Technical guide
Angle is cheaper per tonne and easier to bolt; tube is stronger in torsion and cleaner to look at. The decision usually comes down to one question: is the member loaded in torsion or bending about two axes?
| Property | Square tube (SHS) | Equal angle |
|---|---|---|
| Torsion | Closed section — high torsional stiffness | Open section — very low torsional stiffness |
| Bending, both axes | Equal in both directions | Strong about one axis only |
| Weight for same stiffness | Lighter per unit of bending stiffness | Heavier for the same job in biaxial bending |
| Connections | Needs plates, gussets or flattened ends | Bolts directly, one leg against one face |
| Surface and finish | Smooth, easy to paint and clean | Inside corner traps dirt and water |
| Price per tonne | Higher | Lower |
Rule of thumb from our order book: if the member is a column or carries a twisting load, use tube; if it is bracing or a one-way purlin, angle is cheaper and perfectly adequate.
Tube needs end plates, gussets or crushing (flattening) to bolt to. That is fabrication time — typically 15–30 minutes per joint including welding and drilling. Angle bolts straight through both legs in five minutes.
So the honest comparison is not price per tonne:
total cost = steel price + fabrication + coating + erection timeFor a 20-joint frame, the connection cost difference can exceed the steel price difference.
In torsion, dramatically. In pure one-axis bending, angle is often more efficient per kilo because more material sits far from the neutral axis.
Yes, with a gusset or a flattened tube end. Direct tube-to-angle welds concentrate stress at the corner and are best avoided on fatigue-loaded joints.
Tube — provided vent and drain holes are added. Angle has no enclosed volume, so it is simpler still, but it holds water in the inside corner if not detailed.