RHS vs SHS for Steel Gate Frames

TL;DR

• RHS = rectangular cross-section (e.g. 65x35). Use long-side-vertical for stiles where bending matters in one direction.
• SHS = square cross-section (e.g. 100x100). Use for posts where loads come from any direction.
• Standard grade: AS/NZS 1163 C350 (350 MPa yield). C450 if bending strength is the critical limit.
• Standard wall thickness: 1.6 mm pedestrian, 2.0 mm typical swing, 2.5 mm sliding/tall, 3.0 mm heavy/long-span.
• Section properties matter. Depth in the bending direction multiplies the section’s capacity faster than width. A 65x35 long-side-vertical beats a 50x50 SHS for sag resistance at the same kg/m.
• Most welds are 4 to 6 mm fillets. Reach for full-penetration butt only on splices and high-fatigue joints.

RHS vs SHS at a glance

Section properties primer

Two numbers describe how a steel section resists bending. The second moment of area (Ix) is geometric stiffness, the resistance to rotation. The section modulus (Zx) is geometric strength, related to the stress in the outer fibre at a given bending moment. Both scale with depth (the dimension in the direction of the load) faster than they scale with width.

For a hollow rectangular tube, doubling the depth roughly cubes the Ix. Doubling the wall thickness roughly doubles it. So when you’re looking at a stile that’s sagging, going from 50x25 to 65x35 (40% deeper) increases Ix by about 2.5x. Going from 1.6 mm wall to 2.0 mm wall (25% thicker) only buys you about 25% more Ix. Depth is the cheap fix.

Here’s a worked comparison. Take a 65x35x2.0 RHS used long-side-vertical (so 65 mm is the depth in bending) versus a 50x50x2.0 SHS. The RHS has Ix around 18 cm^4 in the strong direction; the SHS has Ix around 11 cm^4. Same wall thickness, similar weight per metre, almost double the bending stiffness. That’s why the 65x35 RHS in the right orientation beats the symmetrical 50x50 SHS for a tall stile.

The catch: the 65x35 RHS used long-side-horizontal (laid flat) has Ix around 6 cm^4 in the bending direction. Same section, half the stiffness, because depth becomes width. Orientation matters. Specify it on the drawing.

Common sizes for gate work

The masses below are for AS/NZS 1163 mild steel. Multiply by 1.06 for galvanised (the zinc coating adds a few percent). Section properties are approximate and vary slightly by mill.

The strong/weak breakdown for RHS shows the orientation effect. A 50x25 RHS in the strong axis carries 8.4 cm^4 of Ix; the same section turned 90 degrees gives you only 2.7. Always orient the deeper dimension in the direction of the load.

Worked example: bending moment on a 3 m single swing gate

A 3 m wide x 1.8 m tall single-swing battened gate hangs from two hinges on the left post. The gate’s weight tries to rotate it about the lower hinge, with the leading edge wanting to drop. We need a hinge stile that resists that rotation without yielding or visibly bending over time.

Estimate the gate weight first. A battened gate with 50x25 RHS frame, two cross rails, and 65x16 aluminium battens runs about 25 kg per square metre. The gate area is 3 x 1.8 = 5.4 m^2. So gate weight is around 135 kg, or 1325 N.

The centre of mass sits at half the gate width, 1.5 m from the hinge stile. The moment about the hinge stile is force x lever arm: 1325 N x 1.5 m = 1990 Nm, or about 2 kNm.

Now check whether a 65x35x2.0 RHS hinge stile in the strong orientation holds it. The section’s Zx in strong axis is 5.6 cm^3 = 5600 mm^3. Bending stress is moment / Zx: 2 x 10^6 Nmm / 5600 mm^3 = 357 MPa. That’s above C350’s yield point.

Step up to 75x50x2.5 RHS. Zx in strong axis is 10 cm^3 = 10000 mm^3. Bending stress: 2 x 10^6 / 10000 = 200 MPa. That’s a safe ratio, well below yield, with margin for wind load and dynamic effects.

Conclusion: a 3 m single swing gate wants 75x50 RHS hinge stiles, not 65x35. The latch-stile, top-rail, and bottom-rail can stay at the lighter section because they’re not carrying the gate moment. This is also why hinge-stile reinforcement (an internal gusset behind each hinge plate) is worth the workshop time on heavy gates.

