Tensile strength is the highest stress a material can carry while being pulled apart before it breaks. For steel it is usually given in ksi (thousands of pounds per square inch) or MPa, and common structural steels have a specified minimum tensile strength between 58 and 80 ksi.
This guide explains what tensile strength measures, how it differs from yield strength, the values for the structural steel grades used in beams and plate, how it is tested, and where engineers actually use it. Sources were checked on October 31, 2026.
What Tensile Strength Means
Stress is force divided by the area it acts on. Pull on a steel bar and the stress rises as the force rises. Tensile strength, also called ultimate tensile strength (UTS) or simply “ultimate,” is the peak of that curve: the maximum force the specimen carried divided by its original cross-sectional area. In steel specifications and design codes it is written as Fu.
Steel does not snap the instant it passes yield. It stretches, gets stronger as it work-hardens, reaches its tensile strength, and then a local “neck” forms where the bar thins quickly until it breaks. So tensile strength is a measure of how much load the material can take at the very end, not how much it can take while still springing back to its original shape.
Tensile Strength vs Yield Strength
Yield strength (Fy) is the stress at which steel starts to stretch permanently. Tensile strength (Fu) is the stress at which it is about to fail. For structural steel, yield is the number most beam and column design is built on, because a member that yields has already sagged or stretched more than a building can tolerate. Our article on the yield strength of steel covers that side in depth.
| Yield strength (Fy) | Tensile strength (Fu) | |
|---|---|---|
| What it marks | Start of permanent stretch | Maximum stress before failure |
| Main use in design | Bending, compression, gross-section tension | Fracture checks: bolt holes, net sections, block shear, bolts and welds |
| ASTM A992 (W shapes) | 50 ksi min, 65 ksi max | 65 ksi min |
| ASTM A36 | 36 ksi min | 58 to 80 ksi |
The ratio between them matters too. A low yield-to-tensile ratio means there is a large reserve between first yielding and fracture, which is the ductility that lets steel structures bend and redistribute load instead of breaking suddenly. That is why ASTM A992, the standard grade for wide flange beams, caps the yield-to-tensile ratio at 0.85 as well as setting a minimum tensile strength of 65 ksi.
Tensile Strength of Structural Steel Grades
These are specified minimum values, the numbers the mill guarantees and designers use. The plate values come from Roger Brockenbrough’s MTR Survey of Plate Material Used in Structural Fabrication (AISC Engineering Journal, first quarter 2003, read October 31, 2026), which lists the specified properties alongside the mill test data; the A992 values are from our A992 page. MPa conversions use 1 ksi = 6.895 MPa, rounded.
| ASTM grade | Typical products | Min. yield, Fy | Tensile strength, Fu |
|---|---|---|---|
| A36 | Plate, angles, channels, bars | 36 ksi (248 MPa) | 58 to 80 ksi (400 to 552 MPa) |
| A992 | W shapes (wide flange) | 50 ksi (345 MPa) | 65 ksi min (448 MPa) |
| A572 Grade 50 | Plate, shapes | 50 ksi (345 MPa) | 65 ksi min (448 MPa) |
| A572 Grade 60 | Plate | 60 ksi (414 MPa) | 75 ksi min (517 MPa) |
| A572 Grade 65 | Plate | 65 ksi (448 MPa) | 80 ksi min (552 MPa) |
| A588 | Weathering steel plate, up to 4 in | 50 ksi (345 MPa) | 70 ksi min (483 MPa) |
A36 is unusual in having a maximum tensile strength as well as a minimum. More on that grade is in our A36 steel guide.
Actual steel is usually stronger than the minimum
The same AISC survey analyzed 14,900 mill test results for A36 plate 0.188 to 0.75 in thick. The mean tensile strength was 67.89 ksi against the 58 ksi specified minimum, the mean yield point was 46.88 ksi against 36 ksi, and the mean yield-to-tensile ratio was about 0.69. That margin is normal, but designers still use the specified minimums. The actual figures for any delivery are printed on the mill test report that comes with the steel.
How Tensile Strength Is Tested
A specimen is machined from the product, marked with gauge lines, gripped at both ends in a testing machine and pulled at a controlled rate until it breaks. The machine records force and elongation. The peak force divided by the original area is the tensile strength. The scope of ASTM E8/E8M (read October 31, 2026) covers tension testing of metallic materials in any form at room temperature, which it defines as 10 to 38 °C, and includes methods for yield strength, yield point elongation, tensile strength, elongation and reduction of area.
The product specification decides where the specimen is cut and how many tests are run per heat. The ASTM A992/A992M page, for example, describes the standard as covering rolled steel structural shapes for building framing or bridges and states that a tension test is used to evaluate tensile strength, yield strength and elongation.
Where Tensile Strength Matters in Steel Design
Bolted connections and net sections
Drilling or punching bolt holes removes material, so the remaining “net” section of a plate or angle is weaker than the full section. Failure there is a fracture rather than a stretch, so the check uses Fu. The same applies to block shear, where a chunk of steel tears out around a bolt group, and to the bearing of bolts on the edges of holes.
Bolts and welds
Fasteners are rated by tensile strength. Bolt grades and weld filler metals are classified by their minimum tensile strength, for example the “70” in an E7018 electrode refers to a 70 ksi minimum tensile strength weld metal. Our guide to bolt grades lists the tensile strengths behind the head markings.
Ductility and safety
Because structural steel yields well before it reaches tensile strength, an overloaded beam usually gives visible warning by sagging or twisting. That reserve is one reason grade, ratio and elongation limits are written into the standards. None of this replaces design: the size of any beam, column, connection or tension member must be checked by a licensed structural engineer.
Tensile Strength FAQ
What is tensile strength in simple terms?
It is the maximum pulling stress a material can survive before it breaks, measured as the peak force in a tension test divided by the specimen’s original cross-sectional area.
What is the tensile strength of a steel I-beam?
Most wide flange beams sold in the US are ASTM A992, with a minimum tensile strength of 65 ksi (448 MPa). Older or lighter shapes in A36 have a tensile strength of 58 to 80 ksi.
Is tensile strength the same as ultimate strength?
Yes. Ultimate tensile strength, ultimate strength, tensile strength and Fu all refer to the peak stress in a tension test.
Why is yield strength used more than tensile strength?
A beam that has yielded has permanently deformed, which is already a failure for a building. Tensile strength is used for checks where the failure would be a fracture, such as at bolt holes, in bolts and in welds.
How do I convert ksi to MPa?
Multiply by 6.895. For example, 65 ksi × 6.895 = about 448 MPa, and 58 ksi is about 400 MPa.
Reference information only, not engineering advice. Disclaimer.