RHS Portal Frame Span Limits: How Far a Hollow Section Frame Can Reach

A rectangular hollow section portal frame can reach further than most buyers expect — and the limit is rarely the steel’s strength. What stops a hollow section frame at a given span is usually one of five other things: how much deflection the client’s specification allows, the depth and wall thickness that can be economically fabricated, the capacity of the bolted connections, the length that can be transported and lifted, and the point at which a hollow section stops being the cheapest way to carry the load.

So the honest answer to “how far can an RHS portal frame span?” is that the span is decided by the design criteria, not by a catalogue number. A frame designed to a strict deflection limit will be deeper and heavier than one designed only for strength, at the same span. If you are comparing two quotations for the same building and one of them offers a noticeably lighter frame, the first question is not which is cheaper — it is which deflection limit each design was checked against.

This guide sets out what governs the span of a hollow section portal frame, how depth, wall thickness and section shape interact, where RHS is the right choice and where a universal beam or welded plate girder takes over, and which transport and erection limits usually bite before the structure itself does.

What “span limit” actually means in a portal frame

In a single-span portal frame, the span is the clear distance between column centres, and the frame carries the roof load by bending in the rafters and columns. As span increases, bending moments rise quickly, and the frame has to respond somewhere: a deeper section, a thicker wall, a different section family, or an additional column that turns one long span into two shorter ones.

Five separate limits can each become the governing one, and they arrive at different spans. Strength is almost never the first to fail. Serviceability — deflection and movement under load — usually governs the rafter, because a roof that sags visibly and lets water pond is a problem even when the steel is safe. Local buckling of a slender hollow section wall, connection capacity at the eaves and apex, and the practical limit of member length all tend to arrive earlier than buyers assume.

That table is why two engineers can look at the same building and produce frames of different weight without either being wrong. If the specification says nothing about deflection, the designer will use a default, and defaults vary between offices and between countries. Writing the deflection criterion into the enquiry — or asking the supplier to state which one they used — removes the largest hidden variable in the comparison.

Frequently asked: is there a maximum span for a hollow section portal frame?

There is no single number. Practical spans depend on the deflection limit, the roof load, the section depth that can be fabricated and transported, and the cost comparison against other framing. Long spans with hollow sections are common in industrial buildings where clear floor space matters, and the point where a hollow section stops being economical is a design and cost decision for the specific project rather than a fixed ceiling. Ask your engineer or supplier to show the governing check for your own building.

Frequently asked: why is deflection usually the deciding check?

Because deflection limits are written in absolute terms — often as a fraction of the span — while the load carried by the frame is fixed by the building use. As the span grows, the allowed movement grows more slowly than the bending moment, so the rafter has to become stiffer than the strength calculation alone would require. That is why long-span frames often look heavier than the load figures suggest.

Span, depth and wall thickness: how the three trade off

Hollow sections behave differently from open sections, and the difference matters most at long span. A rectangular hollow section has stiffness in both axes, a closed profile that resists torsion well, and a clean appearance. It also has walls that can buckle locally if they are thin relative to the section width, and it is harder to inspect and coat on the inside.

The practical consequence is that depth, not wall thickness, is normally the efficient way to buy span. Doubling the wall thickness of an existing section adds weight everywhere and increases the demand on the connections; increasing the depth raises stiffness with proportionally less material, as long as the wall stays thick enough to prevent local buckling and the connections can be detailed to suit.

The last row deserves a note, because it is the answer buyers usually resist. Adding a column is often the cheapest way to solve a long span, and it is sometimes the only way to solve it economically. If the building’s function truly needs clear floor space — racking layouts, aircraft or large vehicle movement, long production lines — the frame has to pay for it in depth, weight and connections, and that cost should show up in the quotation rather than appear as a value-engineering suggestion later.

High-strength grades are frequently misunderstood at this point. A higher grade raises the stress the steel can carry, which helps strength-governed members, but it barely changes the elastic modulus, so it does nothing for deflection. Offering a higher grade to solve a deflection-governed long-span frame is one of the most common value-engineering mistakes in this market.

When RHS wins, and when it does not

Hollow sections are not automatically the best long-span member. They are excellent in specific situations, and a comparison with the two alternatives — hot rolled open sections and welded plate sections — makes the trade-offs visible.

The decision usually turns on three questions. Is the frame visible to the public or to the building’s users? Does the building need stiffness in both directions from a single member? And is the fabrication base able to weld and drill hollow sections reliably, including the flush end plate connections?

