Working with Hollow Sections in Practice
Hollow sections are everywhere in structural steelwork. Columns, trusses, space frames, crane runways, handrail supports — you name it. They're used because they offer decent strength for their weight and look clean when exposed. The problem is most people treat them like generic steel members and get surprised by the failure modes they bring to the table. When I first started designing with CHS and RHS, I assumed everything would behave according to textbook Euler buckling. It didn't. The connection details, the local buckling behavior, the way torsion shows up where you least expect it — those are the things that will eat your schedule if you don't account for them early.
Where to Find a Hollow Section Steel Design Guide
There's no single authoritative document that covers everything. Eurocode 3 Part 1-8 handles connections, Part 1-1 handles general rules, AISC 360 has its own approach for the US, and CSA S16 covers Canada. Most of the practical knowledge lives in the notes sections and annexes of these documents, scattered across dozens of pages. What most people actually need is a consolidated reference that pulls the relevant clauses together for hollow sections specifically. The EN 1993-1-8 section on CHS and RHS connections is where most people should start if they're working to European standards, and AISC 360 Chapter K is the equivalent for US practice. Both are dense reads and neither spells out the practical compromises you end up making on real projects. Local buckling is the thing that trips people up. With a solid rectangular bar, you don't think about wall thickness. With a hollow section, the slenderness of the walls relative to the overall dimensions changes everything. A 200x200x10 RHS column behaves very differently from a 200x200x6 RHS column even though they have the same outer footprint. The thinner-walled section will local-buckle before it reaches its plastic capacity, and Eurocode's cross-section classification system catches this with classes 1 through 4. Most structural designers skip past the classification tables because they look tedious, then wonder why their member capacities don't match what the software predicts. I ran into this on a project last year. We were designing a series of RHS columns for a light industrial canopy. The spec called for 300x200x10 RHS S355, and the structural model showed ample capacity. But when we got to the connection design, the beam-to-column web needed a stiffener layout that wasn't feasible with that section geometry. The web was too thin to handle the concentrated load without local deformation. I had to step up to a 350x200x10 section just to get the web thickness to a workable level. That's one of those cases where the member might be fine in isolation but fails once you try to connect it to something. The hollow section design guide clauses for joint resistance don't always make this trade-off obvious.
Connection Design Is Where Things Get Real
Hollow section connections are harder than you'd expect. You can't just bolt or weld to a flat surface and call it done. The ends are closed, which means you can't access the inside for standard framing details. Gusset plates, internal stiffeners, reinforced chord faces — these are all part of the conversation. Planar K-joints, N-joints, X-joints in trusses each have their own capacity equations in the code, and the equations change depending on whether you're dealing with CHS or RHS, whether the branch is in tension or compression, and what the overlap ratio looks like. Here's a counter-intuitive point that most beginners miss: a larger branch member isn't always stronger in a hollow section joint. If you increase the branch width beyond a certain ratio relative to the chord width, you start losing capacity because the load path becomes less efficient. The code has parameters like beta (branch-to-chord width ratio) and gamma (chord width-to-thickness ratio) that control this behavior. Push beta too high and you're not gaining anything. The optimum range for most practical joints sits between 0.25 and 0.75 for beta, and keeping gamma below about 20 avoids excessive chord face bending. Welded connections on hollow sections also have a hidden issue. The heat input from welding a branch member to a chord can distort the tube walls, especially on thinner sections. I've seen 8mm wall CHS columns that became slightly oval after welding heavy brace connections. That ovality reduces the effective section properties and makes subsequent member checks invalid. The workaround is to use internal collar stiffeners or to specify a slightly thicker wall than your calculation requires. It's easier and cheaper to add 2mm to the wall thickness than to deal with rework on site.
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Torsion and Unusual Loading Cases
CHS sections have equal polar and planar moments of inertia, which makes them attractive for torsion-heavy applications. But that advantage disappears the moment you introduce an eccentric load or an unsymmetrical connection layout. A CHS column supporting an eccentrically loaded beam will experience combined axial, bending, and torsional stresses, and the interaction equations in the design code aren't as forgiving as pure compression checks. The reduction factors for combined loading can drop your capacity by 20 to 30 percent compared to an axially loaded member of the same size. RHS sections behave differently under torsion. They're not as efficient as CHS, but they're easier to connect to because you have flat faces. This is a genuine trade-off that comes up constantly. If your design calls for multiple connections along the length of a member, the RHS is often the better choice despite its lower torsional stiffness. CHS looks elegant in diagrams but becomes a pain the moment you need to attach something to it.
Practical Sizing and Selection
When selecting hollow sections for columns, start with the effective length and the design load, then check the slenderness ratio. Keep lambda below about 180 for compression members unless you have a specific reason not to. For truss members in tension, you can go higher, but vibration and handling during erection become considerations. A lightweight RHS purlin or girt that passes the strength check might still rattle under wind loading if it's too slender. The most efficient hollow sections for columns tend to be square or near-square. Rectangular sections with a high width-to-depth ratio develop different buckling strengths about each axis, and you end up either over-designing one direction or needing bracing in the weak axis. I usually recommend starting with SHS or square-ish RHS for primary columns unless architectural constraints force a different orientation. For beams and truss chords, rectangular sections make more sense because the strong-axis bending capacity is what matters.
