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Cold Formed Steel Sections

Courses > Structural Steel Design > General Topics of Steel Material and Design > Cold Formed Steel Sections


Introduction on Cold Formed Steel Sections :

Cold-formed steel (CFS) section is the term used for products which are made by rolling or pressing thin gauges of steel sheets into goods. CFS goods are created by the working of thin steel sheets using stamping, rolling or presses to deform the steel sheets into a proper product which are usable. In the construction industry of steel, both the structural as well as the nonstructural parts are formed using the thin gauges of steel sheets. The building materials can be of columns, studs, beams, floor decking, built up sections and other any parts of the structure. CFS construction materials differ in many respects than other steel construction materials like hot rolled steel. The manufacturing of CFS products occurs at the room temperature with the use of rolling/pressing. The buckling property is used to analyze the strength of elements. The construction practices are just like the timber framing where the assembling stud frame using the screws.

The applications of CFS members includes the buildings, bridges, car bodies, storage tanks, highway products, railway coaches, transmission towers, drainage facilities, etc. The thin-walled steel members are of having the thicknesses usually having 0.4 mm to 7 mm. The steel plates and steel bars of thickness 25.4 mm can be made into the cold formed structural shapes. The yield strength of steel sheets used in the CFS construction process should be of at least 280 MPa as there is a trend to use the steel sheets of more yield strength.

There is a change in the mechanical properties of steel material due to the cold working material as it is formed using pressing. The yield strength and ultimate strength of the steel section which is formed from steel plates/sheets are increased. The coating of zinc or galvanizing is made to protect the cold-formed steel sections and this provides the protection against the corrosion in the environment. The thickness of 0.04 mm coating is usually made for internal environment. If the moisture is present for a longer period of time then the thicker coat is provided for more protection.

In comparison to the hot rolled section with the cold rolled sections, CFS have more moment of inertia and section modulus in x-direction and y-direction, therefore the load carrying capacity and moment resisting capacity are higher.

 


Concepts and Formulas of Cold Formed Steel Sections:

 

Advantages of Cold-formed steel members:

There are many advantages of the cold-formed steel members as compared to other members are follows:

  1. Members are light in weight.
  2. Sections have High strength and stiffness.
  3. The erection and installation procedure is very easy.
  4. The conventional connection methods like riveting, bolting or welding can be used.
  5. The construction process does not need any kind of formwork.
  6. The desired shape and desired length are obtained by prefabrication process.
  7. The transportation cost is lower as the lightweight sections are available.
  8. The recycling of this type of material is possible easily.
  9. The termite-proof and rot proof sections are prefabricated.
  10. As compared to the hot rolled section the greater moment carrying capacity is possible due to the thinner wall sections

 

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.

 


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