6

Recently, tube steel manufacturers in the US have begun using the new ASTM A1085 specification for forming tube steel (a.k.a. hollow structural section or HSS) shapes, as opposed to the existing A500 specification.

From what I've read (here and here), the A1085 material spec has the following benefits over the existing A500 spec:

  • tighter tolerances (no need to take reduced wall thickness for design)

  • identical yield strengths for all types of tube members (as opposed to A500, in which the yield strength differs for round and rectangular shapes and depending on the grade)

  • set maximum yield stress of 70 ksi (useful for seismic applications, though I am not well versed in this area)

  • standard Charpy V-notch requirements corresponding AASHTO Zone 2 (I assume useful for the transportation industry for fatigue reasons)

These benefits come with a small premium - I've read 7% to 10% increased cost depending on the mill.

I primarily work as a structural engineer in the nuclear industry. Most of the work I do is in services, where we perform mostly small modifications to plants (i.e., no design of large buildings for the most part, but smaller supports, platforms, and the like).

If I have no need to restrict the maximum yield strength of the material and I have no fatigue concerns, is there any benefit to specifying the newer A1085 tube steel material over the existing A500 specification? For practicing engineers in non- or light-seismic zones, have you realized any benefit using the new A1085 spec?

grfrazee
  • 3,597
  • 1
  • 18
  • 33

2 Answers2

2

Set maximum yield stress of 70 ksi (useful for seismic applications, though I am not well versed in this area)

If I have no need to restrict the maximum yield strength of the material and I have no fatigue concerns, is there any benefit to specifying the newer A1085 tube steel material over the existing A500 specification? For practicing engineers in non- or light-seismic zones, have you realized any benefit using the new A1085 spec?

While I deal primarily in the realm of structural plastics - I have had experience in high seismic zones and will say a set yield strength is of a huge benefit. The new seismic code says that if the concrete floor fails first before the steel fails, that the seismic loads need to be 2.5x higher. This will prevent the ground from ripping up under the part, possible damaging embedded piping, electrical systems, etc. Instead, if the hold down points are considered the weak point - so the structure falls over without tearing up the ground, then seismic loads are reduced drastically (A 2.5x multiplier can be HUGE).

Without this effect, I see no reason to embed a deliberate weak point into the structure. So, in light seismic zones while designing heavy storage tanks I have simply added the seismic multiplier to my anchoring design and ran with it. In heavy seismic zones, the need for a deliberate weak point becomes obvious and that weak point needs to be carefully controlled - this material sounds like it would have a distinct advantage - 10% additional cost v. 250% extra loading.

Community
  • 1
Mark
  • 5,333
  • 16
  • 47
2

AISC's magazine, Modern Steel Construction periodically publishes a guide to specifying grades of steel to help engineers stay aware of developments in the market. Their latest edition, from this February mentions A1085, but still suggests A500 Grade C as the standard. They recommend checking that A1085 is available and affordable in your area - it sounds like you already have. This may not be true in all regions.

Other than the maximum yield strength which isn't useful in your practice, there are a few other convenient features. Firstly, according to this article, it has the same mechanical properties specified regardless of product form. By contrast, A500 Gr C tube has a different Fy for square/rectangular and round tubes. Also, A500 allows actual wall thickness to be reduced by a large variance. This was intended to allow for cheaper manufacture by less accurate methods, but with modern steel mills, tubes are consistently produced undersized. Because of this, calculations require reducing the wall thickness by 7% from nominal. While these issues are not major considerations in specifying steel, they will have some appeal in simplifying the design process and reducing opportunities for error.

The standard also specifies minimum corner radii, which reduces the risk of corner cracks. Previously, AWS D1.1 and the AISC Manual have warned about the risk of corner cracks in square and rectangular A500 members subject to significant stress from welding or galvanizing. I'm not certain if this fear will go away, or just be easier to quantify if using the new standard since A500 is produced with fairly uniform radii already.

The main down side of specifying the A1085 tube is cost, as you point out. Additionally, you may not be able to count on availability right now if you are specifying projects outside of your specific area. One other down side if you do lighter work is that A1085 is not available with 1/8" wall thickness like A500 is.

In your situation, these considerations may be a wash, meaning you should continue to specify A500 to reduce costs as long as it is still available. It seems probable that the distinct advantages for seismic design will lead to widespread use of this new tube standard. If so, it will eventually become the default nationally, and you may have to switch simply because A500 becomes less readily available. In the mean time, it might be appropriate to allow tube steel in your designs to conform with either spec, since the design values are fairly similar.

Ethan48
  • 4,561
  • 19
  • 42