Cold formed steel

This article provides an overview of cold-formed steel as a construction material. The use of cold-formed steel in commercial, industrial and residential buildings is summarized from a historical perspective, including the chronological development of design standards by the American Iron and Steel Institute (AISI).The material properties of cold-formed steels, including chemical composition, yield stress, ductility, and weldability are summarized. The article concludes with a comparison of cold-formed steel to hot-rolled steel.

Introduction

Cold-formed steel (CFS) members have been used in buildings, bridges, storage racks, grain bins, car bodies, railway coaches, highway products, transmission towers, transmission poles, drainage facilities, 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 or 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 of AISI specifications

The use of cold-formed steel members in building construction began in the 1850s. 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). This 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 [1] since 1939. It was revised subsequently 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 University of Missouri of Rolla and Washington University under the directions of Wei-Wen Yu [2] 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 successfully 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 [3] website.

International codes and standards

A list of international cold-formed steel codes and standards is currently being compiled by engineers around the world [4].

History of cold-formed steel

The use of cold-formed steel members in building construction began in the 1850’s in both the United States and Great Britain. In the 1920’s and 1930’s, acceptance of cold formed steel as a construction material was still limited because there was no adequate design standard and limited information on the use of material in building codes. One of the first documented uses of cold-formed steel as a building material is the Virginia Baptist Hospital[5], 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 [6], the joists were adequate to carry the initial loads and spans, based on current analysis techniques. Green 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 this: “These joists from the “roaring twenties” are still supporting loads, over 80 years later!” In the 1940’s, Lustron Homes built and sold almost 2500 steel-framed homes, with the framing, finishes, cabinets and furniture made from cold-formed steel.

History of design standards for cold formed steel

Design standards for hot-rolled steel were adopted in 1930’s, 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 that it could be deformed into any possible shape. Even a small change in the geometry created significant changes in the strength characteristics of the section and it was necessary to establish some minimum requirements and laws to control the situation. Also it was observed that the thin walls underwent local buckling under small loads in some sections and that these elements in were capable of carrying loads even after the local buckling of the members.

Since 1946 the use and the development of thin-walled cold-formed steel construction in the United States have been accelerated by the issuance of various editions of the ‘‘Specification for the Design of Cold-Formed Steel Structural Members’’ of the American Iron and Steel Institute (AISI). The earlier editions of the specification were based largely on the research sponsored by AISI at Cornell University under the direction of George Winter[7] since 1939. It has been revised subsequently to reflect the technical developments and the results of continuing research.

Common section profiles and applications of cold formed steel

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:

• Lightness in weight
• 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
• No formwork needed
• Termite-proof and rot proof
• Uniform quality
• Economy in transportation and handling
• Non combustibility
• Recyclable material
• Panels and decks can provide enclosed cells fro conduits.

A broad classification of the cold-formed shapes used in construction industry can be made as individual Structural framing members or panels and decks.

Some of the popular applications and the preferred sections are:

• Roof and wall systems (industrial, commercial, and agricultural buildings)
• Steel racks for supporting storage pallets
• Structural members for plane and space trusses
• Frameless Stressed skin structures: Corrugated sheets or sheeting profiles with stiffened edges are used it form small structures up to a 30ft clear spam with no interior framework

The AISI Specification allows the use of steel to the following ASTM specifications in the table below:

