Two sheds can look identical on a quote sheet but differ significantly in structural integrity. The frame type is why.
Understanding the difference between Rectangular Hollow Section (RHS), Universal Beam (UB), and C-section framing helps you choose a shed that performs for your intended use, not just one that fits your initial budget.
Key Takeaways
- C-section frames are cold-rolled from thin flat sheet steel and suited to light-duty, smaller-span applications where long-term durability is not the priority.
- RHS structural steel is hot-rolled with thicker wall sections, factory-welded before delivery, and handles wider spans without intermediate posts.
- Two sheds quoted at a similar price can differ substantially in frame weight, weld quality, and effective lifespan.
- Victoria’s permit and engineering requirements vary by frame type, span, and building class, which affects both project cost and approval timeline.
What Is C-Section Steel?
C-section gets its name from the shape of each steel section when viewed end-on. It resembles the letter C. The steel is cold-rolled from flat sheet, typically 1mm to 3mm thick, and frames are bolted together on site rather than welded in a factory.
This makes C-section more affordable upfront. For a domestic garage, a small storage shed, or a light-duty structure on a sheltered site, C-section can be a practical choice. The trade-off is a shorter effective lifespan and less capacity to brace against high wind loads or add heavy door gear and mezzanines later.
What Is RHS and UB Structural Steel?
RHS is a closed, rectangular tube of hot-rolled steel. Wall thickness typically ranges from 2mm to 6mm. Because the section is fully enclosed, it resists twisting and racking forces that open C-sections cannot handle as effectively.
UB (Universal Beam) is the heaviest-duty option. It suits industrial buildings, warehouses, and spans of 50 metres or more. Trusteel Fabrications builds with RHS as its standard structural frame. Frames are welded and prefabricated in-house before delivery, which produces greater rigidity than site-bolted assembly.
How They Compare in Practice
The practical differences appear clearly across three dimensions.
- Span: C-section frames are typically suited to spans under 12 metres. RHS portal frames handle wider clear spans without internal columns, which matters for machinery sheds and equestrian arenas where vehicle or animal movement is the priority.
- Load capacity: A 15-metre-wide farm shed in a high-wind zone places very different demands on a frame than a 6-metre domestic garage. RHS frames carry greater loads and brace more effectively against wind classifications relevant across Victoria.
- Durability: Agricultural environments expose frames to fertilisers, livestock effluent, and temperature cycling. RHS structural steel performs better in these conditions over a 30-year lifespan than cold-rolled C-section of equivalent span.
RHS vs UB vs C-Section — Comparison Table
Feature | C-Section | RHS | UB (Universal Beam) |
Profile shape | Open C-shape | Closed rectangular tube | I-shaped with flanges |
Manufacturing method | Cold-rolled from flat sheet | Hot-rolled, fully welded | Hot-rolled structural steel |
Wall thickness | 1–3mm | 2–6mm | Varies by section size |
Assembly method | Bolted on site | Factory-welded before delivery | Factory-fabricated |
Clear span capability | Suited to spans under 12m | 12–30m and beyond | 30m+ for industrial applications |
Load capacity | Light to moderate | Moderate to heavy | Heavy to very heavy |
Resistance to racking | Lower — open profile | Higher — closed section resists twist | Highest — mass and geometry |
Corrosion performance | More vulnerable on exposed sites | Better — closed section resists moisture ingress | Better — hot-rolled with protective coatings |
Typical applications | Domestic garages, small storage | Farm sheds, machinery sheds, equine, commercial | Large warehouses, industrial buildings |
Cost relative to C-section | Baseline | Higher | Highest |
Trusteel standard | No | Yes | For heavy-span commercial builds |
Note: Wall thickness and span capability figures are indicative ranges for common shed applications. Specific engineering requirements vary by site, wind region, and building class. Trusteel designs to the exact specification required for each project.
When C-Section Is the Right Choice
C-section frames are adequate for domestic garages, small storage sheds, and low-load applications on sheltered sites. The upfront cost saving is real.
The trade-off is reduced flexibility. Adding a span, installing heavy roller doors, or fitting a mezzanine later becomes more difficult with C-section frames. If the shed’s use is likely to evolve, that matters.
When RHS or UB Structural Steel Is the Right Choice
RHS structural steel is the right choice for wider clear-span builds, commercial and industrial sheds, machinery sheds, and equine structures that face daily use. It is also the appropriate frame for any steel shed in a high-wind classification zone or on a rural property where chemical exposure is ongoing.
Factory-welded RHS frames arrive on site as prefabricated units. This affects both structural integrity and construction reliability. What we consistently see on rural properties is that site-bolted frames assembled in variable weather conditions introduce tolerances that factory fabrication does not.
What This Means for Building Permits in Victoria
Victoria’s National Construction Code building classifications affect which frame type requires engineering certification. Larger sheds, commercial buildings, and any Class 7 or Class 8 structure are subject to Victorian Building Authority (VBA) oversight regardless of frame type.
A C-section frame on a larger span may require additional engineer sign-off that adds cost and time to the project. RHS frames engineered by a registered VBA builder arrive with that documentation already managed. Most Victorian councils also require a building permit for any shed exceeding 10 square metres in a residential zone, though requirements vary by council and zone. Check with your local council for the specific threshold that applies to your property.
If you want to know which frame suits your shed’s span, use, and site classification, talk to Trusteel about your shed frame.
Frequently Asked Questions
What is the difference between RHS and C-section shed frames?
RHS (Rectangular Hollow Section) is a closed, hot-rolled steel tube that is factory-welded into a frame before delivery. C-section is cold-rolled from flat sheet and bolted together on site. RHS carries greater loads, handles wider spans, and has a longer effective lifespan than C-section at equivalent dimensions. For commercial, agricultural, or equine builds, RHS is the structurally appropriate choice.
Are Z purlins stronger than C purlins for shed construction?
Z purlins and C purlins are both cold-formed steel sections used to support roof and wall cladding. Z purlins overlap at joins, which provides slightly more continuity across the span. For most shed applications, the difference in purlins matters less than the strength of the primary structural frame beneath them.
Do C-section shed frames require council approval in Victoria?
Council approval requirements in Victoria are determined by shed size, zone, and building class, not frame type alone. Most Victorian councils require a building permit for any shed over 10 square metres in a residential zone. Check with your local council, as exemption thresholds and overlay requirements vary across different zones and planning schemes.
What span widths can RHS portal frames handle compared to C-section?
C-section frames are typically suited to spans under 12 metres without intermediate posts. RHS portal frames routinely handle clear spans of 15 to 30 metres and beyond in commercial and industrial applications. UB (Universal Beam) frames extend this further, making them suitable for large warehouses and industrial sheds with spans exceeding 50 metres.
What are the long-term maintenance differences between structural steel and C-section sheds?
RHS structural steel frames generally require less maintenance over a 30-year lifespan because the closed section resists corrosion and fatigue more effectively. C-section frames on agricultural or equine properties may show degradation earlier when exposed to fertilisers, effluent, or coastal conditions. The upfront cost difference between frame types is often offset by reduced maintenance and replacement costs over the life of the building.