Stiffener and gusset use

Sometimes upsizing the whole stile is overkill. A 50x25 stile is plenty for the rest of the gate, but the load concentrates at the hinge plate. The fix is a local stiffener.

Cut a 6 mm steel gusset to fit inside the stile, behind each hinge mount. Weld the gusset to the inside face of the stile with a 4 mm fillet on its perimeter. The gusset transfers the hinge load over a wider area of the stile face and stops the hinge plate from rocking the local steel.

Cost-wise, a 100 mm x 30 mm x 6 mm internal gusset is about 0.15 kg of steel and 5 minutes of welding. The whole-stile upgrade from 65x35 to 75x50 is about 1.5 kg of extra steel per metre and zero saved welding. For a 1.8 m stile, that’s 2.7 kg vs 0.15 kg. Use the gusset when the rest of the stile is fine and only the hinge zone needs help.

Same logic for the latch stile, the drop-bolt boss, and the gate-stop weld. Local reinforcement is usually cheaper than upsizing the whole member.

When to choose RHS vs SHS

Choose RHS when

• One direction of bending dominates. Vertical sag on a tall gate stile, vertical droop on a long sliding-gate top rail.
• You want depth without weight. 65x35 RHS gives 65 mm of bending depth for the kg-per-metre of a 50x50 SHS.
• Aesthetics call for slim gate frames viewed face-on. A 50x25 RHS stile reads thinner than a 50x50 SHS.
• The member is a long-span beam (cantilever rail, top rail of a 6 m sliding gate).

Choose SHS when

• The member is a post. Equal stiffness in any direction matters because wind load and gate-weight bending act in different directions.
• The member is short and unlikely to bend significantly. Jambs, mullions, joiner blocks.
• You’re matching legacy material. Older gates almost universally used SHS for the simple aesthetic of equal sides.
• The member sees impact load (gate-stop bumpers, bollards). Symmetric bending strength handles asymmetric impact directions.

Recommended frame sizes by gate width

Starting points, not engineering certifications. Wider gates with heavy slatted infill, marine exposure, or cantilever support will need upsizing.

Welding considerations per profile

The frame profile drives which welds you can use and how much prep they need.

RHS-to-RHS mitre at 45 degrees is the standard gate-frame corner. Cut both members at 45 degrees on a cold saw. Tack the frame square. Lay a continuous fillet weld around the full perimeter of the joint, both on the outside and the inside (where reachable). Standard fillet leg is 4 to 6 mm. MIG with a 0.9 mm wire is the workhorse process; 6010 stick electrodes work for site repairs.

SHS post-to-rail joint on a balustrade or fence is usually a butt weld with a fillet around the perimeter. The post arrives at the rail at 90 degrees (or matching the stair rake), bevel the edges if the post is over 4 mm wall thickness, and run a continuous weld around all four sides of the rail-to-post intersection.

Heavier sections (3.0 mm wall and above) need pre-heat in cold weather. Below about 5 degrees C ambient, the rapid heat loss into the surrounding cool steel can cause hydrogen cracking in the weld. Pre-heating the parent metal to 50 to 100 degrees C with an oxy torch or a propane burner before welding eliminates the risk.

HAZ effectsmatter on lighter sections. The heat-affected zone (HAZ) on a 1.6 mm wall RHS is the full wall thickness at the weld. Don’t place high-load fixings (bolt holes, secondary attachments) in the HAZ. See AS/NZS 1554.1 for the welder qualification and weld procedure requirements.

Coating considerations

Hot-dip galvanising is the default outdoor finish for gate work. The process puts the welded gate into a bath of molten zinc at about 450 degrees C, then withdraws it and lets it cool. The coating bonds to the steel in the bath; the rapid cooling sets it.

On lighter sections (1.6 mm wall and below), the thermal stress of the dip and cool can warp long thin members. A 4 m sliding-gate stile in 50x25x1.6 RHS can come out of the bath with a 5 to 10 mm bow along its length. Two ways to mitigate. Spec 2.0 mm wall or thicker for any member over 3 m long. Or, if 1.6 mm is what the customer’s budget allows, talk to the galvaniser before you ship the gate so they put it through the longer end of the dip path and use the slower cool.

