Frequently Asked Questions

1. TAPERED TUBULAR COLUMN 
The architect would like to taper the upper portion of tall tubular columns in a large atrium area. The column base starts at 508 mm diameter and 12.7 mm thickness and tapers to 300 mm diameter in the upper 6.5 m portion. Do you have information on the design and fabrication of tapered round HSS columns? - J.E. 

You are dealing with a hybrid (non-prismatic) column! The nontapered portion can be treated as an HSS but the tapered part is a…pole (hollow truncated cone).  The round HSS 508 x 12.7 is the largest ASTM A500 HSS you will be able to find that is regularly produced in one of Atlas Tube’s plants. For the tapered portion, Professor Jeff Packer says there are specific manufacturers who specialize in cold-rolled tapered tubular poles, mainly for electricity (utility) and lighting industries. The tube is welded up from a plate cut into a trapezoid shape. A well-known producer is Valmont (www.valmont.com) but you can get lots more by just googling “Tapered Poles”. For general information have a look at the AISI website: www.steel.org under utility poles. For design information, try “Design of Steel Transmission Pole Structures”, ASCE/SEI 48-05 from www.asce.org.

2. A 100-TONNE MINIMUM 
We specified 20” diameter HSS at ⅝” thickness but were told we needed a minimum of 100 tonnes for the rolling to take place. Is this true? - R.B. 

Yes, it’s true. Unless they can piggyback your 40 tonnes with someone else’s (called tonnage accumulation), it will not work. But if you had specified ½", you could have been in the next rolling. Even if that size is on a rolling schedule, you won’t necessarily find them lying around on a steel service centre’s floor. Next time you have HSS that you want to use in reasonable quantities and that are outside the available size range given in the Handbook (greater than 16” diameter), don’t hesitate to call the CISC Marketing Director for your region. We can provide general information on availability of such shapes (with a few phone calls). In your case, it was actually possible to find ½” tubes “lying around” in one of Atlas tube’s plants and you were able to use them with some  re-engineering effort. You were then creative in converting the rest into W-shapes and providing a round architectural finish around them. But next time…. 

3. HELICAL WELDS 
In two projects, I needed 30” and 24” diameter tubular shapes respectively for architecturally exposed steel columns (just painted). In both cases, the fabricator proposed using pipes with helical welds after the tender was awarded but were refused by the architect. How common are these and should we be explicit in our tender documents? - J.C. 

Oh. I can understand why the architect wouldn’t be happy with helical welds but as you pointed out, it wasn’t explicitly stated in your tender document. As you also pointed out, ASTM A252, a piles standard, says the product can be formed either with helical or longitudinal welds. A note like “helical welds are not acceptable” should be added in the Architecturally Exposed Structural Steel (AESS) subsection of the Structural Steel section 05120 of the job specification. (Please read about CISC’s AESS documents in this issue. You might have wanted to create your own custom category, or AESS C, for the columns.)

In summary, up to 20” diameter HSS at ½” thickness, ASTM A500 is available. Above that, you join the ranks of pipes, piles and poles, each with their own standards. It is then up to the engineer to decide whether these are acceptable for the project.
  
· In situations where large-diameter or thick tubes are required, many fabricators elect to use API-grade pipes (where API stands for American Petroleum Institute) after close discussion with the engineer. Specifically, this type of large tubing is covered  by ANSI/API Spec 5L which is actually a modified American adoption of ISO 3183: 2007, available for purchase at www.techstreet.com. API sections are commonly available from steel distributors in the southern U.S. However, please note that interior pressure requirements have nothing to do with a structural application.

Alfred Wong adds that some of the production requirements of ASTM A252 piles deviate from those governing the production of CSA G40.21 HSS, including tolerance for straightness, wall thickness and other imperfections. The helical seam in an A252 pile may be a butt seam or a lap seam. These characteristics, among other things, must be accounted for in the structural design.

In the case of poles, don’t forget that, unlike most poles, columns support substantial axial loads.  

Finally, fabricators tell me there can be surface issues with some of these larger tubes; apparently more so with seamless tubes or pipes which are typically less smooth than cold-formed tubes.