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The Wall Thickness Problem A1085 Was Written to Solve
Under ASTM A500, the as-produced wall thickness of an HSS is allowed to run as much as 10% under the nominal (ordered) wall thickness, and the actual manufactured wall is not guaranteed on the mill certificate. Because designers can't assume the full nominal wall is present, AISC 360 requires a 0.93 design wall thickness factor be applied to A500 HSS in every structural calculation - section properties, connection design, and capacity checks are all run against 93% of nominal, whether or not the pipe you receive is actually thinner.
ASTM A1085 requires the minimum wall thickness to equal the nominal wall thickness - no under-tolerance allowance. This is the core reason AISC 360 permits designers to use the full nominal wall thickness (design factor of 1.0) for A1085 sections. In practical terms, an A1085 member of the same nominal size carries more calculated capacity than the equivalent A500 member, simply because the design doesn't have to discount for wall thickness the mill might not have actually delivered.
| Property | ASTM A500 | ASTM A1085 |
|---|---|---|
| Minimum wall thickness vs. nominal | -10% allowed | Equal to nominal (no under-tolerance) |
| AISC 360 design wall thickness factor | 0.93 | 1.0 |
| Corner radius | Not tightly controlled | Controlled, more consistent for gusset/connection fit |
| Yield-to-tensile ratio cap | None specified | Capped (Grade A: 0.85 max) |
| Minimum yield strength, Grade A | - | 50 ksi (345 MPa) |
| Charpy V-notch toughness | Not required | Available as supplementary requirement |
Why the Yield-to-Tensile Ratio Cap Matters in Seismic Design
A500 sets a minimum yield and minimum tensile strength but places no ceiling on how high the actual yield strength can run relative to tensile strength. In seismic-force-resisting systems designed on capacity-design principles - braced frames and buckling-restrained brace connections in particular - the engineer needs the member to yield and deform predictably before the connection or adjacent member reaches its own capacity. An unexpectedly high yield-to-tensile ratio can push a member's actual strength closer to its ultimate strength than the design assumed, undermining that sequence.
ASTM A1085 Grade A caps the yield-to-tensile ratio at 0.85, giving the engineer a bounded, predictable margin between yield and ultimate strength - which is why A1085 is frequently the specified material (not just an acceptable substitute) on seismic-detailed braced frame and moment frame projects in AISC 341 Seismic Provisions work.
Where We See A1085 Specified in Practice
- Seismic-force-resisting HSS bracing where AISC 341 capacity design governs member and connection sizing
- Long-span or heavily loaded HSS columns where the 0.93-vs-1.0 design wall thickness factor makes a measurable difference in section capacity at the same nominal size
- Projects with tight connection tolerances, where more consistent corner radius geometry simplifies gusset plate and cap plate fit-up
- Architecturally exposed structural steel (AESS) work, where more consistent wall and corner dimensions reduce surface irregularity after fabrication
Manufacturing Process
- Coil selection to the chemistry required for the specified grade
- Cold roll-forming of the flat strip into the target tube profile
- High-frequency welding of the longitudinal seam
- In-line weld seam normalizing/heat treatment as required to meet toughness and ductility targets
- Sizing and corner forming to controlled, consistent corner radius
- Full-length wall thickness verification, not spot-checked, to confirm minimum wall meets nominal
- Straightening and cut-to-length
- Mechanical testing - tensile, yield, and yield-to-tensile ratio calculation per heat/lot
- Marking and certification, with mill test report showing actual wall thickness and yield/tensile results
A Real-World Case
A structural engineer on a seismic braced-frame project specified A500 Grade C HSS bracing based on a standard section table, without accounting for the yield-to-tensile ratio requirement embedded in the project's AISC 341 capacity-design calculations. During shop drawing review, the connection design couldn't be verified against A500's unbounded ratio, and the specification had to be revised mid-fabrication to A1085 Grade A to satisfy the ductility requirement the seismic design was actually built on - causing a material re-order and a short schedule delay. On the next phase of the same project, specifying A1085 from the outset avoided the issue entirely.
FAQ
Q: Is ASTM A1085 stronger than ASTM A500?
A: Not necessarily in raw material strength - A1085 Grade A's minimum yield (50 ksi) is close to A500 Grade B's (46 ksi). The real difference is design efficiency: because A1085 guarantees full nominal wall thickness, AISC 360 lets engineers design against 100% of nominal wall instead of the 93% required for A500, which typically results in higher usable section capacity at the same nominal size.
Q: Can A1085 be substituted for A500 on an existing project without re-checking the design?
A: Not automatically. While A1085 generally meets or exceeds A500's properties, section capacities in the structural calculations were likely run using A500's 0.93 design factor. Substituting A1085 doesn't require re-analysis for adequacy (it's conservative in that direction), but the reverse - substituting A500 where A1085 was specified - does require re-checking, especially on seismic projects.
Q: Does A1085 cost more than A500?
A: Typically yes, due to tighter production tolerances and additional in-process wall thickness verification. Many engineers find the higher usable section capacity offsets some or all of the unit cost difference, particularly on seismic or heavily loaded members - we can help quote both for side-by-side comparison on a specific project.
Q: Is Charpy V-notch toughness testing required for A1085?
A: Not as a base requirement, but it's available as a supplementary requirement (Supplementary Requirement S5 in the standard) and is commonly specified on cold-region or seismic projects. Confirm with us at order time if your project requires it.
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