Steel Designation	ASTM Designation	Product	Yield Strength Fy (ksi)	Tensile Strength Fu (ksi)	Fu / Fy	Minimum Elongation (%) in 2-in. Gage Length
Carbon structural steel	A36		36	58-80	1.61	23
	A36		50	70	1.4	21
High-strength low-alloy Structural steel	A242		46	67	1.46	21
Low and intermediate tensile strength carbon steel plates	A283
	A		24	45-60	1.88	30
	B		27	50-65	1.85	28
	C		30	55-75	1.83	25
	D		33	60-80	1.82	23
Cold-formed welded and seamless carbon steel structural tubing in rounds and shapes	A500	Round Tubing
	A		33	45	1.36	25
	B		42	58	1.38	23
	C		46	62	1.35	21
	D		36	58	1.61	23
		Shape Tubing
	A		39	45	1.15	25
	B		46	58	1.26	23
	C		50	62	1.24	21
	D		36	58	1.61	23
High-strength carbon–manganese steel	A529 Gr. 42		42	60-85	1.43	22
	A529 Gr. 50		50	70-100	1.40	21
Hot-rolled carbon steel sheets and strips of structural quality	A570
	Gr. 30		30	49	1.63	21
	Gr. 33		33	52	1.58	18
	Gr. 36		36	53	1.47	17
	Gr. 40		40	55	1.38	15
	Gr. 45		45	60	1.33	13
	Gr. 50		50	65	1.30	11
High-strength low-alloy columbium– vanadium steels of structural quality	A572
	Gr. 42		42	60	1.43	24
	Gr. 50		50	65	1.30	21
	Gr. 60		60	75	1.25	18
	Gr. 65		65	80	1.23	17
High-strength low-alloy structural steel with 50 ksi minimum yield point	A558		50	70	1.40	21
Hot-rolled and cold-rolled high-strength low-alloy steel sheet and strip with improved corrosion resistance	A606	Hot-rolled as rolled cut length	50	70	1.40	22
		Hot-rolled as rolled coils	45	65	1.44	22
		Hot-rolled annealed	45	65	1.44	22
		Cold-rolled	45	65	1.44	22
Hot-rolled and cold-rolled high-strength low-alloy columbium and/or vanadium steel sheet and strip	A607 Class I
	Gr.45		45	60	1.33	Hot rolled (23) Cold rolled (22)
	Gr.50		50	65	1.30	Hot rolled (20) Cold rolled (20)
	Gr.55		55	70	1.27	Hot rolled (18) Cold rolled (18)
	Gr.60		60	75	1.25	Hot rolled (16) Cold rolled (16)
	Gr.65		65	80	1.23	Hot rolled (14) Cold rolled (15)
	Gr.70		70	85	1.21	Hot rolled (12) Cold rolled (14)
	A607 Class II
	Gr.45		45	55	1.22	Hot rolled (23) Cold rolled (22)
	Gr.50		50	60	1.20	Hot rolled (20) Cold rolled (20)
	Gr.55		55	65	1.18	Hot rolled (18) Cold rolled (18)
	Gr.60		60	70	1.17	Hot rolled (16) Cold rolled (16)
	Gr.65		65	75	1.15	Hot rolled (14) Cold rolled (15)
	Gr.70		70	80	1.14	Hot rolled (12) Cold rolled (14)
Cold-rolled carbon structural steel sheet	A611
	A		25	42	1.68	26
	B		30	45	1.50	24
	C		33	48	1.45	22
	D		40	52	1.30	20
Zinc-coated or zinc-iron alloy-coated steel sheet	A653 SS
	Gr. 33		33	45	1.36	20
	Gr. 37		37	52	1.41	18
	Gr. 40		40	55	1.38	16
	50 Class 1		50	65	1.30	12
	50 Class 3		50	70	1.40	12
	HSLAS Type A
	50		50	60	1.20	20
	60		60	70	1.17	16
	70		70	80	1.14	12
	80		80	90	1.13	10
	HSLAS Type B
	50		50	60	1.20	22
	60		60	70	1.17	18
	70		70	80	1.14	14
	80		80	90	1.13	12
Hot-rolled and cold-rolled high-strength low-alloy steel sheets and strip with improved formability	A715
	Gr. 50		50	60	1.20	22
	Gr. 60		60	70	1.17	18
	Gr. 70		70	80	1.14	14
	Gr. 80		80	90	1.13	12
55% aluminum-zinc alloy-coated steel sheet by the hot-dip process	A792
	Gr. 33		33	45	1.36	20
	Gr. 37		37	52	1.41	18
	Gr. 40		40	55	1.38	16
	Gr. 50A		50	65	1.30	12
Cold-formed welded and seamless high-strength, low-alloy structural tubing with improved atmospheric corrosion resistance	A847		50	70	1.40	19
Zinc-5% aluminum alloy-coated steel sheet by the hot-dip process	A875 SS
	Gr. 33		33	45	1.36	20
	Gr. 37		37	52	1.41	18
	Gr. 40		40	55	1.38	16
	50 Class 1		50	65	1.30	12
	50 Class 3		50	70	1.40	12
	HSLAS Type A
	50		50	60	1.20	20
	60		60	70	1.17	16
	70		70	80	1.14	12
	80		80	90	1.13	10
	HSLAS Type B
	50		50	60	1.20	22
	60		60	70	1.17	18
	70		70	80	1.14	14
	80		80	90	1.13	12

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.

