Kim Olson, PE 2020-12-17 09:24:41
Everything you’ve always wanted to know about HSS, but were afraid to ask.
ROUND HOLLOW STRUCTURAL SECTIONS (HSS) have a lot going for them.
Not only are they aesthetically pleasing and favored by architects, but they are also efficient structural members. With their lack of a weak axis, they are superior in compression. Their closed shape makes them preferred when torsionally loaded. When designing connections to round HSS, there are fewer limit states to consider due to the geometric nature of the section. And they can also be filled with concrete to increase compression capacity and provide fire resistance.
From bridges to transmission towers and stadium roofs to handrails and posts, HSS can be used to overcome common design challenges and expand possibilities when building and designing complex structures. In tandem with the many uses for HSS are many questions on its design potential. Here, we’ll explore a few that will hopefully provide a better idea of how you can get the most out of HSS on your projects.
The fabricator on my current job requested a material substitution for A500 Grade B round shapes. What else is available, and what will the impact on my design be?
When it comes to materials, here’s what engineers need to know. Currently, Steel Tube Institute (STI) producers dual-certify all of their products to ASTM A500 Grade B/C, meaning that the material meets the specification requirements for both A500 Grade B and Grade C. The AISC 15th Edition Steel Construction Manual (aisc.org/manual) contains capacity tables are calculated for A500 Grade C to reflect this as the predominant material in the marketplace. Therefore, the design community should design using Grade C, as that is what is being purchased and what has been provided for many years—not only for round sections but for all HSS.
Round sections should be specified as A500. Historically, the belief was that A53 was the most available round section and, therefore, the most cost-efficient. This is not the case. A53 is the standard specification for steel, black lacquer coated, welded, and seamless steel pipe. It is intended for use in mechanical and pressure applications as well as for use in ordinary steam, water, and air lines. ASTM A500 is the standard specification for cold-formed, welded, and seamless carbon steel structural tubing. Available in four grades, A through D, it is intended for use in construction and structural applications. Unlike A53 piping, which is only round, A500 is available in more shape options, most commonly round, square, and rectangular.
In addition to these differences in intended use between the two steel products, many additional details are critically important for engineers, especially as they relate directly to matters of cost and quality. Consider yield strength. No matter the grade, A500 material’s yield strength will be greater than A53 piping. A500 Grade C round HSS specifies a minimum yield strength of 46 ksi, and A53 Grade B piping requires a minimum yield strength of 35 ksi. Although A53 was, at one time, the standard specification for round shapes, specifying A53 for columns or braces of a building results in a thicker, larger section than if using the stronger A500. Structures designed with A500 require less steel by weight; the cost-saving implications are clear.
An A500 round also has a tighter outside diameter (OD) and wall tolerances. When using an A500 round for a building column, you could specify an HSS8.625×0.322 with an outside diameter tolerance of +/-0.75% and a wall tolerance of +/-10%. The A53 equivalent, an 8-in. standard pipe, has an OD tolerance of +/-1% and a wall tolerance of -12.5%. Another word to the wise: A53 pipe is available only in lengths of 21 ft and 42 ft. A500 rounds can be produced in lengths from 20 ft to 75 ft.
When selecting section sizes for structural design, you can be assured of not only the desired cross-sectional dimensions but also the necessary straightness with A500, as producers must also adhere to a straightness tolerance specified in A500. With A53, there is no specification in the standard for how straight the pipe must be.
Here’s why the external characteristics are equally important. Thus far, this discussion has focused on the structural characteristics of A53 and A500, but what happens on the outside matters just as much. When an A53 pipe is specified, part of its material cost is for the sealant that producers use to coat the outside of the pipe. In order to weld to these pipes, a fabricator must remove the sealant, creating an unnecessary cost and extra step in the fabrication process. The bare surface of the A500 tube makes it easier to paint after fabrication is complete. Also, because A53 pipe is produced to carry pressurized steam, water, or gas, the manufacturer must hydrostatically test the product, ensuring that it can withstand pressure when in use. If A53 piping is used in structural applications, the product includes the cost of those tests that a structural application does not require.
From experience, I am finding that not all HSS shapes given in the AISC Manual are actually produced. Is there a list of sections available?