Where the frame is exposed — a showroom, an entrance canopy, a sports hall — hollow sections often justify their extra detailing because the profile looks deliberate rather than industrial. Where the frame will be clad on all sides and the building is a multi-span warehouse grid, open sections are frequently the more economical answer. The right question for the buyer is not “which section type is better” but “which one does the design of my building need”, and that is answered by the span, the load and the deflection limit.

Frequently asked: are hollow sections more expensive than universal beams?

Usually more expensive per tonne for the section itself, and often more expensive to fabricate, because the connections require flush end plates, careful drilling and welding on a closed profile. The comparison only becomes meaningful when the alternative would need a heavier member or additional restraint to achieve the same stiffness and torsional behaviour. Ask for both options priced if the frame will be visible.

What long spans change beyond the rafter

When the span grows, the rafter is the member everybody looks at, but it is rarely the only part of the building that changes. Handling the span properly means checking everything the longer frame leans on.

The column base line is where design mistakes become expensive. A long-span portal frame with a pinned or nominally pinned base still generates horizontal thrust and uplift on the foundations, and those reactions depend on the analysis assumptions. If the frame design changes after the foundations are poured — new section sizes, different base fixity — the anchor bolts may no longer suit. That is why the anchor bolt schedule and the frame design should be finalised together, and why buyers should insist on receiving the anchor bolt layout before the foundation drawings are issued.

Connections: where a hollow section frame earns or loses its cost

A bolted connection between hollow sections cannot use the flat face of an open section, so the design usually relies on flush end plates welded to the tube ends, with bolts arranged around the plate perimeter. This is a well-established detail, and it is also the part of a hollow section frame where the workshop’s competence shows most clearly.

Three details decide whether the connection is straightforward or problematic. First, whether the end plate is welded with full access to all four sides — tubes require welding around a closed perimeter, and poor access invites defects. Second, whether the bolt group fits within the plate without clashing with the weld or the tube wall. Third, whether the tube ends are sealed, because an unsealed hollow section becomes an internal corrosion problem that nobody discovers until the coating fails.

When you review a supplier’s technical proposal for a hollow section frame, ask for the connection detail drawings, not just the member list. A proposal that shows flush end plates, bolt sizes, plate thicknesses and stiffener arrangement is a proposal that has actually been detailed. A proposal that only lists sections and weights has not yet answered the question of how the frame will be joined on site — and connections are where cost and programme risk concentrate.

Frequently asked: can hollow section frames be bolted on site, or do they need site welding?

They can and normally are bolted, using flush end plates welded in the factory and high-strength bolts tightened on site. Factory welding of the end plate keeps the critical weld under workshop conditions. Site welding is sometimes unavoidable for special details, but it shifts quality control onto the site team and should be identified in the quotation rather than discovered by the erector.

Protection, corrosion and fire: the hollow section specifics

Hollow sections present two protection questions that open sections do not. The inside of the tube is inaccessible for inspection and repainting, so the specification has to decide whether the section is sealed, internally treated, or accepted as a closed void with the ends capped. And the outside profile, while easy to paint, has a small surface area relative to its stiffness — which is an advantage for coating cost, since coating is priced by area.

Galvanizing long hollow sections is worth checking early, because a member that does not fit the dipping bath has to be spliced or coated differently — both of which change the design. The same applies to any member whose length exceeds transport limits: if the rafter has to be split into two pieces, the splice becomes a designed connection with its own bolts, plates and fabrication hours, and it should appear in the bill of materials rather than in a site instruction.

Transport and erection limits that arrive before the structural limit

For export projects, the frame’s real limit is frequently the container or the crane, not the steel. Member length is capped by container internal dimensions and by road transport regulations at the destination, and lifting weight is capped by the crane available on site. Neither limit appears in a structural calculation, and both are decided long before fabrication begins.

These constraints interact with the design in a way that is easy to underestimate. A splice position chosen for transport reasons becomes a moment connection with a defined capacity, and its location affects the bending diagram. That is why the transport plan and the structural design should be produced together, and why a supplier who asks about container type and destination access before pricing is doing useful work rather than delaying the quotation.

The site facilities line in that table is the one buyers most often forget to plan. Erection crews need an office, a store for fasteners and sealants, and somewhere secure to keep drawings and documents. On projects where accommodation is short, modular site units are usually ordered separately from the steel package, and the earlier they are planned, the better they align with the erection programme.

