Hollow Structural Sections (HSS) are manufactured hollow steel sections, typically of circular, rectangular, or square cross-section. In the United States, HSS sections conform to either the ASTM Specification A500 or A1085.
The use of HSS is growing with popularity all around the world in the building industry and in the United States accounts for approximately 18% of the structural steel market. Popular uses include architecturally exposed structural steel, staggered truss and conventional truss structures, and basic column or vertical bracing elements.
In 2013 a new ASTM specification for HSS was released, ASTM A1085. The new specification provides improved performance that makes HSS design easier and potentially more effective for the client. A quick comparison between A500 and A1085 can be found in the summary flyer.
Many of the questions that arise relative to the design, specification, detailing and fabrication of HSS are addressed in the following resources. If you have additional questions or just want to talk with one of the AISC Steel Solutions Center advisors about HSS, please contact the AISC Steel Solutions Center at 1-866-ASK-AISC or email@example.com.
Frequently asked questions
ASTM A500/A500M Grade C is most common when specifying square, rectangular and round HSS. These specifications cover cold-formed production of both welded and seamless HSS; ASTM A847/A847M offers atmospheric corrosion resistance properties similar to that of ASTM A588/588M for W-shapes. Pipe-size rounds (P, PX and PXX) are also available in ASTM A53/A53M Grade B material. See FAQs 1.4.6 and 1.4.7 for additional information on HSS and pipe section designations and material grades.
ASTM A1085/A1085M is a new specification for HSS. It offers improved material properties and design wall thickness equal to nominal wall thickness, among other desirable characteristics. For more information see www.aisc.org/a1085.
AISC has producer listings at aisc.org/steelavailability for hot-rolled shapes and HSS of various sizes and weights. Shapes producers have the ability to update these lists on a real-time basis. Contact information for many shapes producers is given on the website. Producers publish rolling schedules. Those schedules indicate which shapes are rolled on a periodic basis and those that are subject to accumulation of orders. Shapes that are rolled on a periodic basis are more readily available than those that are subject to accumulation of orders.
Round HSS are intended to be used as structural members. Pipe, though sometimes used as structural members, is intended to be used for mechanical and pressure applications. As used in the AISC Steel Construction Manual, steel pipe and round HSS are manufactured to meet different ASTM standards. Steel pipe is made to requirements in ASTM A53/A53M Grade B (Fy= 35 ksi).
Pipes up to and including NPS 12 are designated by the term Pipe, nominal diameter (in.) and weight class (Std., xStrong, xx-Strong). NPS stands for nominal pipe size. For example, Pipe 5 Std. denotes a pipe with a 5.563-in. outside diameter and a 0.258-in. wall thickness, which corresponds to the standard weight series. Pipes with wall thicknesses that do not correspond to the foregoing weight classes are designated by the term Pipe, outside diameter (in.) and wall thickness (in.), with both expressed to three decimal places. For example, Pipe 14.000·0.375 and Pipe 5.563·0.500 are proper designations.
Round HSS are usually ASTM A500/A500M Grade C (Fy= 50 ksi). They are available in cross sections matching each of the cross sections for ASTM A53/A53M Grade B steel pipe. For example, an HSS 6.625×0.280 has the same dimensional properties as a Pipe 6 Std. Additionally, ASTM A500/A500M HSS can be obtained in many more sizes with periphery not exceeding 88 in. and wall thickness not exceeding 1 in.
The tolerances on ASTM A500/A500M HSS also tend to be tighter than those on A53. For example, ASTM A53/A53M requires only that the pipe be reasonably straight, where ASTM A500/A500M places a specific tolerance on straightness. Also, ASTM A53/A53M specifically allows dents with depths up to the lesser of 10% of the pipe diameter or ¼ in.
Structurally, there is no difference. The Steel Tube Institute, an organization representing the manufacturers of hollow structural sections, initiated the change from “tube” to “HSS” in 1997 to conform to their designation practices. Thus, “TS” is simply an outdated way to specify “HSS.
Rectangular HSS are designated by the mark “HSS,” overall outside dimensions (in.) and wall thickness (in.), with all dimensions expressed as fractional numbers. For example, a square HSS should be designated as HSS8×8×3⁄8. A rectangular HSS should be designated as HSS5×3×3⁄8. Round HSS are designated by the term “HSS,” nominal outside diameter (in.) and wall thickness (in.) with both dimensions expressed to three decimal places. For example, a round HSS should be designated as HSS5.563×0.258.
Note that ASTM A53/A53M steel pipe designations (e.g., Pipe 5 Std., Pipe 5 x-strong, etc.) are designated differently than round ASTM A500/A500M HSS.
Limits on radii of curved shapes are essentially a function of the capabilities of the bender. AISC does limit the radius of bend for bent plates to prevent cracking during the bending process. Though similar limits would apply to any bent product, such deformations are not generally achievable in HSS. Guidelines for bending plates are found in ASTM A6/A6M-Appendix X4.
Cold-bending guidelines for shapes are also found in the AISC Manual, Part 2 Section:
1. "The minimum radius for camber induced by cold-bending in members up to a nominal depth of 30 in. is between 10 and 14 times the depth of the member. Deeper members may require a larger minimum radius."
2. "A minimum length of 25 ft is commonly practical due to manufacturing/fabrication equipment."
