses on Stiffness, Not Strength

Here is the part that surprises buyers. A well-made FRP side rail can reach 200 MPa tensile strength in the fibre direction under ASTM D638, which sounds competitive with steel's 400 MPa. But stiffness is a different property, and stiffness is what controls a tray.

Glass-reinforced plastic has a flexural modulus of roughly 10–20 GPa against steel's 200 GPa. That is a ten-to-twenty-fold gap. Glass content in a pultruded profile is commonly 60–70% by weight, and the glass carries the load while the resin bonds and protects it — but the composite still sags far more than steel under the same kilos.

Two practical consequences follow. First, FRP tray designs are almost always governed by the L/180 or L/200 deflection limit rather than by material failure, so the tray looks over-built when it is actually at its limit. Second, FRP is anisotropic: the machine direction (pultruded or filament wound) is strong, the transverse direction is weaker, and the splice joints and rung connections need their own check. Steel yields and gives visible warning before collapse; FRP behaves essentially linear-elastically right up to failure, so the safety factor is doing real work rather than being decorative.

Load vs Span: The Numbers Side by Side

The table below shows typical working UDL ranges for a 100 × 300 mm ladder tray at three common support spacings. Treat the ranges as a screening tool, then confirm with tested data.

Support span	FRP/GRP ladder tray (UDL)	Hot-dip galvanized steel (UDL)
1.5 m	90–150 kg/m	180–300 kg/m
3.0 m	40–75 kg/m	75–150 kg/m
6.0 m	Not recommended for standard profiles	30–60 kg/m

The spread inside each cell comes from resin type, glass content, side-rail depth and — critically — whether the deflection criterion is L/180 or L/200. A manufacturer quoting a higher number is usually quoting a looser deflection limit. Ask which one, in writing.

Weight: The Figure That Reshapes the Structure

Weight is where FRP stops being a compromise and starts being an advantage. At 1.8–2.0 g/cm³ against steel's 7.85 g/cm³, the density gap is close to 4:1.

• A 3 m length of 300 mm FRP ladder tray weighs roughly 8–12 kg. The galvanized steel equivalent runs 25–35 kg.
• Two installers can carry and position FRP tray by hand; steel at that size usually needs a lift, a trolley or a third person.
• A 500 m route adds roughly 1.5–2 tonnes of dead load in FRP against 4–6 tonnes in steel. On a retrofit where the structural engineer has almost no spare capacity, that difference decides whether the project can proceed at all.
• Hanger rods, anchors and primary steel can all be sized down. Fewer kilos per metre means fewer kilograms per hanger.
• Seismic demand scales with mass (force = mass × acceleration), so cutting tray mass by around 70% cuts the seismic force the supports must resist in roughly the same proportion.
• Logistics improve too: you ship something in the order of three times the linear metres per truckload at equal tray size, and you do not need a crane on site for most of the route.

Corrosion, Heat and Service Environment

Corrosion is the second axis of the comparison, and here the roles reverse.

Hot-dip galvanizing to ASTM A123 typically deposits 65–85 µm of zinc on 3 mm steel — around 610–705 g/m². Under ISO 12944, that coating performs well in C3 (urban/industrial, low pollution) atmospheres and gives a service life often quoted in the 20–30 year range. Move to C4 or C5-M marine or high-chloride conditions and the same coating thins faster, so the specification jumps to duplex systems, stainless steel or a heavier section as a corrosion allowance. All three cost money.

FRP ignores galvanic corrosion entirely. Isophthalic polyester and vinyl ester laminates handle acids, brine, hypochlorite and chloride-laden air, which is why fiberglass tray shows up in chlor-alkali plants, pickling lines, fertilizer plants and wastewater halls. Two caveats matter. First, UV: outdoor runs need a UV-stabilised resin or a surface veil, or the surface will chalk and the fibres will eventually show. Second, heat: standard polyester laminates are commonly rated to about 90–100 °C, with vinyl ester and speciality resins going higher. Fire performance also needs stating — flame-spread ratings under ASTM E84 and self-extinguishing behaviour under ASTM D635 are achievable but must be specified, not assumed.

One more number worth carrying into the design review: thermal movement. Over a 30 m run with a 40 °C temperature swing, an FRP tray moves roughly 24–36 mm, while the steel tray moves about 14 mm. Long straight runs therefore need expansion provisions, and the supports should not be rigidly clamped where the tray needs to slide.

Where Each Tray Wins

Specify fiberglass when:

• The atmosphere contains chlorides, acids, alkali or solvent vapours
• The route runs through a washdown bay, a brine area or a coastal plant
• The structure is existing, weight-limited or difficult to reach with a crane
• The cables are light to medium power or control cables on modest spans
• Non-sparking, non-conductive behaviour adds safety value

Specify steel when:

• Spans exceed 3 m without intermediate supports
• Cable bundles are heavy and the UDL is high
• The environment is dry and climate-controlled
• Ambient or radiant temperature exceeds what standard resins tolerate
• High point loads are expected during installation

Sourcing: One Composite Supply Chain Behind the Tray

Composite quality is resin handling plus laminate discipline, and those skills transfer across product lines. Hebei Yongchang Composite Material Technology Co., Ltd., based in the Economic Development Zone of Jizhou District, Hengshui City, Hebei Province, runs six fully automatic microcomputer-controlled filament winding production lines across a plant of more than 50,000 m². That same manufacturing base produces cable tray and the GRP/FRP Storage Tank range, which is useful when a project wants one supplier accountable for the whole corrosion-resistant package instead of three.

The overlap matters most below ground. Where a cable trench, pump chamber or buried vault runs beside a Fiberglass Underground Storage Tank, soil moisture, sulphates and stray-current corrosion rule out coated steel for both the tank and the tray. Buying them from one composite shop means one corrosion-resistance specification, one laminate standard and one inspection report.

Vertical routes tell the same story. When a riser tray climbs the side of a GRP/FRP Vertical Storage Tank in a chemical plant, the tray needs to be light enough to bolt onto the shell's support brackets, non-sparking near flammable service, and immune to whatever the tank holds. Steel on that duty needs frequent re-coating; FRP does not.

Specification Checklist

• State the load class, span and deflection limit together — 50 kg/m at 3 m with L/180, for example.
• Ask for a tested load/span table per IEC 61537 or NEMA VE 1, not a marketing figure.
• Confirm the resin system (isophthalic polyester or vinyl ester), glass content and laminate schedule.
• Check UV stabilisation and the fire performance class (ASTM E84 flame spread).
• Match the temperature rating to the cable surface temperature, not just the ambient.
• Cross-check the chemical resistance list against real spill and vapour conditions.
• Verify that hangers, splice plates and fittings carry the same rating as the straight section.
• Allow for thermal expansion on long runs and include earthing continuity for the cable system.

FAQ

Is fiberglass cable tray as strong as steel?

No. At the same span and section size, expect roughly half the uniformly distributed load. Fiberglass compensates with about a quarter of the weight and far better corrosion resistance.

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