Steel products shaped by cold-working processes
Cold-formed steel (CFS) is the common term for steel products shaped by cold-working processes carried out near room temperature, such as rolling, pressing, stamping, bending, etc. Stock bars and sheets of cold-rolled steel (CRS) are commonly used in all areas of manufacturing. The terms are opposed to hot-formed steel and hot-rolled steel.
Cold-formed steel, especially in the form of thin gauge sheets, is commonly used in the construction industry for structural or non-structural items such as columns, beams, joists, studs, floor decking, built-up sections and other components. Such uses have become more and more popular in the US since their standardization in 1946.
Cold-formed steel members have been used also in bridges, storage racks, grain bins, car bodies, railway coaches, highway products, transmission towers, transmission poles, drainage facilities, firearms, various types of equipment and others. These types of sections are cold-formed from steel sheet, strip, plate, or flat bar in roll forming machines, by press brake (machine press) or bending operations. The material thicknesses for such thin-walled steel members usually range from 0.0147 in. (0.373 mm) to about ¼ in. (6.35 mm). Steel plates and bars as thick as 1 in. (25.4 mm) can also be cold-formed successfully into structural shapes (AISI, 2007b).
History.
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 walls were load bearing masonry, but the floor system was framed with double back-to-back cold-formed 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. 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.
History of AISI design standards.
Design standards for hot-rolled steel (see structural steel) were adopted in 1930s, but were not applicable to cold–formed sections because of their relatively thin steel walls which were susceptible to buckling. Cold-formed steel members maintain a constant thickness around their cross-section, whereas hot-rolled shapes typically exhibit tapering or fillets. Cold-formed steel allowed for shapes which differed greatly from the classical hot-rolled shapes. The material was easily workable; it could be deformed into many possible shapes. Even a small change in the geometry created significant changes in the strength characteristics of the section. It was necessary to establish some minimum requirements and laws to control the buckling and strength characteristics. Also it was observed that the thin walls underwent local buckling under small loads in some sections and that these elements were then capable of carrying higher loads even after local buckling of the members.
In the United States, the first edition of the Specification for the Design of Light Gage Steel Structural Members was published by the American Iron and Steel Institute (AISI) in 1946 (AISI, 1946). The first Allowable Stress Design (ASD) Specification was based on the research work sponsored by AISI at Cornell University under the direction of late Professor George Winter since 1939. As a result of this work, George Winter is now considered the grandfather of cold-formed steel design. The ASD Specification was subsequently revised in 1956, 1960, 1962, 1968, 1980, and 1986 to reflect the technical developments and the results of continued research at Cornell and other universities (Yu et al., 1996). In 1991, AISI published the first edition of the Load and Resistance Factor Design Specification developed at Missouri University of Science and Technology and Washington University in St. Louis under the directions of Wei-Wen Yu and Theodore V. Galambos (AISI, 1991). Both ASD and LRFD Specifications were combined into a single specification in 1996 (AISI, 1996).
In 2001, the first edition of the North American Specification for the Design of Cold-Formed Steel Structural Members was developed by a joint effort of the AISI Committee on Specifications, the Canadian Standards Association (CSA) Technical Committee on Cold-Formed Steel Structural Members, and Camara Nacional de la Industria del Hierro y del Acero (CANACERO) in Mexico (AISI, 2001). It included the ASD and LRFD methods for the United States and Mexico together with the Limit States Design (LSD) method for Canada. This North American Specification has been accredited by the American National Standard Institute (ANSI) as an ANSI Standard to supersede the 1996 AISI Specification and the 1994 CSA Standard. Following the successful use of the 2001 edition of the North American Specification for six years, it was revised and expanded in 2007.
This updated specification includes new and revised design provisions with the additions of the Direct Strength Method in Appendix 1 and the Second-Order Analysis of structural systems in Appendix 2.
In addition to the AISI specifications, the American Iron and Steel Institute has also published commentaries on various editions of the specifications, design manuals, framing design standards, various design guides, and design aids for using cold-formed steel. For details, see AISI website.
International codes and standards.
The United States, Mexico and Canada use the North American Specification for the Design of Cold-Formed Steel Structural Members, document number AISI S100-2007. Member states of the European Union use section 1-3 of the Eurocode 3 (EN 1993) for the design of cold formed steel members. Other nations utilize various design specifications, many based on AISI S-100, as adopted by the building codes listed below. Another list of international cold-formed steel codes and standards is maintained (and can be edited with permission) at Cold-Formed Steel Codes Around the World.
Africa.
Ethiopia
Building Codes:
EBCS-1 Basis of design and actions on structures
EBCS-3 Design of steel structures
Americas.
United States
Specification: North American Specification for the Design of Cold-Formed Steel Structural Members, document number AISI S100-2007 published by the American Iron and Steel Institute in October 2007.
Building Code: IBC and/or NFPA may be enforced, but both reference AISI S100.
