Cold-formed steel (CFS) refers to steel products shaped through cold-working procedures performed at or near room temperature, including rolling, pressing, stamping, and bending. Cold formed steel beam sections are usually categorized as slender sections with their design being governed by local buckling modes.
In this article we will discuss the design of CFS beams under pure bending only using AISI S100. The main standard for the design of cold-formed steel structural members in North America is AISI S100. It’s important to note that the complete design of a cold-formed steel (CFS) beam also includes shear checks, web crippling, and bending and shear interactions, which will be discussed in a separate post.
Watch: Cold forming process
Factors influencing design of CFS beams under bending
Three limit states: local, distortional and global buckling control the design of CFS beams under bending
Lateral torsional buckling in cold formed steel beam design
A CFS beam under flexure can fail by global lateral torsional buckling based on the critical moment. For values of critical moment greater than 2.78 times the yielding moment, AISI S100 also allows to use inelastic strength to resist flexure. Lateral torsional buckling (LTB) is the deformation of an unconstrained beam caused by stresses applied away from its longitudinal axis, which includes both lateral displacement and twisting. Unrestrained steel beams are beams whose compression flange is free to move (or displace) laterally and rotate.
Distortional torsional buckling in cold formed steel beam design
Distortional buckling involves both rotation and translation at the cross-sectional corners. This is detected as a distortion of the cross section when one component of the section is “forced out” by a more rigid reaction of the remaining part.
Local buckling in cold formed steel beam design
Local buckling is the phenomenon in which the web of a steel beam or compression flange experiences buckling deformation in a specific localized location rather than being broad. After the appearance of local buckling, the buckled section may further reach to a yielding state. That is why AISI S100 consider local buckling interaction with yield in design
How to calculate strength corresponding to different buckling modes?
There are two ways to compute them:
- Effective width method
- Direct strength method
In this article we have explained Direct Strength Method only as it can be widely applied to most types of section shape.
Direct Strength Method
The Direct Strength Method (DSM) estimates the strength of a cold-formed member by taking into account the member’s buckling loads in local, distortional, and global buckling. The DSM, unlike the Effective Width Method, requires a cross-section elastic buckling analysis rather than calculating the effective section characteristics. Elastic buckling analysis can be carried either manually or via computer-aided numerical analysis [ref].
There are various software tools available to perform buckling analysis. CUFSM developed by John Hopkins University is one such tool, which provides efficient means of computing critical loads for different buckling modes.
Do you consultation of CFS design, get in touch with me here.
Design of CFS beam under bending using AISI S100
Following flow chart explains step by step process of computing design strength of a cold formed steel (CFS) beam under bending (pure flexure). Section F2 of AISI S100 is used to extract various relation shown in this flow chart.
