ASTM A500 GR B Square Hollow Section

ASTM A500 Gr B square tube is a cold-formed, welded structural steel hollow section (HSS) produced from flat-rolled coiled sheet. Square and rectangular Grade B shapes carry a minimum yield strength of 46 ksi (315 MPa) and a minimum tensile strength of 58 ksi (400 MPa). Available from about 1 to 16 in (25 to 406 mm) on a side with wall thicknesses from 0.083 to 0.625 in (2.1 to 15.9 mm), it is the standard closed-section member for structural framing, columns, and machine frames.

ASTM A500 / A500M is the governing specification for cold-formed, welded carbon steel structural tubing in round, square, and rectangular shapes - the products commonly called HSS (hollow structural sections). The name "cold formed welded" tells you how it is made: strip is cold-rolled into a box section and the longitudinal seam is closed by electric-resistance welding (ERW), forming a square or rectangular hollow profile with sharp corners and a uniform wall.

Grade B is the most widely specified strength level in the A500 family for square and rectangular shapes. It balances strength, weldability, and cost, which is why structural engineers, trailer builders, and fabricators reach for A500 Gr B square tube for everything from warehouse frames to equipment supports. The closed cross-section gives it excellent torsional and multidirectional bending resistance per unit weight, far more than an open section of equal material.

ASTM A500 GRB Square Tube Specification

Mechanical Properties

Mechanical properties are the first thing design engineers check, because they size the member against the applied loads. The values below are the guaranteed minimums from the A500 specification. Note that square and rectangular (non-circular) Grade B shapes carry a slightly higher yield than Grade B round tubing.

Elongation values for square/rectangular Grade B apply to walls 0.180 in (4.6 mm) and over; thinner walls have a separate reduction formula. Confirm against the current A500/A500M edition and mill certificate.

Chemical Composition

Grade B square tube is a weldable carbon-manganese steel. The chemistry is controlled within the ranges below, which keep the tube formable during cold rolling and weldable in the field or shop.

Element limits per ASTM A500/A500M Grade B. Copper is added only when a separate corrosion-resistance requirement is specified.

Size Matrix and Wall Thickness

The practical question buyers ask first is "what sizes are available?" ASTM A500 covers HSS up to a maximum total perimeter of 88 in (2235 mm). For a square, that caps the side around 22 in; most commercial ranges go well below that. These are the commonly stocked square sizes and their wall thickness options.

Availability varies by mill. Square tubes are measured to the outside dimension (OD) on each side; wall thickness options above are representative of standard sections.

Dimensional Tolerances

Dimensional tolerances govern how the tube fits into a fabricated assembly. ASTM A500 sets permissible variation on outside dimensions, wall thickness, length, straightness, and cross-section squareness. Representative tolerances are shown below.

Corner radii carry separate limits in the spec; target 2–3× wall for square sections. Confirm exact values against the A500/A500M table for your section.

Sourcing note: A purely black (uncoated) A500 square tube is the default supply condition. If your design is in a coastal or humid environment, ask for hot-dip galvanizing after forming per ASTM A123 - it typically extends service life several times over bare steel. State whether you need galvanizing up front, because it changes both price and lead time.

A500 Grade B vs. A500 Grade C vs. A513

Three specifications dominate inquiries for structural square tube, and they are frequently confused. Here is how they compare at a glance.

A513 is mechanical tubing with tighter tolerances for machinery, but it is not designated as structural HSS the way A500 is. Choose A500 when the code requires a structural hollow section.

In practice, Grade B is chosen when 46 ksi is sufficient and the project or code names A500 Gr B, while Grade C is the newer North American default for many structural jobs because it is widely milled and dual-graded. A513 enters only when the part is a mechanical member rather than a structural framing element. Matching the grade to the governing structural code (e.g., AISC 360) prevents rejections at inspection.

Applications

A500 Gr B square tube is a versatile structural member used wherever a box section provides the needed strength and stiffness. Typical applications include:

Structural framing - columns, beams, and bracing in buildings, warehouses, and mezzanines.

Trailers and truck bodies - frames and side posts where torsional loads are common.

Machine and equipment frames - bases, gantries, and supports with multidirectional loading.

Racking and shelving - storage systems and rack frames.

Sign supports and guardrails - posts and structural supports for outdoor structures.

Test and inspection

 

 

 

 

Request our downloadable A500 Gr B datasheet with complete size/wall/weight tables, section properties, and a mill certificate sample for your inspection.

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FAQ

How do I verify the quality of A500 Gr B square tube before accepting a shipment?

Run an incoming inspection against the tolerance table: measure outside dimension and wall thickness at the center of the flat (wall tolerance ±10%), check corner radius does not exceed 3 times the wall thickness, verify adjacent sides are square within ±2°, check camber within 0.2% of length, and visually inspect the surface and weld seam for cracks, dents, porosity, or weld burrs. Confirm the heat number on the physical tube matches the mill test certificate.

What does a mill test certificate (MTC) include for A500 Gr B square tube?

An EN 10204 3.1 mill test certificate for A500 Gr B lists the heat number, grade, chemical composition (carbon, manganese, phosphorus, sulfur), and mechanical test results including yield strength, tensile strength, and elongation. It records the quantity, dimensions, and confirms that the tube met all tests required by the A500 standard. Traceability from the certificate back to the physical heat is what proves conformance.

Can third-party inspection be arranged for A500 square tube?

Yes. Third-party inspection can be carried out by an independent agency, and the result is reported on an EN 10204 3.2 certificate or a third-party inspection report. Typical checks include dimensional tolerance verification, visual and weld-seam inspection, chemical analysis, mechanical testing to ASTM A370, and review of the mill test certificate. State the inspection requirement and agency on the purchase order.

How does A500 Gr B compare to A36 material for square tube?

A500 Gr B square tube has a minimum yield of 46 ksi (315 MPa) and tensile of 58 ksi (400 MPa), while structural A36 plate and bar carries 36 ksi (250 MPa) yield. For a structural tube section, A500 Gr B is stronger than A36 and is the correct closed-section structural material. A36 is a plate and structural-shape material, not a square tube specification.

What weld-seam defects should I look for and how are they detected?

Look for an uneven weld bead, porosity, incomplete fusion, cracks at the seam, and weld burrs that were not removed. Visually, the seam should be smooth and flush with the tube surface. For internal flaws you cannot see, non-destructive testing such as ultrasonic testing (UT) or eddy-current examination detects weld porosity, inclusions, and cracks within the seam.

Is hydrostatic or non-destructive testing available on A500 square tube?

Hydrostatic testing and non-destructive testing (eddy-current or ultrasonic) are optional in ASTM A500 and performed only when specified in the purchase order. If the application demands it, the tube can be leak-tested hydrostatically or examined with eddy-current or ultrasonic methods over the full length. State this requirement on the order so the mill can schedule it in production.

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