Minimum thickness of uncoated steel sheet

Archive of Older Topics Archive - Product Discussions #2
1

Does anyone have the definitive table for minimum thickness in decimals of an inch of uncoated steel sheet?

Prior to 1970, sheet steel was referred to by gage. ASTM and ANSI currently do not list gage numbers in their standards. Gage (or guage) is ingrained in many vacabularies and is misunderstood as a term of thickness. I use the minimum thickness published by NAAMM which are taken from the Underriters Laboraties, Inc. publication for gage and equivalent thickness.

Minimum thicknesses specified in AIA MasterSpec, published in SMACNA Architectual Sheet Metal Manual - 5th Edition, and NAAMM are at variance with one another. For example, the decimal equivalent of 22 gage (guage) is 0.026" by NAAMM, 0,0299" by SMACNA Appendis A-1, Chapter 9, and 0.0217" by AIA MasterSpec Section 076200_FL.

I guess I could quote the source for my uncoated steel sheet thickness in my spec.

What are others doing?

2

One of the reasons you are finding differing minimums is that there never was any definitive standard for gage. Thus, different organizations and industries had their own determinations of the actual thickness of a given gage. The “conversions”, if you can call them that, are going to be different for each industry. Also, make sure you look at nominal versus minimum thicknesses.

3

The gauge debacle should be recognized as the scandalous dereliction of responsibility that it is. Surely some broad-based industry organization – ASTM, CSI, NIST – should have years ago compiled the definitive cross-reference tabulation of all the various gauge standards for various metals, sheet and wire, coated and uncoated, and the metric and American fractional and decimal equivalents for each, along with a definitive guide to which standard is used for each industry and class of products. I don’t care how “gauge” or “gage” is spelled, but someone should have stepped up to the plate. Who will?

4

And the tolerances…forgot to mention the tolerances, which one seldom sees listed anywhere. As you can see, when it comes to gauge, mine is wearing thin.

5

As a former steel cook in Bethlehem’s open hearths at Sparrow’s Point - and one who haunted all the other mills I could get into on that campus - I can tell you that the final thickness of rolled steel is not a precise quantity.

Actualy, in steel sheet (as opposed to thicker plate) the tolerances are amazingly tight. But the range of thickness variation will still vary from machine to machine, plant to plant, and country to country.

Yes, we need a US industry standard. It could be based on average thickness within a range, but practically speaking what design engineers and construction administrators need is a minimum figure. Nothing less than; no matter how much more than.

As we slowly lurch into SI, it would be wise for all parties to get together and come up with minimum mm thicknesses for each weight of metal. In the end, some plants would have to adjust their habits.

An average and an upper limit could be agreed on also, but it’s the minimum thickness that we need to do our work.

6

Recently, I waded through the gauge to decimal value issue. Just about each group working with a metal has its own thickness measurement.

For steel doors, I use ANSI/HMMA or ANSI/SDI standards. Both documents discourage the use of gauge and list a minimum thickness conversion chart based on published figures from ULI.

For cold formed metal framing, I use the conversion chart in the Steel Stud Manufactuer’s Association (SSMA) ICC-ER-4943P report. It uses a minimum thickness value. It states the basis for it conversion as Section A3.4 of the 1996 AISI Specification.

For sheet metal, I use the charts in the SMACNA Manual. SMACNA uses an approximate thickness rather than a minimum thickness. SMACNA bases its converion on ASTM A 924 to the extent that the Manufacturers Standard Gauge exists.

In each spec section, I identify the appropriate source for the conversion. For my architects and the CA information, I put the gauge in (). From what I’ve read, the use of gauge has been discouraged and most orders for metal products are in a decimal or mil form, particularly for imported metals.

7

Much of the problem would be resolved if we lurched toward “metrification” instead of away from it. I believe that the the ASTM standards for various sheet metals give thicknesses (and tolerances) in decimal inch thickness and metric (decimal mm) thickness as well.

Older versions of the sheet steel ASTM standards had published a chart of decimal thickness correlating to MSG (manufacturer’s standard gauge); however, it was located in the appendix and provided for information only.

