How Far Can a Steel Bar Grating Span: Dual-Criteria Strength and Deflection Check for G325/30/100 (2026)

"Can this grating panel work over a 1.5 m span?" It is the single most common question in an enquiry, and the one most likely to be answered off the cuff — "should be fine", "we've always done it that way". But a welded steel bar grating is a precisely computable component: the bearing bars are plain rectangles, the pitch is fixed, the grade is Q235 or stainless, and only two equations stand between span, load and deflection. This article works through the most widely used specification, G325/30/100 (bearing bar 32 mm deep x 5 mm thick, 30 mm bar centres, 100 mm cross-bar centres), and establishes both governing limits in one pass, with a span-versus-load table you can apply directly.

The result up front: beyond a span of 0.736 m the governing criterion switches from strength to deflection at L/200, and under a 4.0 kN/m2 maintenance-platform live load the maximum span is 1.76 m — not the 1.2 m or 2.0 m that usually gets quoted from memory. And what actually gets gratings into trouble is rarely the distributed load; it is a point load, where the same 2 kN can produce stresses five times apart depending on how many bearing bars you assume share it.

1. Establish the section properties per metre of width

A grating panel is not one beam; it is dozens of parallel bars tied together by cross bars. Rather than checking a single bar, engineers convert the section into properties per metre of panel width, so that a surface load in kN/m2 can be substituted directly.

With bearing bars at 30 mm centres, the number of bars per metre of width is:

For one 32 x 5 mm bearing bar:

Converted to a per-metre-of-width basis:

Material values: Q235 bending design strength f = 215 MPa (thickness 16 mm or less), elastic modulus E = 206 000 MPa. For 304 stainless the yield is 205 MPa with a design value around 137-180 MPa and a slightly lower E (193-200 GPa); the coefficients in this article must then be adjusted accordingly.

2. First governing line: strength, q = 48.92 / L²

A panel resting on two support beams behaves as a simply supported beam under a uniformly distributed load. The maximum mid-span moment is:

The strength condition is M ≤ W·f. The moment the section can carry:

Solving for the allowable distributed load (L in metres):

This line decays with the square of the span: doubling the span cuts the allowable load to a quarter.

3. Second governing line: deflection, q = 36 / L³

A grating that feels "spongy" underfoot is almost never short of strength; it has exceeded its deflection limit. For a simply supported beam under uniform load:

The convention widely applied to bar grating is a deflection limit of L/200, capped at 10 mm (a formal design must follow the project specification and the standard it invokes). Substituting I = 455 111 mm⁴/m and E = 206 000 MPa, and setting δ = L/200:

This line decays with the cube of the span — far steeper than the strength line. Double the span and only one eighth of the load remains admissible. That is why long-span grating is almost always governed by deflection.

4. The crossover: L = 0.736 m

Equating the two lines gives the span at which the governing criterion changes:

Working through common spans and taking the lesser of the two values:

How to read it: at short spans such as 0.6 m, strength governs; from 0.8 m upwards, deflection decides everything. That has a direct purchasing consequence — upgrading to a higher steel grade such as Q345 barely helps, because Q235 and Q345 share the same elastic modulus and deflection is unchanged. To improve deflection you must increase I (deeper bearing bars) or shorten the span.

5. Worked example: how far can a 4.0 kN/m² maintenance platform span

Self-weight first. For G325/30/100 the bearing bars contribute 33.33 bars x 160 mm² x 1000 mm = 5.33×10⁶ mm³ of steel per square metre, which at 7850 kg/m³ gives 41.9 kg/m²; the twisted square cross bars (6 x 6 at 100 mm) add roughly 2.8 kg/m². Total about 44.7 kg/m², i.e. 0.44 kN/m² (edge banding excluded).

Take a maintenance-platform live load of 4.0 kN/m² (2.0 kN/m² where no equipment is present; always follow the design documents). Applying partial factors of 1.3 on permanent and 1.5 on variable actions:

Inverting the deflection equation for the maximum span:

In other words: under a 4.0 kN/m² maintenance live load G325/30/100 can span 1.76 m — at which point deflection is exactly 8.8 mm = L/200, while the strength side still allows 15.8 kN/m² and has ample reserve. If the project demands a 2.0 m span, the allowable load falls to 4.5 kN/m², below the 6.57 kN/m² design value, and the support beams must be spaced more closely.

The same calculation for three common bar sizes at q_d = 6.57 kN/m²:

6. Go deeper, not thicker: efficiency of bar sizes

The table hides a genuinely useful selection rule. Since I = b·h³/12, stiffness depends on bar depth h to the third power but only linearly on thickness b:

• 32x3 → 32x5: cross-sectional area rises from 96 mm² to 160 mm², +67%; I rises from 8 192 to 13 653 mm⁴, also +67%. You get exactly the stiffness you pay for, one for one.
• 32x5 → 40x5: area rises from 160 mm² to 200 mm², +25%; I rises from 13 653 to 26 667 mm⁴, +95%. A quarter more steel buys nearly double the stiffness.

The reason is simply that I scales with h³: 40/32 = 1.25, and 1.25³ = 1.95. When budget is tight and you need a longer span, increase bar depth before increasing bar thickness. Tightening the bar pitch from 30 mm to 20 mm also raises the total I, but it impairs zinc drainage during galvanizing, adds weight and traps debris, so it is rarely the first choice.

7. Point loads: one force, five different answers

With distributed load settled, the remaining exposure is point load — an equipment foot, a trolley wheel, a drum of sealant. This is what generates most of the "one panel sank under my foot" complaints.

