Tube Steel and Standard Steel Pipe Options for a Column for a Shop

In an earlier article we determined the necessary `W Shape’ steel column size for a shop. The load on the column, from the roof, was not particularly large, as steel goes (47,000 pounds), but the height of the column was 25 feet, making it potentially unstable. We found that the W 8 x 31 in A992 50 ksi steel gave us the strength and stiffness (resistance to buckling) we needed to safely carry the load. The `name’ of the shape betrays a couple of its important characteristics, namely that it is about 8 inches across (deep) and weighs 31 pounds per linear foot. In this article we will find alternate steel columns for the same application, in particular, using hollow tube steel (HSS), and Standard Steel Pipe.

The design approach will be the same as for the W Shape; we will investigate a trial size member, determine the allowable axial stress, then the total allowable axial load, and check it against the applied load (47k). We will then pick a larger, or smaller, as needed, section, until we find the lightest weight option that (`just’) safely carries the load.

Square Tube Steel Column

Let’s first get a tube steel column. The steel industry uses the term HSS for tube steel, and the preferred specification is Fy = 46 ksi.

Since tube steel is about the same strength as the W shape (46 ksi versus 50), let’s look for a tube steel section with about the same area, A, as did the W Shape (A = 9.13 in.²).

From Table A5 of the Onouye text, the 9 x 9 x 0.25 has A = 8.59 in.² and r = 3.56 in.

The slenderness ratio is kL/r = 84, which is less than 200, good!

Cc = sqrt ( 2 π² 29,000 / 46 ) = 112.

For this trial size, then, we use E2-1 to calculate Fa.

Fa = [ 1 – (kL/r)² ] Fy / [ 5/3 + 3 (kL/r) / 8 Cc – (kL/r)³ / 8 Cc³ = … 17.3 ksi.

P allowable = Fa x A = 17.3 ksi x 8.59 in.² = 149 k.

This is way more than we need (149k versus 47 needed). Note that the r is much greater for the HSS, driving up the Fa. The greater r reflects the steel being distributed farther from the center of the column, making it more stable. The more stable the column is, other things equal, the greater the allowable stress, and thus, allowable load.

Let’s look for a smaller HSS section.

Try 6 x 6 x 3/8 …

This section is not in the Onouye text; thus we will go to a more robust table of HSS section information, provided by the Steel Tube Institute;

A = 8.08, r = 2.30

kL/r = 130

Cc = 112, again

kL/r is greater than Cc, so use E2-2

Fa = 12 π² E / 23 (kL/r)² … 8.8 ksi.

P allowable = 8.8 x 8.08 = 71k … still way good.

Let’s go smaller still!!!

Let’s try 6 x 6 x 47/71 (3/8) thick … 6 x 6 x 0.25 …

Gonna be close!

For the 6 x 6 x ¼ … A = 5.24, r = 2.34.

kL/r = 128.

Fa = 9.1 ksi from E2-2.

P allowable = 9.1 x 5.24 = 48k. Perfect!

And it weighs … 19 pounds per foot. The total weight is 19 plf x 25 = 475 lb … about 0.5k.

Any possibility of being able to go any smaller?

What about the 5-1/2 x 5-1/2 x 3/8? … or 5-1/2 x 5-1/2 x 5/16? Even if they are strong enough, they weigh more, so, no!

So, 6 x 6 x ¼ in. 46ksi tube steel.

Note that when we were looking at W shapes we were in the 30 pounds per foot range, so, the tube steel is more efficient in these conditions (even though its unit strength is not quite as much).

Steel Pipe Column

The preferred yield strength specification for Steel Pipe is 35 ksi.

Let’s try 6 in. diameter x Standard Weight (similar `size’ as the tube).

From Table A5, Onouye text, A = 5.58, r = 2.25, weighs 18.97 pounds per foot.

kL/r = 133.

Cc turns out to be 128, on account of the different Fy.

Thus we use E2-2, which gives … Fa = 8.4 ksi.

P allowable = 8.4 x 5.58 = 47k. Whoa! PERFECT!

And it weighs the same as the 6 x 6 x ¼ tube steel.

So, for this application, we find the 6 x 6 x ¼ tube steel (HSS) and the 6 in. Standard Pipe sections also safely carry the 47k axial load. The tube and pipe sections both weigh about 19 pounds per foot. The lightest section in W Shapes to carry the load we found to be the W 8 x 31, weighing about 50% more. In this application the tube steel and steel pipe win out over the W section.

References

Design of a Steel Column for a Shop, Jeff Filler, Yahoo! Contributor Network.

Statics and Strength of Materials for Architecture and Building Construction, Fourth ed., Onouye, B., and K. Kane, Pearson Education / Prentice Hall, Upper Saddle River, N.J., 2012.

Manual of Steel Construction – Allowable Stress Design, American Institute of Steel Construction, Chicago, IL.

Hollow Structural Sections – Dimensions and Section Properties, Steel Tube Institute of North America, Glenview, IL.