The following Questions and Answers Numbers 1, 2 and 3 were sent as a letter to all Structural Engineers, Architects, Fabricators, Building Officials and Educators on the CISC Mailing list on September 5, 1995. Question and Answer Number 4 were added when the original letter was posted to the Internet, June, 1996. The information contained herein is still current and applicable.
RE: Hollow Structural Sections
Hollow structural sections (HSS) have been used extensively in Canada for many years. Two of their most common applications are as chord and web members in steel trusses, and as columns. Designers normally specify that HSS meet the requirements of the Canadian Standard CAN/CSA G40.21-M92 "Structural Quality Steels" (or G40.21-92 for projects using Imperial units of measure). G40.21 in turn references CAN/CSA G40.20-M92 "General Requirements for Rolled or Welded Structural Quality Steel" (or G40.20-92 for Imperial units).
This letter addresses enquiries we have been receiving with increasing frequency, about HSS.
Question 1 - Are HSS produced in accordance with ASTM Specification A500 grade C equivalent to HSS meeting CSA Standard G40.21 grade 350W (or 50W in the Imperial system)?
Answer 1 - No. HSS produced in accordance with the requirements of CSA Standard G40.21 must meet the requirements of Table 31 in CSA Standard G40.20. This Table specifies that the mass (or weight in Imperial units) shall not deviate from the published mass by more than -3.5%. By comparison, ASTM A500 permits deviation in the wall thickness of up to -10%. Studies show that significant reductions in the compressive strength of HSS, in comparison with the design values shown in the CISC Handbook of Steel Construction, occur when the wall thickness is reduced beyond the CSA limits. The Handbook values are based on published dimensions.
In addition, CSA standard G40.21, grade 350W specifies an ultimate tensile strength range of 450 to 620 MPa. ASTM A500 grade C specifies only a minimum ultimate tensile strength of 427 MPa. There are also differences in the chemical requirements and elongation requirements listed in ASTM A500 in comparison with CSA G40.21.
As far as round HSS is concerned, ASTM A500 grade C specifies a minimum yield point of 317 MPa, rather than the 350 MPa specified in CSA G40.21.
Clause 5.1.1 of Canadian Standard CAN/CSA S16.1-94 "Limit States Design of Steel Structures" refers to CSA Standard G40.21 for acceptable steel material and products for use with S16.1. Accordingly, if HSS produced in accordance with ASTM A500, grade C are offered as a substitute for HSS specified to meet G40.21, grade 350W, it would be prudent to take into account the possible differences in wall thickness and strength.
Question 2 - Should I specify Class H or Class C, CSA Standard G40.21, 350W?
Answer 2 - It depends. For small jobs, Class C should be specified. For larger jobs, if the higher compressive strength available with Class H can be used effectively, it may be suitable. Designers should check with a local CISC Member Fabricator before making a decision to specify Class H sections.
Class H sections are either hot-formed to final shape, or cold-formed to final shape and then stress-relieved. Class C sections are cold formed and are not stress relieved. Thus, due to the difference in methods of manufacture, Class H sections have a more favourable residual stress pattern. Accordingly, Class H sections can be designed for a larger axial compressive resistance than Class C sections, in accordance with Clause 13.3.1 of CAN/CSA S16.1-94 "Limit States Design of Steel Structures".
Class C sections are the sections normally available in stock, in steel service centres, for example. Class H sections usually require a special mill order, and are more expensive than Class C sections. Thus, if the tonnage of a single size of HSS is large enough to justify a mill order directly from the manufacturer, and the increase in strength offsets a higher cost of material, Class H sections may be the preferred alternative. Otherwise, Class C sections are likely to be a more suitable choice.
Question 3 - Is there any relationship between Class C sections of CSA Standard G40.21 and grade C material of ASTM specification A500?
Answer 3 - No, none whatsoever.
Question 4 - What is the difference between the 1987 and the 1992 editions of CSA Standards G40.20, with respect to wall thickness tolerances for hollow structural sections?
Answer 4 - Table 31 of CSA Standard G40.20-92 "General Requirements for Rolled or Welded Structural Quality Steel" states that "the actual weight of an individual length of hollow structural section shall not deviate from the published weight by more than -3.5% or +10%". The metric version of the standard says the same thing, substituting the word "mass" for the word "weight". Table 31 in the 1987 edition of the CSA G40.20 Standard has exactly the same requirement. This requirement has existed in the relevant standard since it was first published in 1969 as clause 6.1 in CSA Standard G40.15 "General Requirements for Delivery of Welded or Seamless Hollow Structural sections", which was one of a group of steel material standards published by CSA prior to the initial publication of CSA Standards G40.20 and G40.21 in 1973.
The only difference between the 1987 and the 1992 editions of CSA Standard G40.20, as far as hollow structural sections are concerned, is a change in table 32. In the 1992 edition, this table states that "wall thickness shall not deviate by more than -5% or +10% from the nominal wall thickness specified, ....". In the 1987 edition, the maximum deviations permitted were -10% or +10%.
In spite of this change, certification of hollow structural sections in accordance with the requirements of CSA Standard G40.20 with regard to dimensional and weight (or mass) requirements requires that the sections conform to the specified maximum tolerance for weight per foot (or mass) of table 31, which is the same in both the 1987 and the 1992 editions of the standards.
Hugh A. Krentz, P.Eng.
For further information, see: Letter of 5 November 1996