Technical Resources

HSS

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 solutions@aisc.org.

Frequently asked questions

1.3.4. To which ASTM specifications are hollow structural sections (HSS) ordered?

ASTM A500 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 offers atmospheric corrosion resistance properties similar to that of ASTM A588 for W-shapes. Pipe-size rounds (P, PX and PXX) are also available in ASTM A53 Grade B material. See FAQs 1.4.6 through 1.4.8 for additional information on HSS and pipe section designations and material grades.

ASTM A1085 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.

1.3.8. How can shape availability be determined?

AISC has producer listings at www.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.

1.4.6. What is the difference between a round HSS and a pipe?

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
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-in. nominal 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 Grade C (Fy= 46 ksi).
They are available in cross sections matching each of the cross
sections for ASTM A53 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 HSS can be obtained in
many more sizes with periphery not exceeding 64 in. and wall
thickness not exceeding 5⁄8 in.

One important difference, especially from an architectural perspective, is that round HSS will have an outside diameter equal to the nominal diameter, but the outside diameter of a pipe will vary depending on its thickness. The tolerances on A500 HSS also tend to be tighter than those on A53. For example A53 requires only that the pipe be reasonably straight, where A500 places a specific tolerance on straightness. Also, A53 specifically allows dents with depths up to the lesser of 10% of the pipe diameter or ¼ in.

1.4.7. What is the difference between a tube shape (TS) and HSS?

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.

1.4.8. What is the appropriate call-out for 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 (instead of
the old TS8×8×3⁄8). A rectangular HSS should be designated
as HSS5×3×3⁄8 (instead of the old TS5×3×3⁄8). 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 steel pipe designations (e.g., Pipe 5
Std., Pipe 5 x-strong, etc.) are designated differently than round
ASTM A500 HSS.

2.6.4. What are the maximum and minimum curved radii of HSS and W-shapes?

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-Appendix X4.

Cold-bending guidelines for shapes are also found in the AISC Manual. They are summarized below:

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. Cold-bending may be used to provide sweep in members to practically any radius desired.

3. A minimum length of 25 ft is commonly practical due to manufacturing/fabrication equipment. Bending by heat is also a possibility, but it should be noted that this procedure is generally much more expensive than cold-bending.

Note that providers for structural shape (including HSS) curving/bending often advertise their services in Modern Steel. A list of AISC Associate Member Bender-Rollers can be found at http://bit.ly/1ABwTV5. They would be the best ones to contact for determining minimum and maximum curved radii of shapes.

4.1.1. Can the 2005 AISC Specification be used to design structural shapes not listed in Part 1 of the 13th Steel Construction Manual?

Yes. AISC's 13th Edition of the Steel Manual is written based upon the 2005 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 2005 AISC Specification.

4.8.3. Should weep holes be used at the bottoms of Hollow Structural Section [HSS] columns?

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.

5.2.2. Are through-plates always required for singleplate connections to HSS columns?

No. Sherman and Ales (1991) demonstrated that local yielding of the support was not a concern due to the self-limiting nature of simple-shear connection end rotation and that the compressive strength of the HSS column was unaffected by the associated local deformations. However, this same research indicated that punching shear may be of concern for relatively thin supporting material thicknesses. Punching shear can be prevented by selecting an HSS with a wall thickness t w that meets the following criteria:

tw is greater than or equal to (Fy pl)(t pl) / Fuw where Fy pl = the yield strength of the single plate tpl = the thickness of the single plate Fuw = the tensile strength of the HSS wall

Note that this equation differs slightly from that given in Sherman and Ales (1991). Here, the expression is derived at the design strength level (omega factors included) whereas it was previously derived at the nominal strength level (no omega factors). If the actual maximum stress is known, it can be substituted for Fy pl in the above equation for a less conservative result.

The above minimum thicknesses would also be applicable to a welded plate tension connection (uniform stress distribution assumed). However, for cantilevered bracket connections, which do not have self-limiting rotations, yielding must also be checked. Sherman, D.R. and Ales, J.M. (1991), “The Design of Shear Tabs with Tubular Columns,” Proceedings of the 1991 AISC National Steel Construction Conference, AISC, Chicago, IL.

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 2003 (Table 720.1(1) and Article 720.5.1.4), 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. The relevant background information could be found in 

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.

last modified 1 January 2006

12.5.4. What structural shape is the optimal choice for beams in blast-resistant floors?

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.

last updated 5 January 2004

12.5.5. What types of column sections are preferred in blast-resistant design?

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.

last updated 5 January 2004