5 November 1996
TO: Structural Engineers, Architects, Fabricators, Building Officials and Educators
RE: Hollow Structural Sections and Structural Tubing
Hollow structural sections (HSS) have been used extensively in Canada for many years. Two of their most common structural 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 inquiries we have been receiving about possible substitutions of structural tubing produced in accordance with ASTM Specification A500 for HSS specified to meet the CSA Standard. A previous letter dated 5 September 1995 dealt with the differences between ASTM Specification A500 Grade C and CSA Standard G40.21, Grade 350W.
Question - May structural tubing produced in accordance with ASTM Specification A500 be substituted for HSS of the same nominal size and nominal wall thickness produced in accordance with CSA Standard G40.21 Grade 350W (or 50W in the Imperial system)?
Answer - No, not unless approved by the designer or the engineer of record after a detailed engineering review. As explained in the letter of 5 September 1995, there are significant differences between these product standards, such as the specified minimum yield and tensile strengths and tolerances on mass and wall thickness, that must be accounted for in the structural evaluation. These differences can affect the structural capacity of the HSS, such as axial, net section, and connection resistances.
The following table provides general guidance to assist in the selection of a trial substitute section for compression members and tension members in statically determinate structures. The adequacy of substitute sections chosen with the aid of this table must be verified by the designer or the engineer of record.
In preparing the table, it was assumed that the structure has been designed in accordance with the requirements of CAN/CSA Standard S16.1-94 and that CAN/CSA G40.21-92, Class H or Class C HSS have been specified. The table is applicable only to possible substitutions using ASTM Specification A500 Grade C material, and is based on the assumption that the actual wall thickness of the substituted HSS is 10% less than the nominal thickness, as permitted by the ASTM Specification, but that out-to-out cross-sectional dimensions are the same as the originally specified HSS. In all cases, the A500 HSS must have a nominal wall thickness greater than the G40.21 HSS member for which the substitution is being proposed in order to have the same nominal resistance. For compression members designed as Class H, the maximum difference between the column curve prescribed by CSA Standard S16.1-94 for Class H HSS and the basic column curve, has been assumed for this table.
For guidance on the evaluation of the connection resistances using the substituted HSS, please refer to "Design Guide for Hollow Structural Section Connections" by J.A. Packer and J.E. Henderson.
Suggested Increase in Nominal Thickness for A500, Grade C, Structural Tubing When Substituted for:
CSA G40.21, grade 350W, Class C, HSS
Squares and Rectangles
For compression members, 13%
For tension members, 18%
Rounds
For compression members, 24%
For tension members, 24%
Suggested Increase in Nominal Thickness for A500, Grade C, Structural Tubing When Substituted for:
CSA G40.21, grade 350W, Class H, HSS
Squares and Rectangles
For compression members, 52%
For tension members, 18%
Rounds
For compression members, 55%
For tension members, 24%
Notes:
1) 13% and 24% increases account for differences in Fy and thickness.
2) 18% increase accounts for differences in Fu and thickness.
3) 52% and 55% increases account for differences in Fy, thickness and the maximum difference between respective column curves.
Additional engineering evaluations may also be required for beam-columns, beams, joints and connections, and trusses.
For members whose ductile behaviour is a design consideration, such as HSS braces in ductile and nominally ductile concentrically braced frames that are designed for seismic loads, the effects on connections due to the increase in nominal thickness and the change in the ratio of Fy to Fu must be considered.
In conclusion, do not substitute ASTM A500 tubes of the same nominal dimensions for CSA G40.21, 350W HSS specified in projects without a detailed review by the engineer of record. ASTM A500 tubes should not be assumed to have equivalent structural capacity.
CISC assumes no responsibility for the adequacy of substitute HSS selected with the aid of this Table.
Yours truly,
Hugh A. Krentz, P.Eng.
President