Common Mistakes That Cost Time and Money
Specifying sections that aren't commercially available is probably the most expensive mistake. Not every dimension in every grade exists in stock. A 280x200x8 RHS might look perfect on paper, but if it's not a standard size in your region, you're looking at lead times, minimum order quantities, or having to redesign around what's actually in the catalogue. The common standard sizes in Europe follow the EN 10210 and EN 10219 product families, and in the US you're looking at ASTM A500 and A1085. Stick to those ranges and you'll avoid procurement headaches. Another frequent error is ignoring the manufacturing tolerances. Hollow sections come with out-of-squareness tolerances, wall thickness tolerances, and straightness tolerances. When you're designing closely fitted connections, a section that's 2 degrees out of square can make a gusset plate that doesn't fit without grinding or shimming. The code allowances cover this to some extent, but the practical reality on site is that you'll need flexibility in your connection details. Sloped bolt holes, slotted connections, and adjustable stiffeners are worth specifying upfront rather than dealing with field modifications. Corrosion protection is another area where hollow sections cause unexpected problems. Unlike open sections, hollow sections can trap moisture inside if the ends aren't properly sealed. I've seen RHS columns where condensation formed inside during storage and the interior corroded before the member was even installed. The fix is simple — plug the ends with temporary caps or weld end plates during fabrication. It adds maybe ten minutes per member but prevents internal corrosion that you can't inspect or treat later.
Software and Design Tools
Most structural analysis software handles hollow sections fine for member design. SAP2000, RSTAAD, Tekla Structural Designer — they all have hollow section databases and can run the relevant code checks. The gap is in connection design. Member capacities are straightforward. Joint capacities require specialized knowledge and often a separate check. I recommend running the member design through your main software and then doing connection checks either in a dedicated tool like IDEA StaticCa or manually using the code equations. The manual check takes longer but gives you actual understanding of what's happening, and you'll catch issues that automated checkers sometimes miss because they apply default assumptions you didn't intend. For quick hand calculations, the simplified methods in the annexes of Eurocode 3 Part 1-8 are sufficient for preliminary sizing. The exact punchout equations are more accurate but more cumbersome. A practical rule of thumb I use is to size connections to about 1.3 times the calculated demand. This accounts for the uncertainty in the joint models and the fact that connection design is rarely the optimizable part of a structure. You'll rarely get criticism for a slightly oversize connection, but you will get it for a connection that fails during fabrication or erection.
When Hollow Sections Aren't the Right Choice
Despite their popularity, hollow sections aren't always optimal. For heavy columns with very high loads, hot-finished RHS or SHS can be economical, but for extremely heavy sections the material cost per kilonewton of capacity starts to exceed that of rolled I-sections. A HEB column might be heavier and less aesthetically pleasing, but it's often cheaper and easier to connect. For long-span beams where strong-axis bending dominates, an I-section gives you more material where you need it. Hollow sections spread material equally in all directions, which is inefficient when one axis carries most of the load. Fire protection is another consideration. Hollow sections require different fire protection strategies than open sections. Intumescent coatings work on exposed surfaces, but if the section is enclosed in a box column or a concrete-filled tube, the heat dissipation characteristics change significantly. Concrete-filled RHS columns have higher fire resistance because the concrete acts as a heat sink, but this changes the buckling behavior and the connection design requirements. It's a trade-off worth evaluating early in the design process.
A Few Things the Codes Don't Emphasize Enough
Handling and transportation of long hollow sections is often overlooked. An 8-meter RHS beam might be fine in the structure but impossible to transport without intermediate supports. Sagging during transport can introduce permanent deformations that affect the member's capacity. The code doesn't address this because it's a construction issue, not a design issue, but it will bite you if you specify long members without considering how they'll get from the fabricator to the site. Breaking long members into shorter segments with splices is usually the answer, and designing those splices while the rest of the structure is still being planned saves a lot of rework later. Bolted connections to hollow sections also deserve more attention than they get. End-plate connections to RHS columns work well when the column is adequately stiffened. Without stiffeners, the column walls can deform under bolt preload and joint load, reducing the effective pretension and changing the load path. Split tee connections and base plates with stiffeners are the go-to solutions, but they add complexity and cost. If you're designing a bolted frame with hollow section columns, budget extra time for connection detailing. It's not a trivial portion of the project. The bottom line is that hollow section design requires more attention to detail than open section design. The member properties look simple on paper, but the connections, the fabrication details, and the construction sequencing all carry additional complexity. A good Hollow Section Steel Design Guide approach means understanding not just the equations but the practical constraints that the equations don't capture. That practical understanding is what separates a design that works on paper from one that works on site.