Here are some key considerations around sizes and availability. Round HSS can be specified in a wide variety of sizes. Discerning what sizes are readily available is a little trickier. Engineers frequently wonder why there are fewer options for A53 pipe than A500 rounds. A53 pipes are designated to a nominal pipe size (NPS) referring to a “nominal” OD in inches, plus one of three scheduled wall thicknesses (standard, x-strong, and xx-strong). They are sized this way because A53 pipes—designed to carry pressurized steam, air, or water—must work with standardized fittings and valves. There is no such need with A500 tubes, which are therefore designated with much more precision and, accordingly, more efficiency. With A500 rounds, the outside diameter and wall thickness, in inches, are carried to three decimal places.
A good rule of thumb is to specify an HSS member that is equivalent to the NPS sizes. These are listed in Table 1 with the corresponding callout for an HSS. If deviating from those listed, it is best to check the STI Capability Tool at www.steeltubeinstitute.org to see if the section specified is domestically produced and, therefore, commonly available. This can also be done for rectangular sections. You can also search for HSS shape availability in the “Who makes the shapes you need?” search box at www.aisc.org.
If I’m working on a handrail design, what sections should I be looking at?
Table 2 lists the sections commonly available in ASTM A500 Grade B/C for handrail construction.
When sourcing smaller sections, can ASTM A513 be substituted if A500 is “not available”?
This is a multi-step answer. First, challenge the question of availability. Check the Capability Tool and contact STI for assistance. Second, the answer to the substitution request is, “It depends.” ASTM A513 is a mechanical tubing specification intended for applications where dimensional tolerances are critical, but the strength of the member is not paramount. ASTM A513 has no physical requirements (minimum yield, tensile, or elongation), and often A513 material is not provided with a material test report (MTR) indicating these properties. Therefore, if a substitution is requested, it is essential to first perform coupon testing or review the product’s MTR to ensure that it meets the physical requirements you assumed in your design.
I need large round sections for my design. What is available domestically?
The availability of these sections should be a concern when considering specifying them. ASTM A500 is limited to sizes with peripheries less than or equal to 88 in. Anything larger than a 28-in.-OD round section cannot be specified as A500. However, rounds even that large are not produced to the A500 specification domestically. Currently, the largest A500 sections made in the U.S. are 20-in.-OD. It is worth noting that, by the end of 2021, there will be a new domestic mill producing sections up to 28-in. OD. In addition to the periphery limits, A500 also has a limit on the wall thickness. The maximum thickness of an A500 member is 1 in.
If a project requires members that exceed what is currently produced in A500, there is piping produced for other industries that can be used in structural applications, with caution. Most commonly, products that meet specifications like ASTM A252: Standard Specification for Welded and Seamless Steel Pipe Piles, which is used for pipe pile foundations, or API 5L, for the oil and gas industries, can be procured in diameters up to 80 in.
What are some of the notable differences between API 5L and ASTM A500?
• API 5L products come in many grades, denoted by “X65” or “X70,” which refers to the yield strength (i.e., X65 has a yield strength of 65,300 psi).
• While API 5L is produced in very large diameters, the thicknesses of domestically produced pipes are limited to 1 in. Imported material, especially from Asia, is available with walls exceeding 1 in. in thickness, although the availability of such products is often challenging to nail down.
• As API 5L is intended for use as pipelines in the transport of petroleum and natural gas, the tolerances and finishes that are expected for building products do not apply to API 5L products. API 5L is similar to ASTM A53 as both are hydrostatically tested. However, API 5L material is of a much higher quality as it is expected to withstand higher pressures and much higher temperatures than A53 pipe is.
• API 5L is not an approved material per the AISC Specification for Structural Steel Buildings (ANSI/AISC 360, aisc.org/specifications), as specified in Section A3.1a. However, the Commentary to this section states: “Other materials may be suitable for specific applications, but the evaluation of those materials is the responsibility of the engineer specifying them.” It is the engineer of record’s (EOR) responsibility to prove the material used conforms to an ASTM Specification specifically listed in AISC 360-16, Section A3. (The three “Unlisted Materials” articles in the October, November, and December 2018 issues, available at www.modernsteel.com, may prove useful here.)
• API 5L has two product specification levels: PSL 1 and PSL 2. PSL 1 provides a standard quality level for line pipe. PSL 2 includes additional requirements for chemical composition, fracture toughness, a maximum yield strength, and additional nondestructive testing.
• Today, the most common grade of API 5L pipe available for structural applications is Grade B or X42 (PSL 1); however, there are 40 other grades given in API 5L, many of which may be available.
