Table of Contents
Square Hollow Sections (SHS), Rectangular Hollow Sections (RHS), and Circular Hollow Sections (CHS) are widely used in structural frames, columns, bracing, and supporting members. Although all three provide the advantages of closed steel sections, their bending behavior, connection requirements, and fabrication needs differ.
The selection should begin with the member’s structural role, the direction of applied loads, and how it connects to the surrounding structure. Wall thickness, material requirements, and surface protection must then be matched to the required structural performance and service environment.
SHS for Balanced Structural Members and Connections
Square Hollow Sections have equal external width and height, giving them the same bending stiffness about their two principal axes.
This makes SHS suitable for columns, structural frames, and supports where loading may act in more than one direction.
For example, a column supporting beams from two perpendicular directions may benefit from a square profile because its bending properties remain consistent about both axes.
SHS also provides flat external faces for attaching connection plates and brackets. This can simplify fabrication where several members meet at one structural joint.
However, the square shape alone does not determine the required load capacity. Column length, end restraint, material grade, and wall thickness still influence the risk of buckling and local wall deformation.
For projects using conventional welded or bolted connections, available hollow section steel dimensions can be reviewed alongside the connection arrangement before the section size is finalized.
RHS for Members With a Dominant Bending Direction
Rectangular Hollow Sections become particularly useful when bending resistance is required primarily about one axis.
The greater overall depth can provide higher bending stiffness and section modulus about the strong axis when the member is oriented correctly.
This makes RHS a practical candidate for horizontal beams, canopy supports, façade framing, and other structural members where the primary bending direction is known.
For example, a horizontal member supporting downward loads may benefit from an RHS installed with its greater dimension vertical. This increases its effective depth in the principal bending direction.
The trade-off is reduced stiffness about the weaker axis compared with the stronger one.
If substantial loads also act sideways, the design must account for bending about both axes and any required lateral restraint.
RHS also offers broad, flat surfaces for attaching plates and secondary framing. Connection design must still account for local wall resistance, particularly where concentrated loads enter through a relatively thin section wall.
CHS for Axial Loading and Tubular Structures
Circular Hollow Sections have the same bending properties about every axis through the center of the section.
Their circular geometry also provides efficient torsional resistance, making CHS attractive for bracing, tubular trusses, exposed structural frameworks, and members subjected to loads from several directions.
In a tubular truss, for example, CHS can provide an efficient combination of axial resistance and torsional stiffness while maintaining a consistent external profile.
The main consideration is connection fabrication.
Unlike square or rectangular sections, CHS has no flat surface for directly attaching a conventional plate or branch member. Tube-to-tube connections may require profiled cuts, curved weld interfaces, or specially designed connection components.
These requirements affect fabrication time and connection detailing.
CHS should also be specified using the applicable structural product standard rather than assuming that a pipe with similar outside dimensions has equivalent material properties, tolerances, or structural performance.
How Should Wall Thickness Be Determined?
After the section shape is selected, wall thickness becomes a critical part of structural design.
Increasing wall thickness generally increases the steel area and can improve resistance to axial forces, bending, and local buckling. However, the required thickness depends on the member dimensions, material strength, and loading conditions.
A large hollow section with relatively thin walls may experience local buckling before the full strength of its cross-section can be developed.
This is particularly important for compression members and structural elements subjected to substantial bending.
Connections introduce additional requirements. A thin section wall may limit the resistance of welded joints or concentrated loads transferred through connection plates.
For this reason, wall thickness should be checked against both member resistance and connection design.
The governing material standard must also be identified.
ASTM A500 is commonly used for cold-formed structural tubing, while EN 10210 covers hot-finished structural hollow sections and EN 10219 covers cold-formed welded structural hollow sections.
These standards have different requirements for manufacturing, dimensions, tolerances, and material properties. The wall thickness used in structural calculations must follow the applicable design rules rather than relying solely on the nominal thickness stated in a product description.
Once the required thickness is established, the section weight, cutting requirements, and fabrication details can be confirmed.
How Does the Service Environment Affect Surface Protection?
Surface protection should be selected according to the environment in which the structural member will operate.
For dry indoor structures, an appropriate paint system may provide sufficient corrosion protection, depending on the exposure conditions and maintenance requirements.
Outdoor frames, canopies, and structures exposed to persistent moisture often require more durable protection.
Hot-dip galvanizing is a common option because the zinc coating provides barrier and sacrificial protection to the underlying steel.
For more demanding environments, a specified paint system or a combination of galvanizing and compatible coating layers may be considered.
Hollow sections also require attention to their internal surfaces.
Where a completed tubular assembly is hot-dip galvanized, properly positioned vent and drain openings are necessary to allow cleaning solutions and molten zinc to enter and leave enclosed spaces safely.
The fabrication sequence is therefore important. Welding, cutting, and connection preparation should be coordinated with the selected protection process.
If fabrication takes place after coating, affected areas may require approved coating repair.
The specification should define the coating system, applicable standard, required thickness or performance, and inspection requirements. This establishes a clear protection requirement for both the supplied sections and their finished connections.
Matching Hollow Sections to Structural Project Requirements
SHS is a practical choice for members requiring balanced bending properties and straightforward flat-face connections. RHS offers advantages where bending is concentrated about one principal axis, while CHS is well suited to applications benefiting from circular geometry and efficient torsional resistance.
The selected profile must then be checked against wall thickness, material grade, connection details, and the required corrosion protection.
For structural projects requiring square or rectangular hollow sections, Baolai Steel offers products under specifications including ASTM A500, EN 10210, and EN 10219, with options for customized dimensions, cutting, fabrication, and surface treatments such as painting and hot-dip galvanizing.
Confirming the section type, dimensions, material requirements, and finishing process against the approved structural drawings provides a consistent basis for manufacturing and supplying the required hollow sections.