ASTM A500 Carbon Steel Structural Square Tubing: Grades, Properties and Selection
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What ASTM A500 Covers
ASTM A500 is the United States specification for cold-formed welded and seamless carbon steel structural tubing in rounds and shapes. The shape range includes square, rectangular and round hollow sections, which the construction industry refers to collectively as hollow structural sections, or HSS. The specification is the reference document for tubing used in welded, riveted or bolted structures such as building frames, trusses, columns, sign gantries and bridge elements. ASME SA500 is the ASME Code equivalent and carries the same grade structure. Square tubing is popular because it offers equal stiffness about both axes, which suits members under multi-axial loading, and because its flat faces are easy to connect and visually acceptable when left exposed.
Grades A to D and Strength Requirements
A500 defines four grades, each with its own minimum yield and tensile strength. Table 1 lists the minimum values in metric and imperial units.
| Grade | Yield strength, min | Tensile strength, min |
|---|---|---|
| A | 230 MPa (33 ksi) | 310 MPa (45 ksi) |
| B | 290 MPa (42 ksi) | 400 MPa (58 ksi) |
| C | 317 MPa (46 ksi) | 427 MPa (62 ksi) |
| D | 250 MPa (36 ksi) | 427 MPa (62 ksi) |
Grade B is the grade most often ordered for general structural work, while grade C is chosen where the designer needs higher yield and grade D where a high tensile strength with a lower yield is acceptable. The specification deliberately leaves the chemical analysis open rather than specifying a full composition; phosphorus and sulfur are each limited to 0.035 % maximum so that the steel remains weldable and tough in the formed condition.
Cold-Formed Welded and Seamless Product
A500 tubing is cold-formed, which means the strip is shaped and welded below the recrystallisation temperature of the steel. Electric resistance welding is the usual process for the longitudinal seam, and the inside weld flash can be left in place or removed depending on the order. Seamless tubing is produced by piercing and rolling and is specified where the application imposes pressure or fatigue duty. Cold forming raises yield strength through work hardening, and the corners of a square section are strengthened most, which is why the corner radii are controlled by the specification and checked on delivery. Wall thickness and outside dimension tolerances apply to the finished tube and should be quoted explicitly for structural design, because a cold-formed section is commonly ordered by outside dimension and wall thickness rather than by mass.
Design and Fabrication Notes
For compression members the flat width-to-thickness ratio governs local buckling and must be checked against the relevant design code. Square sections from about 20 x 20 mm to 400 x 400 mm, with wall thickness from 1.2 mm to 16 mm, cover most building work, and length is normally supplied at 6 m or 12 m with cut-to-length service. The tubing is readily sawn, drilled, punched and welded; welding should follow a qualified procedure, and filler metal selection is straightforward because the grades are low-alloy carbon steels. For external or humid service, hot-dip galvanising in accordance with ASTM A123 is the normal corrosion protection, with the coating mass specified by the designer. Where a project demands guaranteed notch toughness and tighter dimensional control, ASTM A1085 is the alternative specification, since it requires minimum Charpy impact energy and closer tolerances than A500.
Applications
Typical uses include building columns and bracing, mezzanine and racking frames, roof trusses, crane and conveyor supports, highway sign structures, agricultural equipment frames, trailer beds and scaffolding. In each case the material choice is driven by the required yield strength, the connection detail and the corrosion protection system rather than by the tubing dimension alone.
Frequently Asked Questions
Q: What is the difference between ASTM A500 Grade B and Grade C?
A: Grade B has 290 MPa minimum yield and 400 MPa minimum tensile strength; grade C has 317 MPa minimum yield and 427 MPa minimum tensile strength.
Q: Is A500 tubing welded or seamless?
A: The specification covers both. Most structural square tube is cold-formed and electric resistance welded; seamless tube is ordered for pressure or fatigue duty.
Q: Does ASTM A500 specify the chemical composition?
A: Only limits on phosphorus and sulfur, each 0.035 % maximum. The grades are defined by mechanical properties instead of a full analysis.
Q: How does ASTM A1085 differ from A500?
A: A1085 requires minimum Charpy impact energy and tighter dimensional tolerances, which suits seismic and dynamically loaded structures.
Q: Why does square tubing suit columns?
A: The square profile gives equal stiffness about both principal axes and the cold-formed corners add local strength, so the section performs well under combined axial and bending load.
Q: What corrosion protection is usual for A500 square tube?
A: Hot-dip galvanising to ASTM A123, or a paint or powder coating system specified for the exposure category of the structure.
