Rectangular Hollow Section, SHS and CHS: An Engineer's Guide

A rectangular hollow section (RHS) is a closed steel tube with a rectangular cross-section, one of a family of hollow structural sections that also includes square (SHS) and circular (CHS) profiles. Hollow sections are used for columns, trusses, bracing, canopies, frames and exposed architectural steelwork, and they behave quite differently from open sections such as I-beams. Understanding those differences helps in choosing the right section and in detailing it well.

This article explains the types and designations of hollow sections, the difference between hot-finished and cold-formed products, their structural strengths and weaknesses, how they are connected, and how to deal with corrosion and fire.

Types of hollow structural section

Circular hollow sections (CHS). Round tubes, efficient in compression and torsion with identical properties in every direction. They also have low wind drag, which suits towers, masts and exposed trusses.
Square hollow sections (SHS). Equal properties about both principal axes, with flat faces that are easier to connect to than a round tube. A common choice for columns.
Rectangular hollow sections (RHS). Deeper in one direction than the other, giving more stiffness about the major axis for beams and members under bending, while keeping the torsional and lateral stability of a closed section.
Elliptical hollow sections. A less common architectural option combining some of the visual qualities of a CHS with a preferred bending direction.

Internationally the family is often called HSS (hollow structural sections), particularly in North America, while British and European practice uses CHS, SHS and RHS.

Designations and dimensions

Hollow sections are described by their outside dimensions and wall thickness, in millimetres. An RHS is designated by depth × width × thickness, so a 200 × 100 × 10 RHS is 200 mm deep, 100 mm wide with a 10 mm wall. An SHS is designated side × side × thickness, and a CHS by outside diameter × thickness, for example 168.3 × 6.3 CHS, where 168.3 mm is one of the standard pipe diameters.

Because the outside dimensions are fixed for a given size and wall thickness varies, a family of sections with the same outside size gives the designer a range of capacities without changing the connection geometry. That is useful in trusses and frames where members meet at the same node.

Hot-finished versus cold-formed sections

Hollow sections are made in two ways, under two different European standards.

Hot-finished sections, to EN 10210, are formed or finished at high temperature, around normalising temperature. The process gives uniform material properties around the section and tight corner radii.
Cold-formed sections, to EN 10219, are formed from strip at ambient temperature and welded. Cold forming work-hardens the corners, and corner radii are larger.

The two are not interchangeable without checking. EN 10210 itself notes that, although cold-formed grades can have equivalent mechanical properties, the sectional properties of square and rectangular hollow sections in the two standards are not equivalent, because of the different corner geometry. A substitution on site from hot-finished to cold-formed, or the reverse, should therefore be checked by the engineer. Cold-formed corners can also be more sensitive when welded, particularly for thicker walls, and fabrication guidance places limits on welding in the corner regions.

Steel grades for hollow sections carry an H suffix, for example S355J2H, indicating a hollow section grade with defined impact properties. Our article on steel grades in construction explains the naming system.

Structural advantages of a rectangular hollow section

A closed section distributes material around its perimeter, which gives several advantages over open sections such as an I-beam:

Torsional stiffness. Closed sections are many times stiffer in torsion than open sections of similar weight, so they suit members that twist, such as edge beams, curved members and cantilevered canopies.
Lateral stability. Because of that torsional stiffness, a rectangular hollow section in bending is much less prone to lateral-torsional buckling than a narrow I-section, so it can often span without intermediate restraint.
Compression efficiency. SHS and CHS have the same radius of gyration about both axes, so a column buckles at the same load in either direction, making efficient use of material.
Surface area. Hollow sections have less exposed surface area than equivalent open sections, reducing the quantity of paint or fire protection needed.
Appearance. Clean lines and an absence of ledges make them popular for exposed steelwork, glazing supports and furniture-scale structures.

For simple floor beams carrying large bending moments, however, a universal beam usually remains more economical, because an open I-section puts more material into the flanges where it does most for bending.

Connections and joint design

Connections are where hollow sections need most care. Because the inside is inaccessible, a conventional bolt cannot be tightened from the inside face. Common solutions include:

welded joints between hollow sections in trusses, with branch members welded directly to the chord;
end plates or cap plates welded on in the shop, then bolted on site;
fin plates or gusset plates welded to the section or slotted through it;
proprietary blind bolts that can be installed and tightened from one side.

Joints between hollow sections behave differently from those between open sections. The face of the chord can deform locally, punching shear can occur, and side walls can buckle under concentrated loads. Eurocode 3 includes specific design rules for hollow section joints that consider these failure modes and the geometry of the joint, such as the ratio of branch width to chord width. As a result, choosing a slightly thicker chord wall at design stage can save a lot of stiffening at joints later. Our article on bolted versus welded connections discusses the broader choice.

Corrosion, fire and concrete filling

The inside of a hollow section cannot be painted or inspected once fabricated. A section that is fully sealed by welding, with no moisture inside, does not corrode internally to any significant degree, so the usual approach is to seal ends with welded cap plates. Where sections are hot-dip galvanised, vent and drain holes are required so that molten zinc can flow in and out and to avoid dangerous pressure build-up in the bath; these holes must then be considered in the design and, if necessary, sealed afterwards. In Singapore’s humid climate, unsealed hollow sections that collect rain or condensation are a common durability problem.

For fire, hollow sections can be protected with intumescent coatings or boards in the same way as open sections. They can also be filled with concrete, sometimes with reinforcement. A concrete-filled hollow column has greater load capacity and better fire resistance than the steel alone, and the steel tube acts as permanent formwork. This composite approach is widely used for columns in high-rise buildings.

For new frames, canopies or retrofits using hollow sections, early engineering input on connections and durability avoids expensive changes at fabrication stage. A structural engineering consultancy can size the members and design the joints.

Frequently asked questions

What is the difference between RHS, SHS and CHS?

All three are hollow structural sections. RHS has a rectangular profile, deeper in one direction; SHS is square, with equal properties about both axes; CHS is circular, with identical properties in all directions. The choice depends on the loading, connections and appearance required.

How do you read a rectangular hollow section designation?

An RHS is described by its outside depth, outside width and wall thickness in millimetres, in that order. For example, 200 × 100 × 10 RHS means 200 mm deep, 100 mm wide, with a 10 mm wall.

Are hot-finished and cold-formed hollow sections interchangeable?

Not automatically. They can have similar steel strengths, but corner radii differ, so the section properties of square and rectangular sections are not the same. Substitutions should be checked by the engineer, including any implications for welding.

Do hollow sections rust inside?

A fully sealed hollow section with no water inside does not corrode internally to a significant extent, because oxygen and moisture are limited. Problems arise when ends are left open or holes allow water in. Seal ends, and treat vent holes from galvanising appropriately.

Why are hollow sections used for columns?

SHS and CHS have the same stiffness about both axes, so they resist buckling equally in all directions, making efficient use of steel. They also have a clean appearance and can be filled with concrete to increase capacity and fire resistance.

Designing a canopy, frame or steel retrofit with hollow sections? Talk to our consultancy or get in touch.