ASTM Standard of Structural Tubing

A hollow structural section (HSS) is a type of metal profile with a hollow tubular cross section. In some countries they are referred to instead as a structural hollow section (SHS).

A tube, or tubing, is a long hollow cylinder used to convey fluids (liquids or gases).

Most HSS are of circular or rectangular section, although other shapes are available, such as elliptical.
HSS is only composed of structural steel per code.
HSS is sometimes mistakenly referenced as hollow structural steel. Rectangular HSS are also called tube steel or structural tubing. Circular HSS are sometimes mistakenly called steel pipe though true steel pipe is actually dimensioned and classed differently than HSS. The corners of HSS are heavily rounded, or chamfered, at radii approximately twice the wall thickness. The wall thickness is uniform around the section.
In the UK, the terms are circular and rectangular hollow section (CHS and RHS). However, the dimensions and tolerances differ slightly from HSS.

Abbr.	Corresponding	Application
A500	A500-99	Specification for Cold-Formed Welded and Seamless Carbon Steel Structural Tubing in Rounds and Shapes ASTM A500 covers carbon steel, cold formed welded and seamless structural tube
A501	A501-99	Specification for Hot-Formed Welded and Seamless Carbon Steel Structural Tubing
A847	A847-99a	Specification for Cold-Formed Welded and Seamless High Strength, Low Alloy Structural Tubing with Improved Atmospheric Corrosion Resistance
A618	A618-99	Specification for Hot-Formed Welded and Seamless High-Strength Low-Alloy Structural Tubing