ASTM A500 Grade B and Grade C: When Each Is Required and Why Designers Choose Between Them
Structural hollow sections under ASTM A500 come in four grades, but in practice two of them dominate structural design: Grade B and Grade C. Most structural steel specifications for commercial construction in North America default to one of these, and the choice isn’t arbitrary. The difference in yield strength between the two grades has real consequences for member sizing, connection geometry, and code compliance in specific applications.
The mechanical property distinction
ASTM A500 Grade B and Grade C differ primarily in minimum yield strength and tensile strength:
Grade B: minimum yield strength 46 ksi, minimum tensile strength 58 ksi Grade C: minimum yield strength 50 ksi, minimum tensile strength 62 ksi
Grade C’s yield strength is about 9% higher than Grade B. That difference allows a designer to specify a smaller section — lighter wall, smaller outside dimension — and still meet the same member capacity requirement. For columns and bracing members where axial load governs design, that translates directly into less steel in the structure.
Grade A and Grade D exist for applications with lower strength requirements or specific notch toughness needs. They appear in some specialty applications but are rarely encountered in ordinary building design.
Why Grade B has historically been more common
For most of ASTM A500’s history, Grade B was the default for structural HSS in commercial and industrial buildings. It’s what the standard references produced, what service centers stocked, and what connection design tables in the AISC Steel Construction Manual were historically developed around.
The 2016 and later editions of the AISC specification moved toward recognizing Grade C explicitly as the preferred designation for structural HSS in many applications. AISC has updated its standard provisions to align with Grade C as the design basis where Grade C is specified, and the Manual’s connection tables now address both grades.
In practice, if your structural engineer specifies ASTM A500 without a grade designation, your material supplier may provide Grade B or Grade C depending on what’s available in their inventory. That ambiguity creates substitution situations that procurement should address upfront.
When Grade C specifically matters
Grade C becomes the specified material when the higher yield strength is needed to make a connection work or a member size feasible:
Seismic design: AISC 341 (Seismic Provisions for Structural Steel Buildings) includes requirements for expected yield strength in seismic force-resisting systems. For special moment frames and concentrically braced frames using HSS members, the design relies on the member yielding in a controlled manner. Grade C’s higher minimum yield (50 ksi) versus Grade B (46 ksi) affects the expected yield stress calculations used in seismic capacity design.
Column design at higher loads: for buildings where HSS columns carry significant gravity load and the available size range in Grade B doesn’t provide adequate capacity without going to a heavier section, specifying Grade C allows the same outside dimension with a different wall thickness to work.
Long spans and truss members: in spanning applications, Grade C gives the designer more efficiency per pound of steel, which can reduce fabrication cost if the lighter section reduces material weight noticeably.
Interchangeability between grades
Grade C is not freely interchangeable with Grade B in either direction without engineering review. Substituting Grade B where Grade C is specified may be unconservative — the design may have relied on the higher yield value. Substituting Grade C where Grade B is specified is generally conservative from a strength standpoint, but high-strength grades can behave differently in connection ductility and weld heat-affected zones.
Any proposed grade substitution on a project should go through the engineer of record for review. The procurement team’s assumption that “higher grade is always acceptable” is not reliable in seismic design, where the relationship between expected yield and design demands is explicit rather than simply “stronger is better.”
Checking what your project actually specifies
The safest starting point is reading the structural notes and material specification section of the project drawings rather than assuming a default. Engineers who work to the current AISC provisions are more likely to specify Grade C explicitly. Engineers who have been using older specifications may still default to Grade B. Either way, the specified grade should be on the drawings, and the delivered material should match it — confirmed by the mill test report on receipt.
