The performance difference between rectangular tube and square tube in engineering applications needs to be comprehensively analyzed from multiple mechanical perspectives such as strength, stiffness, stability, and bearing capacity.
Bending strength:
Rectangular tube: When subjected to bending load along the long side direction (height direction), the section inertia moment is larger, and the bending resistance is significantly better than that of square tube.
For example, the bending strength of 100×50mm rectangular tube in the long side direction is higher than that of 75×75mm square tube.
Square tube: The inertia moment is the same in all directions, and the bending performance is symmetrical, but its value is usually smaller than that of the long side direction of the rectangular tube under the same cross-sectional area.
Conclusion: If the load direction is clear (such as beam structure), the rectangular tube is better; if the load direction is variable, the square tube is more balanced.
Torsion strength:
The torsion constant of the square tube is higher, the torsion stress distribution is more uniform, and the torsion resistance is better than that of the rectangular tube. For example, the torsion resistance of the 75×75mm square tube is significantly stronger than that of the 100×50mm rectangular tube.
Conclusion: When the torsional load is dominant (such as the transmission shaft), square tubes are better.
Bending stiffness:
Stiffness is proportional to the moment of inertia. Rectangular tubes have higher stiffness in the long side direction, which is suitable for scenarios that need to resist unidirectional deflection (such as bridge beams).
Square tubes have symmetrical bidirectional stiffness and are suitable for multidirectional loads (such as columns).
Conclusion: Stiffness requirements depend on the load direction. Choose rectangular tubes for unidirectional loads; choose square tubes for bidirectional loads.
Local buckling:
Rectangular tubes usually have a larger width-to-thickness ratio, and thin-walled parts are more prone to local buckling, especially under compression or shear loads.
Square tubes have better local stability due to their symmetrical cross-section.
Overall buckling (Euler buckling):
Buckling load is related to the minimum radius of gyration of the cross-section. The radius of gyration of square tubes is the same in all directions, while the radius of gyration of rectangular tubes in the short side direction is smaller, making them more prone to buckling.
Conclusion: Square tubes are preferred for compressive members (such as pillars); if the long side direction of the rectangular tube is constrained, it can be compensated by design.
Axial compression:
Bearing capacity is related to cross-sectional area and slenderness ratio. Under the same cross-sectional area, square tubes have a higher bearing capacity due to their larger turning radius.
Combined load (combined compression and bending):
Rectangular tubes can take advantage of the optimized layout when the bending moment direction is clear (such as vertical load on the long side); square tubes are suitable for bidirectional bending moments.
Material utilization:
Rectangular tubes are more efficient and save materials when subjected to unidirectional bending; square tubes are more economical under multi-directional loads.
Connection convenience:
Due to the symmetry of square tubes, node connections (such as welding and bolts) are simpler; rectangular tubes need to consider directionality.
Application scenarios:
Rectangular tubes: building beams, crane arms, vehicle chassis (clear load direction).
Square tubes: building columns, space trusses, mechanical frames (multi-directional loads).