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10/1/2003
A pipe may be substituted for an HSS provided one takes into account the significant differences in Fy. Although not a normal structural-quality steel, pipe produced to ASTM A53 can be used for structural and architectural applications. Please refer to pp 6-96 and 6-97 of the Handbook, 7th edition, situated just before the HSS tables of part 6. A round HSS in Canada is produced to the G40.21 grade 350W whereas a pipe produced to ASTM A53 has an Fy of 205 or 240 MPa! Generally, round shapes tend to be used for architectural purposes and hence are usually not selected on maximum strength but on adequate proportions for the expressed environment. That means one will unlikely have to increase thickness and/or diameter in direct proportion to the decreased Fy.
As for the difference in looks, you should know that in general, round HSS tend to be produced using a welding process, whereas pipes tend to be the result of an extrusion process. In the first case, there will be a welded seam along the HSS, whereas in the second case, the shape will be seamless. The surface of the welded HSS tends to resemble that of a rolled shape, whereas the pipe may exhibit a light texture akin to an orange peel. A welded seam will tend to be visible even after grinding, depending on the coating process used, as one can only grind perpendicular to a surface. Although there is variance, of final texture in extruded shapes and on the coating system used, the final look is likely to reflect the initial relief of the surface. All this being said, both HSS and pipe may be welded or seamless! And personally, I think both have aesthetic appeal.
Now, should I get into the difference between ASTM A500 grade C and G40.21 grade 350W issue? First of all, there are no visual differences between the two. However, please apply great caution if HSS produced to A500 is offered as a substitute. As mentioned in the same above Handbook page references, “it would be prudent to assess the influence of the differences that arise from possible differences in wall thickness and material strength”. Because there is a thickness tolerance in ASTM A500 but no weight tolerance, as is the case in G40.21 grade 350W steel, the ASTM A500 may have a lower resistance for what appears to be the same shape. So? Check the price differences. If not significant, adopt the next thickness up. Check whether the sections in question are fully utilized for maximum resistance. If you have some reserve, than an increase is unlikely necessary. This is a situation where the Canadian standard is more stringent, so just keep a technical eye open, and read a couple of our info bulletins on the subject, which were written in the 90s but are still current, technically speaking:
Hollow Structural Sections and Structural Tubing
Questions about HSS
An additional, and most useful source are pages 11-14 of a book written by an international guru on HSS matters, Dr. Jeff Packer, from the University of Toronto, in particular, Hollow Structural Section Connections and Trusses.
In terms of availability of the different sections, there is some variation, and it is not the same for different diametre ranges. Check with your local service centre for current availability. For large quantities (i.e. over 50 to 70 tonnes) an order can be placed directly to the structural tubing mill. In Canada, there are half a dozen HSS producers. For an extended list of service centres and steel mills in your region, check our members page.
With respect to prices, it would appear that the ASTM A53 pipe is more expensive, by as much as 30% in some cases as compared to HSS rounds, depending on the diameter requested. Why? I wasn't able to find an answer. Perhaps a reader can help out.
Did you know that all HSS start out round? In a recent visit to Copperweld mill in Brampton, producers of HSS, we found out that 80% of their structural tubes finish square, 17% rectangular and only 2 to 3% round! Their most popular section this year is the 10" square, followed by the 8" and 12". Obviously, there is a close relationship with availability. HSS with a diameter greater than 16" generally require special ordering. Did you also know that often, for a cost premium of about 10% for a class H HSS, you get a 20% increase in strength? Class H sections are subjected to a thermal stress relief process to gain on their class C counterparts. They are most often used for a column application, and are definitely advantageous for ductile requirements in seismic zones. Incidentally, you cannot find class H sections in the US.
Finally, according to an informal survey, there is approximately the same amount of G40.21 grade 350W used on Canadian construction sites, as ASTM A500 grade C tubes so remember our comments about substitutions! Well, I hope we've answered some of the questions you've wanted to ask about HSS and structural pipe.
This question appeared in the "Ask Dr. Sylvie" column of Advantage Steel no. 18, Fall 2003.
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