Similar to other engineering practices cold-formed steel industry also used both imperial and metric versions of the units for measurements. Imperial units are predominantly used in the North American region and Canada while the Metric units are common in places such as Australia. Often there arises confusion amongst users who use a different unit rather than the units commonly used within their region. This confusion prevails commonly while specifying the thickness of cold-formed steel members. Thickness designations in the cold-formed steel (CFS) industry are critical for ensuring proper material usage and compliance with design standards so that the sections meet the minimum thickness requirement for the design of cold-formed steel elements.
In general engineering practice, imperial units use Mils, in (inches) and Gauge to specify thickness while the metric system uses mm (millimetres) to measure and specify thickness.
The Importance of Thickness Designations in Cold-Formed Steel Design
Base metal thickness and Design thickness are two important terms used in the design of cold-formed steel members as per different country codes of practice. Framing standards such as AISI-S240 play an essential role in specifying the thickness designation for various structural components such as floor joists, clip angle etc for design purposes. Different thickness designations are explained below in detail.
Base Metal Thickness (BMT)
Base metal thickness refers to the actual, uncoated thickness of the steel sheet. It excludes any additional material from coatings like galvanization or paint. Engineers rely on this value for structural calculations since coatings do not contribute to the material’s strength. BMT is mainly used in the Australian (AS 4600) and Eurocode (EN 1993 1-3) standards for the design.
Practical Example
A steel sheet with:
- Base Metal Thickness (BMT): 0.95 mm.
- Coating Thickness: 0.05 mm (0.025 mm per side).
- Total Thickness: 1 mm.
In this case, the structural design calculations will use 0.95 mm, ignoring the coating as the coating does not affect the structural design capacities. The image below shows the thickness of the coating on the steel surface.
Source: Bluescope technical bulletin
Design Thickness
Design thickness accounts for both the base metal thickness and tolerances allowed by manufacturing processes. This value is often specified in design codes to facilitate the designer. Design thickness and Base Metal thickness are often specified in the American standards to help designers use the appropriate thickness in their design calculation. The table below shows an example of the same from AISI S240.
Benefits of providing the design thickness include the following.
- Ensures correct tolerances in designing cold-formed steel structures.
- Provides engineers with a reliable basis for design, accommodating real-world variations and ensuring the members are optimally designed.
Note: The design thickness is always equal to or slightly greater than the base metal thickness, depending on manufacturing tolerances.
Best Practices for Managing Thickness Designations
Understanding the conversion factors between imperial and metric units is crucial in choosing the correct thickness. These play a vital role in the design, procurement and manufacturing stage. Cold-formed steel governing organisations such as the American Iron and Steel Institute (AISI) and Steel Framing Industry Association (SFIA) publish technical guides and manuals where the conversion factors are readily available. Designers must ensure that these values are followed during the thickness conversion process from one unit to the other. Table showing typical thickness conversions from the SFIA technical guide is shown below.
The table shows the thickness in Mils, in (inches) and Gauge no for commonly used thicknesses within the cold-formed steel industry. Conversion from inches to millimetres can be done by multiplying inches by a factor of 25.4.
Example calculation
1 in = 25.4 mm, therefore 0.0179 in = 0.45466 mm
For convenience, all values in the table above have been converted into millimetres and are presented below for ready reference.
| Design Thickness (Mils) | Minimum Thickness (mm) | Design Thickness (mm) | Reference Gauge No. |
| 18 | 0.45466 | 0.47752 | 25 |
| 27 | 0.68326 | 0.71882 | 22 |
| 30 | 0.75184 | 0.79248 | 20-Drywall |
| 33 | 0.83566 | 0.87884 | 20-Structural |
| 43 | 1.08712 | 1.14554 | 18 |
| 54 | 1.36652 | 1.43764 | 16 |
| 68 | 1.71958 | 1.81102 | 14 |
| 97 | 2.45364 | 2.58318 | 12 |
| 118 | 2.9972 | 3.15468 | 10 |
Naming Conventions and Implications in Design and Construction
Standardising the correct naming conventions to represent thickness is important for communication between stakeholders within a project. This includes architects who arrive at the room dimensions, engineers who design the structural elements for specified thicknesses, procurement team for sourcing the correct thickness from the steel supplier, manufacturing professionals who load the correct thickness of coil into the roll formers and finally the installation team erecting the fabricated structural elements with correct thickness. Therefore, the designation of the members must be in such a way that all the stakeholders understand the importance of the naming conventions and their significance during the entire construction process.
Standards such as AISI S202 and 220 states that “structural members and non-structural members shall use a four-part product designator that identifies the size (both web depth and flange width), style and thickness. A typical example is shown below representing material thickness of a C-section.
By choosing a standardised naming convention across the industry all stakeholders are well informed and the margin of error in choosing the wrong thickness of steel reduces significantly, thereby increasing efficiency in design and construction.
Scottsdale’s Approach to Steel Thickness
Scottsdale’s bespoke cold-formed steel engineering software packages such as ScotSteel and ScotStruct offer a great range of options for our customers and end users to choose between different units at any stage of the design process. The application and project settings within the software allow the user to design using the Imperial OR Metric system giving the much-needed flexibility on international and local projects. Screenshot below shows the various options supported by ScotSteel in both imperial and metric units.
In addition, ScotSteel and the entire suite of software packages support the four-part product designator for all sections supported by Scottsdale and Knudson series of roll formers. Screenshot below shows a clear description of the section and corresponding thickness with BMT specified along with the grade of steel within ScotSteel.
Scottsdale also has ICC certifications for all the sections and the respective reports can be downloaded from the attached link – Download ICC certifications.
Conclusion: Mastering Thickness Designations
In the cold-formed steel industry, understanding the nuances of thickness designations is essential for successful project execution. Whether you’re working with mils, millimetres, gauge, or inches, knowing how to interpret and apply these measurements ensures accurate material selection, design and fabrication. Moreover, distinguishing between base metal and design thickness is critical for accurate structural design.
By integrating best practices and following the standards, industry professionals can navigate these complexities confidently, paving the way for efficient, sustainable, and safe construction practices.
