Cold Form Steel Construction’s Material Technology
That are cold formed steel structures…!!!
So what is cold formed steel ? •
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Cold-formed steel (CFS) is the common term for products made by rolling or pressing thin gauges of sheet steel into goods. Cold-formed steel goods are created by the working of sheet steel using stamping, rolling, or presses to deform the sheet into a usable product.
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Cold-formed steel products are just what the name connotes: products that are made by bending a flat sheet of steel at room temperature into a shape that will support more load than the flat sheet itself.
The history … •
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The use of cold-formed steel members in building construction began in the 1850s in both the United States and Great Britain. In the 1920s and 1930s, acceptance of cold-formed steel as a construction material was still limited because there was no adequate design standard and limited information on material use in building codes. One of the first documented uses of cold-formed steel as a building material is the Virginia Baptist Hospital, constructed around 1925 in Lynchburg, Virginia.
The history (continued…) •
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The walls were load bearing masonry, but the floor system was framed with double back-to-back coldformed steel lipped channels. According to Chuck Greene, P.E of Nolen Frisa Associates, the joists were adequate to carry the initial loads and spans, based on current analysis techniques. Greene engineered a recent renovation to the structure and said that for the most part, the joists are still performing well.
The history (continued…) •
A site observation during this renovation confirmed that "these joists from the 'roaring twenties' are still supporting loads, over 80 years later!" In the 1940s, Lustron Homes built and sold almost 2500 steel-framed homes, with the framing, finishes, cabinets and furniture made from cold-formed steel.
The process of making …
The benefit … As compared with thicker hot-rolled shapes, coldformed light members can be manufactured for relatively light loads and/or short spans. Unusual sectional configurations can be produced economically by cold-forming operations, and consequently favorable strength-to-weight ratios can be obtained. Nestable sections can be produced, allowing for compact packaging and shipping. Load-carrying panels and decks can provide useful surfaces for floor, roof, and wall construction, and in other cases they can also provide enclosed cells for electrical and other conduits. Load-carrying panels and decks not only withstand loads normal to their surfaces, but they can also act as shear diaphragms to resist force in their own planes if they are adequately interconnected to each other and to supporting members.
The benefit (continued …) •
Compared with other materials such as timber and concrete, the following qualities can be realized for coldformed steel structural members 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Lightness High strength and stiffness Ease of prefabrication and mass production Fast and easy erection and installation Substantial elimination of delays due to weather More accurate detailing Non-shrinking and non-creeping at ambient temperatures Formwork unneeded Termite proof and rot proof Uniform quality Economy in transportation and handling Non-combustibility Recyclable material
Typical stress–strain properties A main property of steel, which is used to describe its behavior, is the stress–strain graph. The stress–strain graphs of cold-formed steel sheet mainly fall into two categories. They are sharp yielding and gradual yielding type illustrated below in Fig.1 and Fig.2, respectively.
These two stress–strain curves are typical for cold-formed steel sheet during tension test. The second graph is the representation of the steel sheet that has undergone the coldreducing (hard rolling) during manufacturing process, therefore it does not exhibit a yield point with a yield plateau. The initial slope of the curve may be lowered as a result of the prework. Unlike Fig.1, the stress–strain relationship in Fig.2 represents the behavior of annealed steel sheet. For this type of steel, the yield point is defined by the level at which the stress–strain curve becomes horizontal. Cold forming has the effect of increasing the yield strength of steel, the increase being the consequence of cold working well into the strain-hardening range. This increase is in the zones where the material is deformed by bending or working. The yield stress can be assumed to have been increased by 15% or more for design purposes. The yield stress value of cold-formed steel is usually between 33ksi and 80ksi. The measured values of Modulus of Elasticity based on the standard methods usually range from 29,000 to 30,000 ksi (200 to 207 GPa). A value of 29,500 ksi (203 GPa) is recommended by AISI in its specification for design purposes. The ultimate tensile strength of steel sheets in the sections has little direct relationship to the design of those members. The load-carrying capacities of cold-formed steel flexural and compression members are usually limited by yield point or buckling stresses that are less than the yield point of steel, particularly for those compression elements having relatively large flat-width ratios and for compression members having relatively large slenderness ratios. Studies indicate that the effects of cold work on formed steel members depend largely upon the spread between the tensile and the yield strength of the virgin material.
Hot-rolled versus cold-rolled steel and the influence of annealing Hot rolled
Cold rolled
Yielding strength
The material is not deformed; there is no initial The yield value is increased by 15%–30% due strain in the material, hence yielding starts at actual to prework (initial deformation). yield value as the original material.
Modulus of elasticity
29,000 ksi
29,500 ksi
Unit weight
Unit weight is comparatively huge.
It is much smaller.
Ductility
More ductile in nature.
Less ductile.
Design
Most of the time, we consider only the global buckling of the member.
Local buckling, Distortional Buckling, Global Buckling have to be considered.
Main uses
Load bearing structures, usually heavy load bearing structures and where ductility is more important ( Example Seismic prone areas)
Application in many variety of loading cases. This includes building frames, automobile, aircraft, home appliances, etc. Use limited in cases where high ductility requirements.
Flexibility of shapes
Standard shapes are followed. High value of unit weight limits the flexibility of manufa cturing wide variety of shapes.
Any desired shape can be molded out of the sheets. The light weight enhances its variety of usage.
Economy
High Unit weight increases the overall cost – material, lifting, transporting, etc. It is difficult to work with (e.g. connection).
Low unit weight reduces the cost comparatively. Ease of construction (e.g. connection).
Research possibilities
In the advanced stages at present.
More possibilities as the concept is relatively new and material finds wide variety of applications.
Material properties
Cold Formed Steel Test Method •
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Mechanical Testing of Steel Products (ASTM A 370) Standard Specification for Steel, Sheet, Carbon, and High-Strength, Low-Alloy, Hot-Rolled and ColdRolled, General Requirements (ASTM A 568) Standard Specification for Steel Sheet, Carbon, Metallic- and Nonmetallic-Coated for Cold-Formed Framing Members (ASTM A 1003)
