The Atlas Tube advantage: HSS specification and its applications in structural design

What HSS is and how it is manufactured

Hollow Structural Sections are cold-formed products, made differently from hot-formed structural steel such as plates, wide flanges, channels, and angles. The most common manufacturing method is electric resistance welding (ERW): flat hot-roll coil is passed through a series of rolls to form a round section, then an electric current passes through the steel, heating it to near-molten temperature. The two edges are forced together under pressure, creating a metallurgical bond without any deposited weld metal. Once welded into a round, the section is then formed into the final square or rectangular shape through additional roll stands.

Brad explained that the weld produced by ERW is required to be stronger than the rest of the section. This is proved through destructive testing: round sections are crush-tested with the weld at 90 degrees to the applied force, and cone tests are forced into square or rectangular ends to verify that cracking occurs at corners (where residual stress is highest) and not at the weld. Production teams monitor the weld in real time and can adjust parameters immediately if needed.

Because the cold-forming process strain-hardens the steel, HSS does not exhibit the distinct yield plateau seen in hot-rolled material. Yield strength is therefore determined using the 0.2 percent offset method rather than a clearly defined yield point on the stress-strain curve.

HSS specifications: choosing the right one

ASTM A500 is the most commonly specified standard for HSS and the one referenced as preferred in the AISC manual. The key update in 2021 removed Grade A, unified the minimum yield strength at 50 ksi for both round and square sections, and eliminated Grade B as a standalone product. North American producers now manufacture only Grade C, dual-certifying material to cover both B and C. Brad recommended that engineers update any drawings still showing Grade B: they are receiving Grade C regardless, but designing to the lower Grade B strength forgoes capacity at no cost difference.

One important design consideration: A500 has a wall-thickness tolerance of plus or minus 10 percent. Because mills order coil on the thinner side and manufacturing introduces further reduction, finished sections tend to be under nominal thickness. AISC addresses this through a design wall thickness equal to 0.93 times the nominal, already incorporated into the AISC manual section property tables.

ASTM A1085 was introduced in 2013 to raise the performance standard for demanding applications. It features wall-thickness tolerances roughly half those of A500, eliminating the need for the 0.93 reduction factor, a maximum yield strength of 70 ksi, and a required Charpy V-notch test of 25 ft-lbs at 40 degrees Fahrenheit. The tighter tolerances and capped yield bring the expected-strength factor (used in seismic connection design) down from 1.3 to 1.25. A1085 is not stocked at service centers; it is a mill-order product best suited to larger projects and is most readily available in larger section sizes for bridge and seismic applications.

ASTM A53 is a pipe specification intended for mechanical and pressure applications, not structural steel. Brad explained that Grade B carries only a 35 ksi yield strength, has a minus 12.5 percent wall-thickness tolerance, and describes straightness only as "reasonably straight." More importantly, A53 functions as a catch-all specification: material that fails higher-grade testing can be downgraded to A53, so actual yield strength is highly variable. Brad cautioned specifically against using A53 in seismic force-resisting systems, where unintended overstrength in a brace or column can prevent the intended yielding hierarchy.

Column efficiency: how HSS compares to wide flanges at longer unbraced lengths

Because a square or round HSS has no weak axis, its radius of gyration is the same in all directions. This means buckling load is the same regardless of the plane in which buckling might occur, simplifying column design and often producing more efficient use of steel at longer unbraced lengths.

Brad presented column curves comparing an HSS 8x8x1/4 (weighing slightly less than a W10x33) to the wide flange at various unbraced lengths. At 15 feet, both carry approximately the same axial load. At 30 feet, the wide flange drops to around 60 kips while the 8x8x1/4 retains over 120 kips. A 10x10x1/4 HSS, which weighs the same as the W10x33, carries nearly four times the load at 30 feet. The advantage of HSS is most pronounced at longer unbraced lengths where lateral buckling governs open sections but not closed ones.

Sustainability and availability

Approximately 70 percent of HSS produced in North America is made from electric-arc furnace coil rather than basic oxygen furnace coil. EAF production uses recycled scrap, reducing the global warming potential of the finished product. Engineers who want to specify lower-GWP material can include EAF coil as a requirement in their project specifications.

Atlas Tube operates seven production facilities across North America and uses quick-change mill technology to roll changeovers in 60 to 90 minutes, allowing more frequent production runs of more sizes. Brad noted that an availability chart on the Atlas Tube website shows how often each section size is rolled, helping engineers avoid specifying sizes that are rarely in stock. Standard lengths run from 35 to 48 feet through service centers; custom lengths to the nearest inch are available as mill orders for projects with repetitive column heights.