Sealed cavities are a worse problem. Any unwelded gap or unvented cavity inside the gate is going to fill with molten zinc, then explode as steam if there’s any moisture or trapped air. Always vent hollow sections. The standard is one 8 mm hole per metre of cavity, drilled in an inconspicuous location.

For powder coat over galvanise (the duplex finish), the galvanise has to be cleaned and primed before the powder cures. The standard is a sweep blast plus an etch primer, followed by the powder coat. Costs add up: galvanise alone runs $5 to $8 per kg. Powder coat alone runs $3 to $5 per square metre. The duplex system (gal + powder) is roughly the sum, but it lasts 2 to 3 times longer than either alone in coastal exposure.

Material costs and stock lengths

Standard Australian stock from major merchants (Onesteel, BlueScope, Coil Steels) runs in 6 m and 8 m lengths. Some merchants stock 12 m on heavier sections. Prices vary with steel commodity cycles; the relative ratios stay consistent.

Indicative wholesale prices (mid-2026) per kg of mild-steel RHS / SHS C350: $1.80 to $2.20 for plain black, $2.40 to $2.80 for pre-galvanised. Hot-dip galvanise after fabrication adds another $1.50 to $2.50 per kg of galvanised weight (the gate after galvanising weighs about 6% more than before).

Offcut and waste is a real cost on multi-leaf jobs. A 6 m stick used to make a 1.8 m stile gives you three stiles plus a 600 mm offcut. For a 2.0 m stile, you only get two per stick, and the offcut is bigger. Plan stock orders against the cutting list to minimise waste. Most fab shops aim for 10 to 15% waste; below that takes careful nesting.

Lead time runs 1 to 5 days from a major Australian merchant for stock items. Galvanising adds another 5 to 10 days through the queue at most galvanisers. Powder coat is 3 to 5 days after galvanising. Total job time from cut-to-coat is typically 3 to 4 weeks, longer in summer when galvanising plants are busiest.

Common section-selection mistakes

• Picking section for aesthetics first, structure second.A 50x50 SHS hinge stile on a 3 m wide gate looks symmetrical to the post but doesn’t carry the bending moment. Spec 65x35 RHS oriented for bending and accept the visual asymmetry.
• Forgetting that sliding gates need stiffer bottom rails than the same-width swing gate. Sliding-gate bottom rails carry rolling load plus dead weight; swing-gate bottom rails carry just dead weight. A 4 m sliding gate wants a 100x50 bottom rail; a 4 m swing gate gets by with 65x35.
• Specifying 1.6 mm wall on cantilever rails. The cantilever rail is the structural backbone of the gate. It needs 2.5 mm wall minimum, often 3.0 mm on commercial leaves. 1.6 mm flexes under top-roller load.
• Mixing C350 and C450 grades on the same frame, then welding without a procedure. Different yield strengths mean different weld-procedure parameters. Either spec C350 throughout or get a specific procedure from a welding engineer for the mixed-grade joint.
• Orienting RHS the wrong way.A 65x35 RHS used long-side-horizontal has 6 cm^4 of Ix where you need 18 cm^4. Always orient the deeper dimension in the load direction. Mark it on the drawing if there’s any chance of confusion in the workshop.
• Forgetting vent holes for galvanising. Sealed cavities trap moisture, then explode in the zinc bath. Drill 8 mm vent holes at one end of every hollow internal cavity.

Frequently asked questions

References & related

• AS/NZS 1163:2016 Cold-formed structural steel hollow sections
• AS/NZS 1554.1 Welding of steel structures
• AS/NZS 4680 Hot-dip galvanised (zinc) coatings
• AS 3715 Metal finishing thermoset powder coatings
• AS/NZS 1170.2 Wind actions
• Glossary: RHS
• Glossary: SHS
• Glossary: CHS
• Glossary: Galvanised steel
• Glossary: Hot-dip galvanising
• Glossary: Powder coat
• Glossary: MIG welding
• Glossary: Fillet weld
• Glossary: Mitre joint
• Glossary: Heat-affected zone
• Glossary: Concreted post
• Guide: Which type of gate to choose
• Guide: Standard pedestrian gate sizes Australia
• Guide: How to measure up sliding gates on raked ground