Fig.1 (Wei Wen Yu, 2000)

Fig.2 (Wei Wen Yu, 2000)

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.2, the stress-strain relationship in Fig.1 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. The exceptions are bolted and welded connections, the strength of which depends not only on the yield point but also on the ultimate tensile strength of the material. 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.

Ductility criteria

Ductility is defined as ‘‘an extent to which a material can sustain plastic deformation without rupture.’’ It is not only required in the forming process but is also needed for plastic redistribution of stress in members and connections, where stress concentration would occur. The ductility criteria and performance of low-ductility steels for cold-formed members and connections have been studied by Dhalla, Winter, and Errera at Cornell University. It was found that the ductility measurement in a standard tension test includes local ductility and uniform ductility. Local ductility is designated as the localized elongation at the eventual fracture zone. Uniform ductility is the ability of a tension coupon to undergo sizeable plastic deformations along its entire length prior to necking. This study also revealed that for the different ductility steels investigated, the elongation in 2-in. (50.8-mm) gage length did not correlate satisfactorily with either the local or the uniform ductility of the material. In order to be able to redistribute the stresses in the plastic range to avoid premature brittle fracture and to achieve full net-section strength in a tension member with stress concentrations, it is suggested that:

• The minimum local elongation in a - 1–2 in. (12.7-mm) gage length of a standard tension coupon including the neck be at least 20%.
• The minimum uniform elongation in a 3-in. (76.2-mm) gage length minus the elongation in a 1-in. (25.4-mm) gage length containing neck and fracture be at least 3%.
• The tensile-strength-to-yield-point ratio Fu /Fy be at least 1.05.

Weldability

Weldability refers to the capacity of steel to be welded into a satisfactory, crack free, sound joint under fabrication conditions without difficulty . Welding is possible in cold formed steel elements, but it shall follow the standards given in AISI S100-2007, Section E.

1.When thickness less than or equal to 3/16” (4.76mm):

The various possible welds in cold formed steel sections, where the thickness of the thinnest element in the connection is 3/16” or less are as follows

• • Groove Welds in Butt joints
  • Arc Spot Welds
  • Arc Seam Welds
  • Fillet Welds
  • Flare Groove Welds

2.When thickness greater than or equal to 3/16” (4.76mm):

Welded connections in which thickness of the thinnest connected arc is greater than 3/16” (4.76mm) shall be in accordance with ANSI/AISC-360. The weld positions are covered as per AISI S100-2007 (Table E2a).

Minimum material thickness recommended for welding cold-formed steel (CFS) connections

Application	Shop or Field fabrication	Electrode method	Suggested minimum CFS thickness
CFS to Structural steel	Field-fabrication	Stick-welding	54 mills to 68 mills
CFS to Structural steel	Shop-fabrication	Stick-welding	54 mills to 68 mills
CFS to CFS	Field-fabrication	Stick-welding	54 mills to 68 mills
CFS to CFS	Field-fabrication	Wire-fed MIG (Metal Inert Gas) welding	43 mills to 54 mills
CFS to CFS	Shop-fabrication	Wire-fed MIG (Metal Inert Gas) welding	33 mills

Application of cold-formed steel in buildings

Cold-formed steel framing

Cold-formed steel framing (CFSF) refers specifically to members in light-frame building construction that are made entirely of sheet steel, formed to various shapes at ambient temperatures. The most common shape for CFSF members is a lipped channel, although “Z”, “C”, tubular, “hat” and other shapes and variations have been used. The building elements that are most often framed with cold-formed steel are floors, roofs, and walls, although other building elements and both structural and decorative assemblies may be steel framed.

