A few years ago, we received drawings for an overseas industrial building where the structural engineer had specified S355 steel for the main frame. During material preparation, the client asked a common question: “Can we use ASTM A572 Grade 50 instead? The strength looks almost the same.”
The question sounds simple. The answer is not always yes.
After more than a decade of fabricating steel structures for international projects, we have seen the same issue many times. S235, S355, ASTM A36 and ASTM A572 are among the most familiar structural steel grades, but they belong to different standards. Comparing them requires more than looking at yield strength.
S235 and S355: Common European Structural Steels
S235 and S355 are widely used under the European EN 10025 standards.
The number gives engineers an immediate indication of the steel’s strength. For thinner material, S235 has a nominal minimum yield strength of 235 MPa, while S355 starts at 355 MPa. The specified minimum strength may decrease as material thickness increases, so the actual standard should always be checked.
For an ordinary light industrial building, S235 may be sufficient for many components. When loads increase, S355 steel becomes more attractive.
We often see S355 specified for main columns, rafters and other heavily loaded members in industrial buildings. Its higher yield strength can allow engineers to optimize member sizes and, in some cases, reduce the total steel weight.
There is another detail that should not be overlooked: S355 is not just one grade. S355JR, S355J0 and S355J2, for example, have different impact toughness requirements. For a building in a cold climate, this distinction can be just as important as yield strength.
Where Does ASTM A36 Fit?
Move from a European project to an American-standard project and the material specification changes.
ASTM A36 is a familiar choice for general structural fabrication. For many products covered by the specification, its minimum yield strength is around 36 ksi, or 250 MPa, depending on product form and thickness.
In our workshop, materials in this strength range are commonly associated with connection plates, base plates, stiffeners and general structural components where higher-strength steel is not required.
A36 is popular because it is widely recognized, practical to fabricate and has good weldability when proper welding procedures are followed.
For more demanding structural members, however, engineers often turn to higher-strength steels.
ASTM A572 Grade 50: Higher Strength for Structural Members
ASTM A572 steel is a high-strength, low-alloy structural steel specification available in several grades. A572 Grade 50 is one of the most commonly encountered.
Its minimum yield strength is typically 50 ksi, or about 345 MPa, within the applicable thickness range.
This is why A572 Grade 50 is frequently compared with S355. Their yield strengths are close:
| Grade | Standard | Typical Minimum Yield Strength* |
|
S235 |
EN 10025 | 235 MPa |
| S355 | EN 10025 |
355 MPa |
|
ASTM A36 |
ASTM | 36 ksi / about 250 MPa |
| ASTM A572 Gr.50 | ASTM |
50 ksi / about 345 MPa |
*Actual requirements depend on product type, thickness and the applicable edition of the material standard.
But similar strength does not make two materials identical.
Can S355 and A572 Grade 50 Be Substituted?
This is where practical experience matters.
On one industrial project, a specified steel grade was difficult to source in the required sections. A locally available grade had a very similar yield strength, and replacing it appeared to be an easy solution.
Instead of changing the material directly, we prepared the available material specifications for review. The structural engineer checked the mechanical properties, chemical composition and project requirements before approving the substitution.
That extra step avoided a much bigger problem later.
When comparing S355 with ASTM A572 Grade 50—or S235 with ASTM A36—engineers should consider more than yield strength. Tensile strength, chemical composition, toughness, weldability, material thickness and certification requirements can all affect whether substitution is acceptable.
For this reason, a fabricator should never change the specified steel grade simply because another material has a similar strength value.
Choosing the Right Steel Grade
For most projects, material selection should start with the structural design and applicable standard.
If the building is designed to European standards, S235 or S355 may be specified. American-standard projects may use ASTM A36, A572 or other ASTM materials. Environmental conditions, loads, welding requirements and local material availability should then be considered.
At Havit Steel Structure, we also review material availability during the drawing and procurement stages. For export projects, doing this early is important. Finding out after fabrication begins that a particular section or grade is difficult to source can affect both cost and delivery time.
Higher strength does not automatically mean better steel.
The right choice is the structural steel grade that meets the engineering requirements, fabrication conditions and applicable standard while remaining practical to source and manufacture.
For an experienced steel fabricator, that balance is far more important than simply choosing the biggest number on the material certificate.
