What Grade C Structural Tube Is
ASTM A500 is the standard specification for cold-formed welded and seamless carbon steel structural tubing, produced in round, square and rectangular shapes. Grade C sits near the top of the strength range defined by the specification: for square and rectangular sections it is assigned a minimum yield strength of 50 ksi, approximately 345 MPa, and a minimum tensile strength of 62 ksi, approximately 427 MPa, which are the values adopted in structural design practice.
The tubing is made by cold forming steel strip into a hollow section and welding the longitudinal seam, or by producing a seamless hollow. Cold forming raises the yield strength of the corners and gives the section accurate, repeatable dimensions, while the welded seam runs the full length of the member and is normally left in the as-welded condition.
Properties and Specification Requirements
| Item | Requirement for Grade C |
|---|---|
| Minimum yield strength | 345 MPa, about 50 ksi, for square and rectangular sections |
| Minimum tensile strength | 427 MPa, about 62 ksi, for square and rectangular sections |
| Section shapes | Round, square and rectangular |
| Production route | Cold formed welded or seamless |
| Wall thickness tolerance | Minus 10% of nominal wall |
| Design treatment | Design wall thickness taken as 0.93 times nominal in structural codes |
| Chemical control | Carbon, manganese, phosphorus, sulphur and copper limits apply |
| Tests | Tension test and flattening test on specimens from the finished tube |
The specification does not require impact testing in the base grade, so a project that needs notch toughness for low temperature service must call for it as a supplementary requirement. Mechanical values also differ between round sections and shaped sections, so the grade designation alone does not fix the strength: the section type must be stated with it.
Where Grade C Tube Is Used
Columns, beams, trusses and bracing in commercial, industrial and institutional buildings, including warehouses, logistics centres and airport structures.
Bridge elements such as pedestrian bridge trusses, highway sign supports and gantry frames.
Frames, booms and supports for agricultural, mining, construction and materials handling machinery.
Transport infrastructure including guardrails, lighting columns and traffic signal structures.
Architecturally exposed structural steelwork such as canopies, atria, staircases and facade framing, where the clean surface and uniform section are part of the design intent.
The reason the grade is used so widely in these applications is the strength to weight ratio of the hollow section. A square or rectangular hollow member resists bending about both principal axes, so a lighter section can carry the same load as a heavier open profile, which reduces the dead load of the structure and the load carried into foundations.
Design, Fabrication and Corrosion Protection
Design section properties using the reduced design wall thickness of 0.93 times nominal, and account for the corner radius of the cold-formed section.
Welding follows the structural welding code applicable to the project, with procedures qualified for the grade and thickness range being joined.
Cold bending is practical at moderate radii; the formed zone should be checked for wall thinning and for local distortion, particularly in heavy walls.
Cut ends and weld areas should be dressed before galvanizing or painting, and vent and drain holes are needed in closed members that will be hot dip galvanized.
For exposed architectural work, specify the surface finish and the protection system in advance, because the as-supplied surface condition varies between mills.
Inspection and Ordering Points
Verify the grade, the section shape and size, the nominal wall thickness and the length against the purchase order and the mill certificate.
Check the tension and flattening test results and the chemical analysis recorded on the certificate.
Measure wall thickness at several points around the section, and check the outside dimensions, corner radius, straightness and twist against the tolerance tables of the specification.
Confirm whether welded or seamless tubing has been supplied where the design depends on it.
Store tubing clear of the ground and protect it from standing water to avoid corrosion before fabrication.
FAQ
Q: What is ASTM A500 Grade C?
It is a strength grade of the cold-formed structural tubing specification, with a minimum yield strength of 50 ksi, about 345 MPa, and a minimum tensile strength of 62 ksi, about 427 MPa, for square and rectangular sections.
Q: How does Grade C compare with Grade B?
Grade C is the higher strength grade. Grade B is assigned a lower minimum yield strength for the same section type, so Grade C carries more load for the same section size.
Q: Is Grade C tubing welded or seamless?
The specification covers both cold-formed welded and seamless tubing, so the required product form must be stated when ordering.
Q: Why is a reduced wall thickness used in design?
Because the specification permits the wall to be 10% below nominal, structural design codes require section calculations based on a design thickness of 0.93 times the nominal wall.
Q: Does Grade C tube have guaranteed impact toughness?
No. Impact testing is not part of the base specification and must be ordered as a supplementary requirement when notch toughness is needed.
Q: What surface protection does the tube need?
The material is supplied bare, so protection depends on the application: painting, galvanizing or another coating system is selected according to the exposure and the design life of the structure.
