Difference between Square Tube and Rectangular Tube

Basic Overview of Square and Rectangular Tubes:

Square tube is a kind of shaped steel tube that is slotted and bent on a steel plate and then welded into it. It is usually called square tube or square hollow structural steel tube. The square tube has a square cross-section and has the advantages of clear corners, precise dimensions, smooth surface, and corrosion resistance. It is widely used in construction, machinery manufacturing, furniture manufacturing, billboard production and other fields.

Rectangular tube is a shaped steel tube similar to square tube, but its cross-sectional shape is rectangular. Rectangular tubes are cut into shape from steel plates and then made through welding or seamless manufacturing processes. Rectangular tubes have the characteristics of light weight, high strength, no deformation, and corrosion resistance. They are one of the important materials in construction, bridges, machinery manufacturing, solar brackets and other industries.

Differences between Square Tubes and Rectangular Tubes:

1. Shape and Mechanical Properties
Square tubes are tubing with four equal sides, a square cross-section, and right angles. Rectangular tubes, on the other hand, have a rectangular cross-section with unequal lengths for their long and short sides, and also right angles.

Square tubes have the same moment of inertia along any central axis, resulting in equal bending and torsional resistance in all directions. Rectangular tubes, with their rectangular cross-section, have a much greater moment of inertia along their strong axis (long side) than their weak axis (short side), giving them a distinct directional bending resistance. This is the most important difference in structural design, directly determining their applicability in different scenarios.

2. Size Differences
Both square and rectangular tubes can be customized to meet specific needs, with side lengths ranging from a few millimeters to over a meter. Common product lines cover a full range from small decorative applications to large structural uses. Commonly used square tube sizes range from 20*20mm to 400*400mm, while rectangular tube sizes range from 20*30mm to 200*400mm. Both square and rectangular tubes are typically made of steel, with carbon steel and stainless steel being the most common.

In the field of architecture and structure, the American standard ASTM A500 (carbon steel structural tubes) and the the European standard EN 10219 (cold-formed welded hollow structural sections) are the most frequently cited general standards. Additionally, there is ASTM A513, used for mechanical structures, etc.

3. Differences in Applications
The selection of square and rectangular tubes is primarily based on their different cross-sectional mechanical properties:

Square tubes: Due to their symmetrical structure, they exhibit better balance when bearing forces from all directions. Therefore, they are often used in scenarios requiring uniform load bearing, such as supporting columns and frames in building structures.
Typical applications: load-bearing columns, space trusses, display racks, guardrails, and mechanical frames requiring a symmetrical appearance.

Rectangular tubes: Due to the difference between their long and short sides, they are more suitable for applications requiring unidirectional bending loads, such as bridge engineering. Rectangular tubes can better adapt to the stress distribution of bridges, improving structural stability.
Typical applications: Beams in buildings, roof purlins, guide rails for solar panel supports, and components serving as main load-bearing beams in equipment.

4. Differences in Manufacturing Processes
In modern industry, the vast majority of structural square and rectangular tubes use the same cold-bending and welding forming process (such as high-frequency resistance welding). The strip steel is passed through a series of forming rolls, gradually bent into a circle, then the seam is closed and welded, and finally rolled into the final square or rectangular shape by sizing rolls.

A small number of seamless square and rectangular tubes are produced through hot extrusion or piercing for special needs. Therefore, the shape itself does not determine the manufacturing process; the process is mainly determined by the manufacturer's equipment, product application, and cost.

Advantages and Disadvantages of Square Tubes and Rectangular Tubes:

1) Square Tube

Advantages of Square Tubes:
a. Isotropic and Highly Stable: The square cross-section exhibits identical characteristics along both principal axes, resulting in uniform stress distribution. This makes it extremely stable as a compression column, with no weak axis.
b. Neat and Aesthetically Pleasing Appearance: The symmetrical geometry enhances the aesthetics of modern architecture and product design, making it suitable for exposed structures.
c. Relatively Convenient Connections: All four sides are flat, facilitating welding, bolting, or the use of standard connectors.
d. Superior Torsional Resistance Compared to Rectangular Tubes: While not as robust as round tubes, it is more symmetrical than square tubes, resulting in stronger torsional resistance.

Disadvantages of Square Tubes:
a. Lower Bending Efficiency as Beams: For the same weight or cross-sectional area, its bending section modulus is lower than that of rectangular tubes. In other words, it is less efficient than rectangular tubes when used as beams.
b. Internal Dead Corners: Unsuitable for fluid transport due to high flow resistance and a tendency to sediment.
c. Potentially Lower Space Utilization: In some frames requiring a flattened design, it may not be as space-efficient as rectangular tubes.

2) Rectangular Tube

Advantages of Rectangular Tubes:
a. Extremely high bending efficiency: When used as beams, rectangular tubes, with their long sides upright, achieve a section modulus far exceeding that of square tubes of the same weight, resulting in the most economical material utilization.
b. High space utilization: The flat shape allows for close fit to walls or equipment, saving space and enabling flexible layout.
c. Optimal connection convenience: Possessing two wide planes, it is easier to weld, drill, and install.
d. Good economy: It is the most mainstream profile in structural applications, with large-scale production and high cost-effectiveness.

Disadvantages of Rectangular Tubes:
a. Anisotropy: The moments of inertia differ along the two principal axes, resulting in a strong axis and a weak axis. When used as columns, stability is poor if the force is not applied along the strong axis.

b. Weak torsional resistance: Under torque, stress concentration is significant at the corners, making its torsional resistance the weakest among the three basic profiles (round, square, and rectangular). 

c. Corners are stress concentration points: Under repeated loading, corners are prone to becoming the initiation points of fatigue cracks.

Conclusion:
Square and rectangular tubes differ in shape, mechanical properties, dimensions, applications, and manufacturing processes. The choice of steel tube shape depends on the specific application scenario and requirements. Generally, square tubes are suitable for load-bearing columns, multi-directional load-bearing components, and precision machining and manufacturing; while rectangular tubes are suitable for structural arrangements of bending beam components (building beams, curtain wall joists, equipment frames, vehicle chassis).

Square and rectangular tubes are complementary tools in the hands of structural engineers. In complex steel structure systems, square tubes often serve as stable "columns," while rectangular tubes often serve as efficient "beams." Working together, they constitute a safe, economical, and efficient structural system.

Go here to learn more about "Square Tube Sizes" or "Square tube vs. Round tube"