Cold-Formed Steel Design
Requires: Design Level
- Assumptions and Limitations
- Design Parameters
- Custom Cold-Formed Shapes
- Missing Component Error
VisualAnalysis designs light-gauge cold-formed steel members used for beams, columns, braces, wall components, headers, etc. VisualAnalysis cold-formed design checks are performed with a built-in, specially licensed version of the CFS 8.0 program from RSG Software (www.rsgsoftware.com). Checks are performed according to the following specifications:
- 2012 NAS/AISI (ASD and LRFD)
- 2012 NAS/Mexico (ASD and LRFD)
- 2012 NAS/Canada LSD
- 2010 NAS/AISI USA (ASD and LRFD)
- 2010 NAS/AISI Mexico (ASD and LRFD)
- 2010 NAS/AISI Canada (LSD)
- Shapes are chosen from a database from several manufacturers.
- Optional inclusion of cold work of forming effects.
- Provides flexible bracing options.
- Custom cold-formed shapes (see below)
Assumptions and Limitations
- Carbon steel materials are assumed to comply with the 'Applicable Steels' listed in the AISI design specification. E = 29,500 ksi during design checks!
- The effects of holes are ignored for shear strength calculations.
- Design checks assume there are no shear stiffeners.
- Lateral braces are assumed to comply with AISI requirements.
- For sections that have web holes, the holes are assumed to be circular.
- For elements with holes under a stress gradient, the effective width is determined by treating the elements adjacent to the hole as unstiffened elements.
- When a member is used as part of a stud wall system, the design check does not include the strength contribution of the sheathing.
- Web-crippling checks are not performed.
- Connections are not designed or checked.
- Checks are limited to shapes found in the IES Shape Database that are also associated with a '.scl' shape library file. See also: Cold-Form Customizations
- Currently there is no way to "double-up" members. You may either model and check the members independently, or you will need to provide custom member shapes for the IES shape database.
- For 2010 specifications, Lm (distortional brace length) is conservatively assumed to be 240 inches, there is no way to set it in VisualAnalysis, you would need to check in CFS directly to use a different value.
- There is currently no support for stainless steel design (ASCE 8) in VisualAnalysis even though the CFS program has this feature.
When you create a Design Group for cold-formed steel design, you need to specify the design parameters that will control the unity checks and design of members. These parameters are available in the Project Manager under the Modify tab. Cold-form parameters include the group name, bracing, size constraints, and shape category. In addition, there are parameters that are specific to cold-formed steel.
Effective Length Factors
The effective length for buckling about each axis will vary depending upon member end conditions and will be calculated automatically. Effective length factors can be manually specified if desired. Kz is for buckling about the strong axis, Ky is for buckling about the weak axis, The subscripts refer to the member element's section-coordinate system, where x is the longitudinal axis, z is the strong bending axis, and y the weak bending axis. These parameters are ignored if there is no axial force in the member.
Cb factors are calculated automatically, but may be overridden. Note that VisualAnalysis calculates these coefficients based on the moments on a single member element, so if the physical beams or columns are broken into several elements the Cb factors may not be calculated correctly. Use the Combined Member feature in VisualAnalysis to help the design software calculate these factors more accurately, or override them if they are incorrect.
Moment magnification factors depend on the stiffness of the frame and the end moments on the member. These Cm factors are calculated automatically, but can be overridden here.
Moment Reduction Factor
This is required by the code in some situations. See AISI code section C3.1.3. The value must be greater than or equal to zero and less than 1.0. There are also times when it does not apply, in which case this option will be grayed out. Note that unsymmetrical sections, such as eave struts, require continuous bracing in all directions to be properly designed.
Strength Increase for Cold-Working
You may optionally use of higher material strength values due to the strain hardening that occurs in steel during the cold-working process.
By default, member unbraced length is the equal to the member length between nodes. See Bracing in VisualAnalysis for a complete description of bracing member elements for design checks.
Torsional unbraced length defaults to the larger (less conservative) of the top and bottom flange unbraced length. The idea here is that a lateral brace also restrains twisting of the cross section.
Double-click on a member in the Design View to get a detailed report of the checks that were made. Report column headings are described below:
Member Name The name of the member being checked.
Load Case # The index of the load case from which the analysis results used in making the check were obtained. There is a table at the top of design reports which associates load case indices with the actual name of the load case.
Offset The location along the member length at which this check was made.
Unity Check The ratio of applied to allowable load for this check. A check value greater than one indicates failure.
Notes One or more flags indicating that there is extra information regarding the check. The particular information associated with a given flag is shown in the "Notes" section below.
The checks correspond to code provisions as shown below:
Combined axial & bending check – Equation 1
ASD Tension/Bending AISI C5.1.1-1
ASD Compression/Bending AISI C5.2.1-1
LRFD Tension/Bending AISI C5.1.2-1
LRFD Compression/Bending AISI C5.2.2-1
Combined axial & bending check – Equation 2
ASD Tension/Bending AISI C5.1.1-2
ASD Compression/Bending AISI C5.2.1-2
LRFD Tension/Bending AISI C5.1.2-2
LRFD Compression/Bending AISI C5.2.2-2
Combined shear & bending check
ASD AISI C3.3.1-1
LRFD AISI C3.3.2-1
The report may include warnings, errors, or just notes about special conditions that were observed during the checks.
Custom Cold-Formed Shapes
It is possible to use VisualAnalysis for the design of custom cold formed shapes, or for designing shapes that were not originally included in the IES Shape Database. You will need to have the shapes defined in a .scl file, which is a shape library file produced by the CFS program from RSG Software (www.rsgsoftware.com). The .scl file you create or obtain may be imported directly into the database using the IES ShapeBuilder. These custom shape files need to be stored in specific locations with other IES data on your machine in order for VisualAnalysis to find them and use them.
- American Iron and Steel Institute, Cold-Formed Steel Design Manual, various editions.
- RSG Software, Inc., 2803 NW Chipman Road, Lee's Summit, MO 64081. Mr. Bob Glauz, owner. IES licenses the CFS 'engine' from RSG Software for cold-form checks. The full CFS product is available for purchase for more advanced uses such as creating .scl files for new shapes to be added to the database. Contact RSG at www.rsgsoftware.com or 816-524-5596.
Missing Component Error
The CFS design module requires a Microsoft component that is no longer installed with Windows 7 or 8 or 10. You may download the Microsoft Visual Basic runtime 5.0 'MSVBVM50.exe' file:
Be sure to use Run As to run this Microsoft installer on the same Windows account that is used to run the IES software product!