If you review an older copy of Architectural Graphic Standards (mine is 3rd. ed., and no, it was not mine in college) and look up wire gauge in the index, you will see a table with colums for “U. S. Standard”, “Washburn & Moen, Roebling American Steel & Wire Co.”, “Brown and Shape (B&S)”, and “Circular Mills”. The table notes that U. S. Standard is generally used for “steel, iron, stainless steel, monel, plates and sheets”. Washburn is used for steel wire, B&S for coper wire & nickel silver, and Circular Mills for copper wire in place of B&S. My recollection is that the table for MSG in the ASTM document and the column for U. S. Standard gauge in this table indicated slightly different thickness for sheet steel gauge.

When you delve into a bit more, you find gauges for sheet steel and galvanized sheet steel, leaving one to wonder if the thickness of sheet steel for 20 ga. material is the same as galvanized steel sheet. I have always wondered how all of this evolved and what would ever be done to sort it all out.

MasterSpec’s standard for referencing sheet metal thickness is congruent with ASTM’s standard, but most of the published product literature (steel doors, lockers, cabinets, toilet partitions, studs, etc.) persists in using gauge. Many times I am not quite sure what the thickness actually is (or should be). Most Drawing annotations I see from our office (and other design professionals’ offices) also use gauge.

I have sometimes done what Ms. Brown does, putting the gauge in parentheses behind the decimal for convenience of use, but I am not always comfortable with it.

8

“most of the published product literature (steel doors, lockers, cabinets, toilet partitions, studs, etc.) persists in using gauge”

That’s just my point. How many of them even name the gauge system they use? The actual thickness represented by gauges is a mess, if not a complete mystery, and no one - NO ONE - has shown any leadership in resolving the problem. I sometimes think manufacturers prefer to keep their “real” thickness values obscure.

Every equivalency table I’ve seen - and I’ve seen a bunch - leaves something to be desired. For example, the tables in the 2000 Architectural Graphic Standards are from the Industrial Perforators Association. There are separate tables for steel, galvanized steel, “long terne” (whatever that is), “stainless-USS gauge,” and Monel. But none of them list metric equivalents! Moreover, it devotes a whole page to a detailed “Guide for Selecting Carbon Steel for Perforating Applications” – but it lists thickness in gauges only, not dimensions!

Until we know (with some degree of confidence) what the “real” thickness of various gauges is, how can we be expected to convert them to metric? Perhaps the US Metric Association (USMA), the American National Metric Council (ANMC), the Construction Metrication Council (CMC) and other such organizations are as guilty as anyone for the stonewalling.

I suspect those in a position to deal with the problem (ASTM, CSI, NIST, USMA, ANMC, CMC, etc.) just hope gages will go away if they wait long enough. But gauges won’t go away until accurate, uniform, comprehensive and authoritative standards for thickness and equivalency are readily available. “Lurching toward metrification” just ain’t gonna be complete until this happens.

9

Steel sheet has not been manufactured in “gauge” thicknesses since, I believe, 1970. Every time the Masterspec Architectural Review Committee reviews a section involving a sheet metal material (and that’s a lot of them), we have to try really hard NOT to reopen this discussion, since there’s really not anything new to say about it.

Lets look at hollow metal as an example. The Steel Door Institute has long ago stopped using gauge. NAAMM newer guide specifications use decimal inches. So, its not to be blamed on the industry organizations, or Masterspec, or the steel manufacturers–it falls squarely on the individual door and frame manufacturers who will not conform in their marketing literature. These are companies that claim to manufacture products that conform to one or both of these very standards. And ironically, their purchasing departments are buying the raw materials in decimal inches, because they have to.

I would also argue that architects abet the perpetuation of use of gauge by continuing themselves to use it. (Yes, I am know the argument that the CA folks will not know what to do because the submittal will be in gauge. I say REJECT IT!)

10

I understood ‘gage’ or ‘guage’ was a response to the number of (draws) or rolling operations necessary to achieve a marketable, defined product. Since that incorrectly implies there is no relationship with the progress of technology (eg production efficiency has improved), gage or guage is entirely superfluous and counterproductive when finite agreements are intended.

Even in the old days, gage or guage was highly dependent on the manufacturer’s production processes. Therefore they had some internal system (a gage or guage) as a physical model of what their machines were supposed to produce for a given number of (passes). I’m not saying 30 gage or guage literally meant 30 passes for any given material.

So, use decimal inches (until we accept metric). Gage or guage is for railroad tracks and locomotives, not metal products.

I think we get inappropriate reinforcement because no electrician goes around asking for “sixteen-thousandths inch drawn copper wire conductor”.

11

Thanks to all who posted in this thread.

