Within the construction industry there is often confusion over gauges, gauge numbers and the related thickness. The industry has been trying to move away from gauge numbers, without complete success. The following information will show why sheet steel products should be specified to the decimal thickness.
Manufacturers’ Standard Gauge For Steel Sheets
The most common gauge system used in Canada for structural sheet steel products is the Manufacturers’ Standard Gauge (MSG). The MSG for steel sheets was developed having a DEFINITE THICKNESS equivalent for each gauge number. In the standard gauge system the density of steel is taken as 489.6 lbs/ft3, or 40.80 lbs/ft2/in. However, since sheet weights are calculated on the basis of specified width and length, with all shearing on the over side, and also since sheets are somewhat thicker at the centre than they are at the edges, a further adjustment was made to obtain a closer approximation for inter-changeability between weight and thickness. Over a long period of time this value for sheets has been found to be close to 2.5 per cent heavier than 40.80 lb/ft2/in. A figure of 41.820 lb/ft2/in is the one commonly used to express the relationship between weight and thickness for steel sheet.
The Galvanized Sheet Gauge
The Galvanized Sheet Gauge is an older system used primarily by the trades consuming non-structural galvanized steel and is a measure of the zinc coated sheet thickness. It was developed in the early days of galvanizing before sophisticated wipers were available and, consequently, zinc thicknesses were thicker than today. The GSG system was used on some of the older gauge charts published years ago that unfortunately still seem to get used even today.
Thickness Definitions
There are a number of terms used for sheet steel products that need to be explained because they will affect the thickness of product that could be delivered to the job site.
Nominal Thickness: When sheet steel is produced by the steel companies it is manufactured to a target or “nominal” thickness. As with all manufacturing processes, variations in the final thickness of the sheet are unavoidable. However, the thickness is controlled very tightly to ensure that it does not fall below the minimum thickness (as described below).
Base Steel Thickness: The thickness of the sheet steel material without any coatings.
Coated Thickness: The thickness of the steel sheet including any metallic coatings (i.e. zinc or aluminum-zinc alloy) and paint coatings.
Design Thickness: The design thickness is the thickness of the base steel only, and is used by the engineer to determine the structural properties of the cold formed product. This is the thickness that a manufacturer will list in their product catalogues and load tables.
Minimum Thickness: The minimum thickness of structural sheet steel building products delivered to the job site will be the design thickness minus the maximum allowable under-tolerance specified by the CSA-S136 Standard or the material specification, whichever is the more restrictive. The minimum thickness allowed by the CSA-S136 Standard is 95% of the design thickness.
Tanks a lot. The Gauge Number And Decimal Thickness table is very clear.
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