3 Common Types of Metal Steel Tubes Pipes

Round (CHS), square (SHS) and rectangular (RHS) steel tubes are the three structural hollow sections behind most fabricated steelwork, and they are not interchangeable. A square tube is not a round tube pressed flat: the forming sequence and the way stress spreads through the wall are completely different, so the section you choose decides how a member carries load. Where a run is bolted together at a flanged joint, the sealing itself is done by flange gaskets, not by the tube profile.

The choice comes down to three questions: how the cross-section looks, how it behaves under load and where it belongs in the structure. The comparison below answers all three and ends with the standards worth stating on a purchase order.

Shape and Stress Distribution

All three are cold-formed hollow profiles, but geometry sets very different mechanical behavior.

• Round section (CHS) — the traditional tube shape. It is isotropic: section modulus and moment of inertia are the same in every load direction, and the wall has no sharp corners.
• Square section (SHS) — four equal sides with defined corners, either radius-rounded or right-angled. The side length is the single governing dimension.
• Rectangular section (RHS) — unequal height and width, ordered as height x width (for example 200 x 100 mm), so the two axes are designed separately.

CHS round section: section modulus and moment of inertia are the same in every load direction.

SHS square section: four sides of equal length with defined corners.

RHS rectangular section: with the long side vertical, the bending modulus on the strong axis is very high.

Where Each Section Works Best

Load path decides the application. Round sections take internal pressure and torsion, square sections make efficient columns, and rectangular sections make efficient beams. The same logic reaches the ends of a pipe run, where the section is welded or bolted to pipe flanges and the sealing surface is machined on the flange, not on the tube.

• Fluid and pressure duty: water lines, natural gas pipelines and hydraulic oil lines are almost always round. Internal pressure acts in every direction and the circular wall spreads that stress most evenly; square sections cannot match this pressure resistance.
• Rotation and transmission: conveyor rollers, barbells and shafts. With no preferred direction, the round section gives better dynamic balance in rotation.
• Long-span structures: pipe trusses in stadiums and airport terminals. Joints take more work than with square sections, but the smaller windward area helps resist wind load and torsional deformation.
• Columns and posts: square sections in steel-structure buildings. Equal sides give the same slenderness ratio on the X and Y axes, so a freestanding column does not stay stiff in one direction while buckling in the other.
• Light structures and frames: guardrails, flagpoles, landscape sculptures, simple carports, machine frames and photovoltaic supports. Flat faces take sealant and fixing screws easily, and square members do not roll around during assembly.
• Beams in tight spaces: rectangular sections inside floor slabs or suspended ceilings and in pipe-rack frames, where height provides rigidity and width provides a mounting surface.

Note: section dimensions, corner radius and yield strength are governed by the ordered standard (GB/T 6728, EN 10219, ASTM A500).

Ordering, Forming and Inspection

Round tubes normally run on a continuous forming line, while square and rectangular sections are formed from them by squeeze sizing and reshaping. Cold forming work-hardens the corners and thins the wall there, which is exactly where failures start. Five details decide whether a section performs as designed.

• State section dimensions, wall thickness and the governing standard — GB/T 6728, EN 10219 or ASTM A500. Corner radius and yield strength requirements differ significantly between them.
• For heavy loads or low-temperature service on square sections, add non-destructive testing of the corners, the critical weak point of the profile.
• For burial or high internal pressure, choose seamless steel tube or high-frequency straight-seam welded pipe and avoid square sections.
• For rectangular beams, specify the load direction: on the strong axis it is a main beam, laid flat it is only a bracing member, and the capacity differs several times.
• Connections: welding and bolting square or rectangular members on site is faster than with round tubes, which need complex profile cuts at intersections.

Frequently Asked Questions

Is a square tube just a round tube pressed flat?

No. Square and rectangular sections are formed from a round tube by squeeze sizing and reshaping, not by pressing it flat. Rolling sequence, corner work hardening and wall thickness distribution make them different products.

Which section resists internal pressure best?

The round section. Internal pressure acts equally in all directions and the circular wall spreads that stress most evenly, so square sections cannot match this pressure resistance.

Why does a rectangular beam deflect less than a round tube?

Placed with the long side vertical, the section reaches a very high bending modulus on the strong axis. With the same 10 kg of steel, a rectangular beam bends far less than a round tube.

Does the orientation of a rectangular section matter?

Yes. On the strong axis it works as a main beam; laid flat it only braces, and load capacity differs several times. Always state the direction of load when ordering.

Which standards cover structural hollow sections?

GB/T 6728 (China), EN 10219 (Europe) and ASTM A500 (United States) are commonly specified. Corner radius and yield strength requirements differ significantly, so give the standard together with size and wall thickness.