Types of cold-formed steel sections:

Cold-formed steel elements are having types of stiffened or unstiffened. The stiffened elements obtained by an element supported by webs along both longitudinal edges. While the unstiffened element is obtained when the element supported along any one longitudinal edge only and the other edge can have the displacement.

The intermittently stiffened element is made by thin element divided into two or more narrow sub-elements and this is prefabricated by the introducing intermediate stiffeners in the rolling process. If the stiffener is adequate then, in that case, the edge stiffened element will be treated as the local buckling coefficient value of 4.0. While if the edge found to be inadequate or partially adequate then, in that case, its effectiveness is disregarded and the element is regarded as unstiffened.

 Local Buckling in cold-formed steel sections:

The local buckling is an important aspect in case of the cold-formed steel elements, the thin steel elements are used and it buckles before the yielding process occurs. As the plate becomes thinner than lower shall be the load for which element buckles.

When one edge of the steel plate is free to move while the other edge is supported then the plate buckles at considerably lower load and the value of ‘K’ reduces to 0.425. This property shows that the plates with the free edges could not perform properly under local buckling. To avoid the local buckling of CFS sections, the free edges are provided with a lip and that will be constrained to remain straight and shall not be able to free to move.

 Behavior after post – critical analysis of CFS:

While considering the channel section when subjected to the uniform bending by the moment application at the end of cold-formed steel section. In this case, the thinner plate at the top layer will be under flexural compression and buckles. Such kinds of buckling sill have the ripples on the top layer of the section and along the length of the section. The top portion of the plate is supported along the edges and its central portion and this is far from the support location, will deflect and distributes the load on the stiffer edges. The regions near edges are prevented to deflect up to the same extent. Therefore the applied moment is largely resisted by the regions near edges. The region located near the centre is stressed a little bit and therefore less effective in resisting the applied moment. If the theoretical point of view is considered then the flat plates will buckle very soon at the elastic critical load.

The plate elements which are not flat perfectly will be deforming out of the plane under the incremental loading if applied. A nonuniform state of stress will exist all across the loading region of the cold-formed steel sections.

The effect of local buckling will be evaluated with the use of the concept of effective width. The least effective section in resisting the applied stresses is a very low stressed region at the center of the element. The regions near supports are very effective and considered to be fully effective. The effective width of the cold-formed section under compression is dependent on the width/thickness ratio, support conditions, and applied stress.