Canada
Specification: North American Specification for the Design of Cold-Formed Steel Structural Members, document number CAN/CSA S136-07 as published by Canadian Standards Association.
Building Code: The National Building Code of Canada is the model code adopted with amendments by individual Provinces and Territories. The Federal government is outside the jurisdiction of the Provincial/Territorial authority but usually defers to the legislated requirements within the Province/Territory of the building site.
Brazil
Specification: NBR 14762:2001 Dimensionamento de estruturas de aço constituídas por perfis formados a frio - Procedimento (Cold-formed steel design - Procedure, last update 2001) and NBR 6355:2003 Perfis estruturais de aço formados a frio - Padronização (Cold-formed steel structural profiles, last update 2003)
Building Code: ABNT - Associação Brasileira de Normas Técnicas (www.abnt.org.br)
Chile
NCH 427 - suspended because it was written in the 1970s. Cold-formed steel sections were based in part on AISI (U.S). The local Institute for Building code INN has specified in recent Codes for seismic design that designers must use the last edition of the AISI Specification for cold formed steel and the AISC for hot rolled, in their original versions in English until some traduced adaption will be issued here .
Argentina
CIRSOC 303 for Light Steel Structures where cold formed steel is included. That Specification, now more than 20 years old, is being replaced by a new one, which will be, in general, an adaption of the current AISI one. The former CIRSOC 303 was an adaption of the Canadian code of that time. At this time CIRSOC 303 was very old, now CIRSOC 301 is in revolution to be aligned with the American codes (LRFD design). In the near future both codes will be aligned also in designations and terminology.
Asia.
Philippines
National Structural Code of the Philippines 2010, Volume 1 Buildings, Towers, and other Vertical Structures, Chapter 5 Part 3 Design of Cold-Formed Steel Structural Members is based on AISI S100-2007
India
Specification: IS:801 and IS:811, Indian standard code of practice for use of cold-formed light gauge steel structural members in general building construction, Bureau of Indian Standards, New Delhi (1975). (currently under revision)
Building Code : see - model code National Building Code of India
China
Specification: Technical Code of Cold-formed Thin-wall Steel Structures
Building Code: GB 50018-2002 (current version)
Japan
Specification: Design Manual of Light-gauge Steel Structures
Building Code: Technical standard notification No.1641 concerning light-gauge steel structures
Malaysia
Malaysia uses British Standard BS5950, especially BS5950:Part 5; AS4600 (from Australia) is also referenced.
Europe.
EU Countries
Specification: EN 1993-1-3 (same as Eurocode 3 part 1-3), Design of steel structures - Cold formed thin gauge members and sheeting. Each European country will get its own National Annex Documents (NAD).
Germany
Specification: German Committee for Steel Structures (DASt), DASt-Guidelines 016: 1992: Calculation and design of structures with thin-walled cold-formed members; In German
Building Code: EN 1993-1-3: 2006 (Eurocode 3 Part 1-3): Design of steel structures – General rules – Supplementary rules for cold-formed members and sheeting; German version
prEN 1090 2: 2005 (prEN 1090 Part 2; Draft): Execution of steel structures and aluminium structures – Technical requirements for the execution of steel structures; German version
EN 10162: 2003: Cold-rolled steel sections – Technical delivery conditions – Dimensional and cross-sectional tolerances; German version
Italy
Specification:
UNI CNR 10022 (National Document)
EN 1993-1-3 (Not compulsory)
United Kingdom
Eurocode for cold-formed steel in the UK. BS EN 1993-1-3:2006: Eurocode 3. Design of steel structures. General rules.
Oceania.
Australia
Specification: AS/NZS 4600
AS/NZS 4600:2005 Similar to NAS 2007 but includes high strength steels such as G550 for all sections. (Greg Hancock)
Building Code: Building Code of Australia (National document) calls AS/NZS 4600:2005
New Zealand
Specification: AS/NZS 4600 (same as Australia)
Common section profiles and applications.
In building construction there are basically two types of structural steel: hot-rolled steel shapes and cold-formed steel shapes. The hot rolled steel shapes are formed at elevated temperatures while the cold-formed steel shapes are formed at room temperature. Cold-formed steel structural members are shapes commonly manufactured from steel plate, sheet metal or strip material. The manufacturing process involves forming the material by either press-braking or cold roll forming to achieve the desired shape.
When steel is formed by press-braking or cold rolled forming, there is a change in the mechanical properties of the material by virtue of the cold working of the metal. When a steel section is cold-formed from flat sheet or strip the yield strength, and to a lesser extent the ultimate strength, are increased as a result of this cold working, particularly in the bends of the section.
Some of the main properties of cold formed steel are as follows:
A broad classification of the cold-formed shapes used in the construction industry can be made as individual structural framing members or panels and decks.
Some of the popular applications and the preferred sections are:
The AISI Specification allows the use of steel to the following ASTM specifications in the table below:
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 cold-reducing (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