Note that cold bending may be used to provide sweep in members to practically any radius desired. Bending by heat is also a possibility, but this procedure is generally much more expensive than cold bending.
More Curved Steel resources include recommended detailing, geometry, a glossary of technical terms, and a list of AISC Associate Member Bender-Rollers who would be the best companies to contact for determining minimum and maximum curved radii of shapes. Providers for structural shape (including HSS) curving/bending often advertise their services in Modern Steel, as well. Additional Guidance can be found in AISC Design Guide 33: Curved Member Design and AISC Design Guide 36: Design Considerations for Camber.
4.1.1. Can the AISC Specification be used to design structural shapes not listed in Part 1 of the 13th Steel Construction Manual?
Yes. AISC's Steel Manual is written based upon the AISC Specification for Structural Steel Buildings and provides design aids and charts for commonly used structural shapes: those listed in ASTM A6/A6M as well as hollow structural Sections [HSS] (square, rectangular, and round). This coverage is not intended to exclude the use of other shapes that may be available. Other shapes and combinations thereof may be designed and used within the requirements of the AISC Specification.
If the HSS assembly is fabricated as an airtight enclosure, weep holes need not be provided because any moisture in the contained air will quickly be used and corrosion cannot progress. When non-airtight HSS columns are exposed to the weather or to temperature changes that can cause interior condensation, weep holes should be provided. If, however, a column is protected from the elements and is neither subject to drastic temperature change nor a humid environment, weep holes may not be necessary. Note that HSS members need weep holes if they are to be galvanized.
No, through-plates are not required for all single-plate connections to HSS columns. The AISC Steel Manual, Part 10, Design Considerations for Simple Shear Connections to HSS Columns Section provides recommendations. The Through-Plate Connections Section states:
“Through-plate connections should be used when the HSS wall is classified as a slender element [b/t ≤ 1.40(E/Fy)0.5 or 33.7 for Fy = 50 ksi for rectangular HSS; D/t ≤ 0.11E/Fy for round HSS and Pipe] or does not satisfy the punching shear limit state. A single-plate connection is more economical and should be used if the HSS is neither slender nor inadequate for the punching shear rupture limit state.”
Single-Plate Connections to HSS Section states:
“As long as the HSS wall is not classified as a slender element, the local distortion caused by the single-plate connection will be insignificant in reducing the column strength of the HSS (Sherman, 1996). Therefore, single-plate connections may be used with rectangular HSS when b/t ≤ 1.40(E/Fy)0.5 or 33.7 for Fy = 50 ksi. Single-plate connections may also be used with round HSS as long as they are nonslender under axial load (D/t ≤ 0.11E/Fy).
Yielding (plastification) of the HSS face has not been a governing limit state in physical
tests. Punching shear (shear rupture), however, should be checked as follows:
Fu = specified minimum tensile strength of the HSS member, ksi
Ra = required shear strength (ASD), kips
Ru = required shear strength (LRFD), kips
e = eccentricity, taken as the distance from the HSS wall to the center of gravity of the
bolt group, in.
lp = length of the single-plate shear connection, in.
t = design wall thickness of HSS member, in.
Φ = 0.75
Ω = 2.00”
11.4.8. How can one determine a fire rating for a system that has not been prequalified, such as a concrete-encased steel column?
Concrete-encased steel columns have been "pre-qualified" by many fire tests. These columns are of generic design (non-proprietary); therefore, they are not listed in the UL directory.
However, most building codes, e.g., IBC 2021 (Table 721.1(1) and Article 722.214.171.124), and ASCE/SFPE 29-99 (Article 5.2.4) contain formulas/specifications to determine the fire resistance of concrete-encased columns. These formulas/specifications are based on extensive experimental data from standard (ASTM E119) fire resistance tests.
Concrete-filled HSS columns are another example of generic construction that has not been listed in the UL directory. Article 5.2.3 of ASCE/SFPE 29-99 specifies a simple method to determine the fire resistance of concrete-filled hollow steel columns.
V. K. R. Kodur, and D. H. MacKinnon, "Design of Concrete-Filled Hollow Structural Steel Columns for Fire Endurance", Engineering Journal, First Quarter, 2000, pp. 13-24.
The choice of structural members supporting a slab depends on the load magnitude and where it is expected to act. If the blast load is expected only on the top of the slab, such as a slab over a basement, then either a W-shape or hollow structural section (HSS) is likely to be effective. If the maximum blast load is as likely to act on top of the floor slab as on its lower surface, then both shapes are likely to be effective. When the underside is loaded, the support beams will be loaded both on the bottom and on their sides. The net direct load on the webs of W-shapes is likely to be minimal. Where significant torsion effects are likely, HSS are preferred for their superior torsion resistance.
Military manuals for blast-resistant design base procedures on material properties increased by approximately 10% to account for strain-rate effects. Columns designed to resist high blast loads usually have sufficiently small slenderness ratios, and buckling occurs plastically rather than elastically. Also, because dynamic-impulse load tends to suppress the occurrence of buckling, it is conservative to adapt static formulas to the dynamic case. The choice of structural shape will depend on a number of factors, like whether the column is subjected to an axial load, or to flexural and axial load. Since in the latter case the load can come from any direction, it is useful to use a shape that has equal flexural strength in all directions, such as a round or square HSS.
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