Although cold-formed steel is used for several products in building construction, framing products are different in that they are typically used for wall studs, floor joists, rafters, and truss members. Examples of Cold-Formed Steel that would not be considered framing includes metal roofing, roof and floor deck, composite deck, metal siding, and purlins and girts on metal buildings.

Framing members are typically spaced at 16 or 24 inches on center, with spacing variations lower and higher depending upon the loads and coverings. Wall members are typically vertical lipped channel “stud” members, which fit into unlipped channel “track” sections at the top and bottom. Similar configurations are used for both floor joist and rafter assemblies, but in a horizontal application for floors, and a horizontal or sloped application for roof framing. Additional elements of the framing system include fasteners and connectors, braces and bracing, clips and connectors.

In North America, member types have been divided into five major categories, and product nomenclature is based on those categories.

• S members are lipped channels, most often used for wall studs, floor joists, and ceiling or roof rafters.
• T members are unlipped channels, which are used for top and bottom plates (tracks) in walls, and rim joists in floor systems. Tracks also form the heads and sills of windows, and typically cap the top and bottom of boxed- or back-to-back headers.
• U members are unlipped channels that have a smaller depth than tracks, but are used to brace members, as well as for ceiling support systems.
• F members are “furring” or “hat” channels, typically used horizontally on walls or ceilings.
• L members are angles, which in some cases can be used for headers across openings, to distribute loads to the adjacent jamb studs.

Cold-formed steel framing categories (CFSF); AISI S201-07 Product Data Standard

In high-rise commercial and multi-family residential construction, CFSF is typically used for interior partitions and support of exterior walls and cladding. In many mid-rise and low-rise applications, the entire structural system can be framed with CFSF.

Connectors and fasteners in cold-formed steel framing

Connectors are used in cold-formed steel construction to attach members (i.e. studs, joists) to each other or to the primary structure for the purpose of load transfer and support. Since an assembly is only as strong as its weakest component, it is important to engineer each connection so that it meets specified performance requirements. There are two main connection types, Fixed and Movement-Allowing (Slip). Fixed connections of framing members don’t allow movement of the connected parts. They can be found in axial-load bearing walls, curtain walls, trusses, roofs, and floors. Movement-Allowing connections are designed to allow deflection of the primary structure in the vertical direction due to live load, or in the horizontal direction due to wind or seismic loads, or both vertical and horizontal directions. One application for a vertical movement-allowing connection is to isolate non-axial load bearing walls (drywall) from the vertical live load of the structure and to prevent damage to finishes. If the structure is in an active seismic zone, vertical and horizontal movement-allowing connections may be used to accommodate both the vertical deflection and horizontal drift of the structure.

Connectors may be fastened to cold-formed steel members and primary structure using welds, bolts, or self-drilling screws. These fastening methods are recognized in the American Iron and Steel Institute (AISI) 2007 North American Specification for the Design of Cold-Formed Steel Structural Members, Chapter E. Other fastening methods, such as clinching, power actuated fasteners (PAF), mechanical anchors, adhesive anchors and structural glue, are used based on manufacturer’s performance-based tests.

Hot-rolled versus cold-rolled steel and the influence of annealing

		Hot Rolled	Cold Rolled
Material Properties	Yielding Strength	The material is not deformed; there is no initial strain in the material, hence yielding starts at actual yield value as the original material.	The yield value is increased by 15%-30% due to prework (initial deformation).
	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 manufacturing 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.

Annealing, also described in the earlier section, is part of the manufacturing process of cold-formed steel sheet. It is a heat treatment technique that alters the microstructure of the cold-reducing steel to recover its ductility.

Conclusion

A study of the history and various properties of cold-formed steel that has been done provide useful information to be compared with the properties of hot-rolled steel sections. The characteristics of cold-formed steel makes it unique and widely applicable and economically feasible. There are more possibilities of cold-formed sections in medium rise buildings and short spans, which need not require heavy sections. It can cater to the majority of such requirements more economically than hot rolled sections.

References

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