Toolkit and consumables belong on the same checklist. If the frame arrives with high-strength bolts but no bolt tensioning equipment, or with cladding but no sealant, the site stops while somebody sources materials locally. The bill of materials and the shipping documents should let a site manager confirm, before dispatch, that everything needed to close the building is on board.

Frequently asked: how do transport limits change the steel weight of a long-span frame?

They can increase it. A splice added for transport introduces a connection that must be designed and fabricated, and in some cases the splice detail is heavier than a continuous member would have been. The weight increase is usually modest, but the cost effect is larger because connections consume fabrication hours. Where possible, transport planning and section design should be optimised together rather than sequentially.

How to brief a supplier so the span can actually be checked

The most common reason a quotation comes back with a heavy or cautious frame is an incomplete brief. A supplier who has to guess the deflection limit, the roof load or the intended use will protect themselves by over-designing, and the buyer pays for that caution in tonnage.

Send those seven inputs and the design question becomes answerable. Withhold them and the supplier is pricing a range, which is why some quotations arrive with a note that the tonnage is an estimate subject to final design. That note is honest, but it also means the number cannot be compared with a competitor’s quotation that was priced against a complete brief.

Useful background reading before you brief a supplier: how RHS portal frame design choices affect sections and connections, what drives the price of a steel portal frame, how the quotation process runs from enquiry to contract, and how factory quote inclusions change the cost per square metre. For the wider decision set, the steel structure knowledge index groups the technical guides by topic.

Common mistakes with hollow section portal frames

• Comparing prices without comparing deflection limits. Two frames at the same span can differ in weight simply because one was checked to a stricter movement criterion.
• Assuming a higher steel grade will solve a stiffness problem. Grade raises strength, not stiffness, and deflection-governed members do not benefit.
• Agreeing the foundation before the frame. Anchor bolts cast to the wrong layout are an expensive problem to correct on site.
• Ignoring internal protection of tubes. Unsealed hollow sections corrode from the inside where nobody can inspect or repair them.
• Discovering transport limits after fabrication. Splices designed at the last minute cost more than splices designed at the quotation stage.
• Forgetting the erection package. Bolts, sealant, touch-up paint, lifting equipment and site facilities all have to arrive before they are needed.

None of these mistakes requires advanced engineering to avoid. They require the buyer to treat the span as a design decision with commercial consequences, rather than as a dimension on a drawing.

Frequently asked questions

How do I know whether my long-span frame should be RHS or an open section?

Ask the designer which member the building’s stiffness, torsional restraint and appearance requirements point to, and ask both options to be priced if the frame will be visible. The decision is usually driven by whether the frame is exposed, whether one member must be stiff in both directions, and how much connection fabrication the project can absorb.

Does a longer span always mean a heavier steel frame?

Steel weight per square metre generally rises with span, because the rafters and columns have to become deeper and heavier to control bending and deflection. The rate of increase depends on the deflection limit, the roof loading and whether the layout allows an internal column. A two-span arrangement usually uses less steel than the same total width in a single clear span.

Can a hollow section frame be galvanized?

Yes, provided the sections are fabricated with suitable vent and drain holes and the members fit the galvanizing bath. Members that are too long for the bath need to be spliced or protected by another system, which is a design decision that should be made at quotation stage, not at the coating shop.

What does the supplier need from me to design the frame properly?

Clear span and bay spacing, eave height and roof pitch, imposed roof load with the governing design code, the deflection limit, column base fixity, the opening schedule, and the destination and transport mode. With those inputs the frame can be designed once rather than estimated and revised.

Should I ask for the structural calculation report with the quotation?

It is reasonable to ask whether the calculation report can be issued with the quotation or with the shop drawings, and to ask which checks governed the members. A supplier who can explain the governing check has designed the frame; a supplier who can only quote a section size may still be working from a precedent.

Final recommendation

Treat the span of a hollow section portal frame as the outcome of five checks, not a fixed capability. Strength usually is not the governing one; deflection, connection capacity, local buckling and transport length decide the frame long before the steel runs out of capacity. Once those are understood, the conversation with a supplier becomes technical and specific — and the quotation comes back with a frame that was designed for the building rather than a section size chosen by habit.

If you are planning a clear span building, send the span, bay spacing, eave height, roof pitch, load requirements and destination. We will come back with the frame arrangement, section schedule, connection details and the transport plan behind it, so the quotation can be compared properly against any alternative.