ASTM A500 carbon steel square tubing

ASTM A500 carbon steel square tubing is cold-formed welded carbon steel structural tubing produced to ASTM A500/A500M — Standard Specification for Cold-Formed Welded and Seamless Carbon Steel Structural Tubing in Rounds and Shapes. It is the specification most commonly cited for hollow structural sections (HSS) in North American practice, a status reflected in Steel Tube Institute producer guidance and in AISC design tables built around A500 product.

Square tubing offers a symmetrical section with equal stiffness about both axes, flat faces that are straightforward to detail and connect, and a clean architectural appearance from every angle. That combination makes it a default choice for columns, trusses, bracing, frames and architectural steelwork — anywhere a hollow section is preferred over an open profile.

Square sections are commercially supplied in the two most widely available strength classes, Grade B and Grade C. Grade C is the most common current production grade for square and rectangular HSS, and much of the market is supplied dual-certified B/C on the same heat. Grades A and D also exist within the specification: Grade A is the lowest strength class and is rarely stocked, while Grade D is the only grade that requires heat treatment and normally carries special-order lead times.

Size Range and Tolerances

• Product: cold-formed welded and seamless carbon steel structural tubing
• Shapes: round, square, rectangular and special shapes
• Size limits: periphery up to 88 in. and specified wall thickness up to 1.000 in. 
• Welded product: manufactured from flat-rolled steel by the electric-resistance-welding (ERW) process; the longitudinal butt joint is welded across its full thickness so that the structural design strength of the section is assured
• Qualification basis: tension and flattening testing, chemistry limits and workmanship
• Grades: A, B, C and D — grade names are strength classes, not manufacturing routes, so welded and seamless product can both be certified to the same grade

How Square Tubing Is Made

Flat-rolled steel strip is uncoiled, progressively roll-formed into a square section, and the longitudinal edges are joined by high-frequency electric resistance welding, with the weld flash removed externally and, where specified, internally. The section then passes through sizing rolls that establish the final dimensions and corner radii before being cut to length. The result is a hollow section with consistent wall thickness, tight corner geometry and a weld that is continuous across the full wall thickness.

Key Advantages of A500 Square Tubing

1. Structural specification, not a mechanical-tubing one. A500 is qualified on tension and flattening testing and chemistry limits, making it the appropriate choice for load-bearing work rather than surface-critical mechanical applications.
2. High strength-to-weight ratio. A closed square section delivers bending and torsional stiffness with substantially less steel than an equivalent open profile, reducing both weight and fabricated cost.
3. Balanced two-axis behaviour. Square sections perform identically about both principal axes, which simplifies detailing and avoids the weak-axis penalty of rectangular or open sections.
4. Flat faces make connections easy. Bolted, welded and gusseted connections, base plates and end caps are all straightforward to detail on flat surfaces.
5. Full-thickness ERW seam. The longitudinal butt joint is welded across the wall thickness, so the design strength of the section is not limited by an unwelded seam.
6. Dual certification is common. Much of the market ships material certified to both Grade B and Grade C, which simplifies substitution and reduces the risk of a size being unavailable in the required grade.
7. Excellent finish options. Square tubing is well suited to hot-dip galvanizing and painting for outdoor and humid service, and it presents a clean architectural appearance when left exposed.
8. Efficient fabrication. It cuts, punches, drills, notches and welds readily, and its closed shape requires no priming of internal surfaces before painting.

Typical Applications

• Building columns and gravity members
• Roof trusses, space frames and long-span structures
• Bracing, struts and structural supports
• Handrails, guardrails and stair stringers
• Highway sign posts and overhead sign structures
• Communication towers and antenna masts
• Equipment frames, machine bases and machine guards
• Agricultural and transport equipment frames
• Mezzanine and storage racking structures
• Architectural and exposed steelwork