• One of the additional requirements PSL 2 stipulates a yield-to-tensile-strength ratio of 0.93 maximum for Grade B and X42 up to X80. This is important, as some of the connection strengths given in Chapter K of AISC 360-16 are rooted in the ductility of the material that produces the anticipated connection deformation. The maximum yield-to-tensile ratio for the materials used in the development of AISC 360-16, Chapter K, is 0.80.
• If a large section is required, but the stringent chemical and testing requirements of the API 5L specification are not, it may be prudent to call out the section as “ASTM A500 Grade C or approved equivalent.” This allows a mill that has not gone through the rigorous certifications necessary to obtain an API license to produce the material needed for a construction application and may save significant project costs.
Is the substitution of ASTM A252 for ASTM A500 OK?
ASTM A252 is a material specification for steel pipe piles for foundations, where the steel either acts as the permanent load-carrying member or as the form for cast-in-place concrete piles. STI does not recommend the substitution of ASTM A252 for ASTM A500 unless extreme care is taken. A few items of note follow:
• ASTM A252 can be specified in one of three grades. Yield strengths vary from 30 to 45 ksi, and tensile strengths vary from 50 to 66 ksi.
• There are no chemical composition requirements in ASTM A252.
• Although A252 is frequently produced in large sections, the specification does not speak to the tolerance for sections exceeding 24 in. in diameter and/or ½ in. thick.
• The tolerances in A252 are more lenient than A500 for wall thickness, and it has no tolerance for straightness.
• Similar to API 5L, ASTM A252 is not an approved material per AISC 360-16.
If A252 is substituted, the EOR should account for its thinner wall, the lower yield strength, and the variable chemistry that may affect the members’ weldability.
What are the key considerations concerning the fabrication of round HSS?
When considering the total cost of a structure, the fabrication is a significant portion of the steel package cost. The handling of the material during fabrication is a contributor to the overall fabrication cost. It should be noted that round HSS can be more challenging to handle in the shop as they have a tendency to roll. Marking and adding pieces to quadrants at 90° to each other on a round piece is not quite as easy as it is on a rectangular section. Additionally, when connecting a round section to another round section, like in the truss shown in Figure 1, the cut necessary to connect the branch member to the chord is complex. This fabrication step has been simplified with the implementation of lasers into fabrication shops. However, without lasers, it can be quite complicated. While round sections are often the most efficient per weight, it may be more cost-effective to use a square or rectangular section to aid in the fabrication costs.
What are the key considerations concerning weldability?
The requirements for the chemical composition of the commonly specified structural steels and, in particular, the limiting values of carbon equivalents have been selected to facilitate weldability. The American Welding Society’s AWS D1.1: Structural Welding–Steel, in Table 3.1, lists prequalified steel materials and grades that have been selected because they have historically displayed good weldability. ASTM A53, A500, A1085, and API 5L Grade B, X42, and X52 are all listed as approved base metals for prequalified WPS (welding procedure specifications). Steel grades not listed in Table 3.1 may be new and have simply not been incorporated into D1.1 or, as is the case for ASTM A252, have been excluded because their mechanical properties and chemical compositions are not sufficiently defined. For these materials to be used, a special qualification test is required.
What do I need to know about weld seams?
ASTM A500 round sections are produced with a straight seam weld, with the outside weld bead scarfed, or cut smooth with the outside surface. Specifiers should be aware that other specifications for round sections, particularly large pipes, are also produced with a spiral weld. In a spiral weld, the weld seam wraps around the member like a spiral. This is of particular importance if the member is to be used in an architecturally exposed condition as an architectural element (AESS). In that case, it is likely necessary that the member should be specified on the contract documents that it shall be “straight-seam welded.” (Note that spiral-welded HSS shapes are not common in building construction and may not be readily available.)
What about the tolerances of these various materials?
All of the materials mentioned in this article have different tolerances innate to their specifications (Table 3). As noted above, it is vital to ensure that assumptions made about the material in the development of the code meet or exceed what is provided in the actual member to be used. Additionally, there are other requirements, such as straightness, that may be worth investigating if an alternate material is sourced for a project.
There are countless questions about all aspects of HSS. Hopefully, the ones we’ve covered here will give you more clarity on the nuances and benefits of designing with HSS—and possibly spark more questions. If the latter occurs, email me!
Kim Olson (firstname.lastname@example.org) is the HSS technical consultant with the Steel Tube Institute.
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