The difficulty is that a point load does not spread across the full metre of width. It passes through the cross bars into a limited number of bearing bars. The number of participating bars, n, depends on cross-bar stiffness, contact patch width and bar pitch; practice commonly assumes 3 to 5 bars (3 where conservative, 5 where a wide contact patch or a base plate exists).

For a mid-span point load P (kN) shared by n bars at L = 1.0 m:

Substituting W1 = 853.3 mm³, I1 = 13 653 mm⁴, E = 206 000 MPa, L = 1000 mm gives two remarkably handy expressions:

Tabulated (L = 1.0 m, deflection limit 5.00 mm, Q235 design strength 215 MPa):

Three findings worth keeping:

• A 2 kN load on G325/30/100 at 1.0 m span deflects 4.94 mm — essentially on the L/200 = 5.00 mm limit. That is why maintenance platforms feel soft: nothing has failed, there is simply no reserve. Reduce the span to 0.8 m and deflection falls immediately to 2.53 mm.
• As n goes from 1 to 5, the stress from the same 3 kN falls from 879 MPa to 176 MPa — a factor of five. So "can the grating carry this unit" depends entirely on whether the equipment foot sits on a spreader plate, and how wide it is. A 200 x 200 x 10 mm steel base plate usually beats upgrading the grating specification, and costs far less.
• Under point load, strength and deflection arrive together (at 3 kN with n = 3, σ is 1.36 times 215 MPa and δ is 1.48 times 5.00 mm), unlike the distributed case where deflection governs long before strength.

8. Do not fail outside the calculation: galvanizing and grade

A correct calculation is not the same as a durable installation. Two further checks matter before delivery.

Hot-dip galvanizing thickness

A 5 mm bearing bar falls in the 3-6 mm thickness class, where the average coating should be at least 70 μm with a local minimum of 55 μm. This is the class most often under-coated: the same zinc consumption looks brighter on thinner steel. Inspect with a magnetic gauge on the side face of the bar, not the top — the top face runs thicker because of zinc run-off and is not representative.

Bar pitch and zinc drainage

30 mm bar centres balance load capacity against processability. Tightening to 20 mm restricts zinc flow between bars, encouraging pooling and dross that then needs manual cleaning, while adding weight and trapping debris. When more capacity is needed, deepen the bar rather than tighten the pitch — the same conclusion as section 6.

Material for coastal and wastewater environments

Using PREN = Cr + 3.3·Mo + 16·N, grade 304 comes out around 19.6 and 316 around 25.5. In coastal exteriors, wastewater treatment plants and pool plant rooms, grades with a PREN below 24 carry a markedly higher pitting risk; specify 316/316L stainless grating, or accept a defined maintenance regime on galvanized carbon steel. Where stainless grating meets carbon steel support beams, insulate the interface with gaskets or rubber strip to prevent galvanic corrosion.

9. Five common mistakes

1. Selecting the span from a strength table and ignoring deflection → consequence: G325 at 1.5 m appears safe at 21.7 kN/m² when deflection is already exhausted at 10.7 kN/m², and a 4.0 kN/m² live load feels obviously soft → fix: for any span over 0.736 m, check deflection with q = 36/L³.
2. Trying to solve deflection with Q345 → consequence: strength rises about 40% while deflection does not move at all, since E is unchanged → fix: deflection responds only to I and L — deepen the bars or add supports.
3. Spreading a point load across the full metre width → consequence: a 3 kN equipment foot computes to a comfortable σ = 59 MPa, when the real figure over three bars is 293 MPa, 36% over the limit → fix: check point loads separately with n = 3 (or 5 with a base plate) and prefer a spreader plate.
4. Installing the panel with the bars running the wrong way → consequence: bearing bars must run perpendicular to the supports; reversed, the twisted cross bars become the primary load path and capacity almost vanishes → fix: mark the bar direction on the drawing and verify every panel on site; cross bars always run parallel to the supports.
5. Forgetting self-weight and partial factors → consequence: comparing 4.0 kN/m² directly against the allowable value ignores 0.44 kN/m² of self-weight and the 1.3 / 1.5 factors, so the real design value is 6.57 kN/m², not 4.0 → fix: combine into a design value before reading the table.

10. Conclusions and selection guide

In one line: G325/30/100 is governed by q = 48.92/L² for strength and q = 36/L³ for deflection, crossing over at L = 0.736 m; the maximum span for a 4.0 kN/m² maintenance platform is 1.76 m; and the real risk is point load, where σ = 293·P/n and δ = 7.41·P/n mean the same force can give answers five times apart depending on how many bars share it.

Frequently Asked Questions

What uniformly distributed load can G325/30/100 carry over a 1.0 m span?

Take the lesser of the two governing values: strength allows 48.9 kN/m² while deflection at L/200 allows 36.0 kN/m², so the governing figure is 36.0 kN/m², controlled by deflection. That is roughly 3.7 tonnes per square metre as an equivalent uniform load, but a design must apply the 1.3 permanent / 1.5 variable combination before using it.

Why does the grating still feel soft after upgrading to Q345 steel?

Because deflection depends only on the elastic modulus E and the second moment of area I, not on strength. Q235 and Q345 both have E = 206 000 MPa, so the deflection is identical after the upgrade. There are only three ways to reduce deflection: increase I (deepen the bearing bars, since I scales with the cube of depth), shorten the span, or add intermediate supports.

How do I decide whether an equipment foot on a grating needs a spreader plate?

Estimate with σ = 293×P/n and δ = 7.41×P/n (P in kN, L = 1.0 m). A 3 kN foot shared by three bars gives σ = 293 MPa against 215 MPa and δ = 7.41 mm against 5.00 mm, so action is required; a plate spreading the load over five bars gives σ = 176 MPa and δ = 4.44 mm, which passes. A spreader plate is cheaper and more effective than upgrading the grating specification.