From this time forward, I will follow the procedure highlighted by Linda in her last paragraph, however I will use the NAAMM steel table for minium thicknesses.

Currently, I am in the fortunate postion of not having to specify light gage metal framing or hollow metal doors and pressed steel frames. At my previous office, I did spec the new steel stud designations of SSMA.

At my current employer, I specify a lot of sheet metal flashings and trims for low slope & steep slope roofing and wall flashings used in rainscreen cladding systems (stucco, fiber-cement siding, vinyl siding, etc).

Wayne

12

In another thread (Specifications/Metric Conversion), Phil Kabza mentions Arcom’s “MasterMetric,” a 44-page document that appears to have the comprehensive coverage of gages I have sought. Up until now, it may have been one of MasterSpec’s best-kept secrets.

It is still deplorable that it is proprietary, not an openly available standard promulgated by the NIST.

13

Just found eFunda: Design Standards - click on gages - it appears to be free for the first visit and for a set time, but for a quick look, it’s good. Click on wire gages or sheet metal gages. Even better, I found http://www.rollformedshapes.com/gagechart.htm - a free chart - uncoated gage reference to thickness with tolerance range.

14

just to confuse the issue, take into account that different manufacturers have different thicknesses for their standard thickness. I’ve been caught a few times by specifying a decimal thickness, and finding out that three different steel stud manufacturers have three different interpretations of what that nominal thickness would be.

15

I was sitting through a box lunch presentation from National Gypsum on UL Rated Assemblies (an excellent presentation!) when a slide was shown on minimum bare metal thickness for steel studs. There are 2 tables in the UL Fire Resistance Directory (“Design Information Section,” Article IV, paragraph 3).

The first table is for load bearing studs and references ASTM C 955 (from 20 to 14 gauge). These second table is for non-load bearing studs and is based on the 1997 UBC (from 25 to 16 gauge). The first table gives the thickness for 20 gauge studs as 0.0329 in. while the second indicates 0.033 inches; OK, rounding. However, the thickness indicated in the tables are 0.0428 and 0.044 inch for 18 gauge and 0.055 inches and 0.0677 for 16 gauge. A few pages back is a gauge/minimum thickness for steel deck which has altogether different values (0.034 in. for 20 ga., 0.045 in for 18 ga., and 0.057 for 16 ga.) based on AISI specifications.

I am not sure what edition the information on the slide was from; I looked it up in our 2003 edition (can’t find the more current volumes).

While I am not surprised that different industries would have different interpretations and slight variations in “guage” thickness (e.g., metal lockers, steel doors, steel flashing), I am surprised to learn that UL sanctions 2 different standards for essentially the same product.

You might want to crosscheck metal thicknesses for steel studs in fire-resistive assemblies, expecially in areas where building code officials actually read the specifications.

And they talk about building science…

16

“Gage” is still an active term in the mechanical contracting world. The California Building Codes still refer to gage thickness of metal. The 2001 Mechanical Code includes a table which list the nominal thickness and recommended minimum thickness of bare steet sheets and galvanized. Reference is listed as ASTM A-568-74 Table 3 & Table 23, & ASTM A-525-79 Table 4 for hot-dip. I don’t think the industry will change unless SMACNA does.

17

“‘Gage’ is still an active term…”

Right. But California Codes have to cite references that are 29 and 24 years out of date, respectively (the latter of which was withdrawn entirely in a major consolidation* in 2003). Though the current (2003?) version of A568 uses both “gage” and “gauge,” it does not appear to use either word in the sense of sheet thickness, and it does not include a table of gage equivalents.

*(See http://64.233.187.104/search?q=cache:L_2XWyAohZ8J:www.astm.org/SNEWS/JANUARY_2003/mahmon_jan03.html+"ASTM.org"+"A+525"&hl=en)

18

Jim,

ASTM A525 was withdrawn in 1994 - no replacment. ASTM A568-04 does not referenece gage thickness; it uses decimal thickness.

If California code is using reference standards as old as you say, I doubt that the manufacturers are offering products that actually meet the old standard.

19

http://ci.temecula.ca.us/cityhall/fire/code/html/PDF/Appendix_VI-K.pdf (Code gage/inches table.)

These are very good points… quite informative and thought provoking.

The California Fire Code and Mechanical Code list complete thickness tables for comparison of “gage” and “inches”, and reference the table of thicknesses as corresponding to the (applicable) code requirement. Therefore they have conclusively closed the circle. But you are correct; the references are still there…