An Overview of Cold-Formed Steel Structures

In building construction, there are primarily 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 or strip material. The manufacturing process involves forming the material by either press-braking or cold roll-forming to achieve the desired shape. Examples of the cold-formed steel are corrugated steel roof and floor decks, steel wall panels, storage racks and steel wall studs.

Press-braking is often used for production of small quantity of simple shapes. Cold roll-forming is the most widely used method for production of roof, floor and wall panels. It is also used for the production of structural components such as Cees, Zees, and hat sections. Sections can usually be made from sheet up to 60 inches (1.5m) wide and from coils more than 3,000 feet (1,000m) long.

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.

A shape that is cold worked to a higher degree will have a higher yield strength, which may make it more appropriate for certain applications. Sometimes cold work is added by overbending and straightening. The end result is that a more complex shape may have higher strength than a less complex shape. Other factors that can affect cold working include roll pressure, corner radius and the properties of the steel.

Depending on the type of cold-forming and the thickness of the steel, the amount of strength increase could reach 20% to 50%. However, the ductility of the steel is reduced as result of cold-forming process.

Generally, the grades of carbon steel and high strength low alloy steel used for cold-formed steel products are characterized by two main properties: the yield point and the tensile strength. Other important properties are ductility, hardness and weldability.

The yield point of the steels commonly used for cold-forming ranges from 33 to 55 ksi (230 to 380 MPa), and may be higher. Tensile strength and ductility are important because of the way they relate to formability, and because of the local deformation demands of bolted and other types of connection. In members that include bolted connection or that, because of special design, may be subject to high stress concentrations, the tensile strength often must be taken into account. The ratio of tensile strength to yield strength for cold-formed steels commonly ranges from 1.2 to 1.8. However, steels with a lower ratio can be used for specific applications.

The differences between cold-formed and hot-rolled steel are not just in the thickness and the shapes. Since cold-formed steel members are formed at room temperature, the material becomes harder and stronger. Its lightweight makes it easier and more economical to mass-produce, transport and install.

One of the main differences between designing with cold-formed steel shapes and with hot-rolled structural shapes is that with the hot-rolled, one is primarily concerned about two types of instability: column buckling and lateral buckling of unbraced beams. The dimensions of hot-rolled shapes are such that local buckling of individual constituent elements generally will not occur before yielding.

This is not the case with cold-formed members. Here local buckling must also be considered because, in most cases, the material used is thin relative to its width. This means that the individual flat, or plate, elements of the section often have width to thickness ratios that will permit buckling at stresses well below the yield point.

There are two types of thin, compressed sheet elements. One type is stiffened by other components along both longitudinal edges. Such elements are called stiffened compression elements, and you see them in the compression flange of the C-section beam, or in all four faces of the box column. The other type is called unstiffened compression elements that are stiffened only along one of the two longitudinal edges. The other edges remain free. These elements appear in the compression flange of the channel section beam, and in all flanges of the I-section column. The behavior of these two types of elements is similar, though not identical.

The concept of effective width is the key to the design of cold-formed steel. Considering the element as the compression flange of some member (see Figure 6), the total compressive force is the area under this stress distribution curve, times the thickness of the element. What is needed in design is only this total compressive force. It is convenient to replace the actual variable stress distribution with a fictitious uniform stress distribution, of the same intensity of the edge stress, f, as in the real element as indicated by the dotted lines.

In order to get the same total compression force in the fictitious (dotted) as in the real (solid) distribution, the areas under the two must be equal. This means adjusting the width of each of the two fictitious rectangles, b/2, until the combined area of the two rectangles is equal to that under the solid curve. This width is known as the effective width, b.

Then, effective section properties, such as area, section modulus, and moment of inertia, can be calculated by using for each compression element its effective width, b, instead of its real width, w.

The basic equation for the effective width, b, are the result of very extensive research, and are shown below:

When a compression component has stiffeners on along two parallel edges, it is considered as a fully stiffened or partially stiffened section depending on the stiffness of the stiffeners. When a compression component has the stiffener along one edge only, it is considered as an unstiffened section. The flanges of C-shaped steel studs are examples of fully or partially stiffened sections while the flanges of U-shaped steel track are examples of unstiffened sections.

Cold-formed shapes can be used for entire buildings and for complete roof, floor and wall systems. They can also be used as individual framing members such as studs, joists and truss members.

From structural standpoint, the cold-formed steel can serve as both primary structures and secondary structures. An example of the cold-formed steel used as primary structures is the steel stud load-bearing wall. Steel studs providing the lateral support to exterior wall finish are usually secondary structures since they rely on the primary structure for support.

Floor decks, roof decks and wall systems are the prime applications for cold-formed steel in panel form. These are made in both simple, or single-component types, as well as more complex units made up of several elements.

Permanent metal deck forms are fabricated from lightweight steel in thickness that vary from 0.020 to 0.075 inches (0.50 to 1.9mm) and depths of 2, 3, and 4-1/2 inches (51, 76 and 115mm), depending on slab thickness and design span. Permanent metal deck forms are usually galvanized.

Permanent metal deck forms are designed to support the wet weight of concrete, a construction load, reinforcing steel and the weight of the form itself.

Composite metal deck forms also provide the tension reinforcement for the slab. The embossments provide interlock between the deck and the concrete. The ribs are generally trapezoidal in shape for roof and floor deck applications, although they may vary by manufacturer. The panels may be punched with a pattern of holes to absorb sound.

Some deck systems use cellular cold-formed steel shapes to permit lightweight floors that reduce deadweight. These floor systems can provide electrical power, communications and data cable distribution as well as heating and air conditioning ducts.

This eliminates separate space requirements in floors, walls and ceilings for ductwork or conduit systems. The modular construction possible with cold-formed steel sections permits access to many points in the deck, making it easy to rearrange wiring or ventilation outlets.

There are two basic types of standing seam roof systems:

Leaving the old roof in place means avoiding the release of potentially hazardous materials into the environment and eliminating the cost of disposing of the existing roof. Most importantly, a new metal roof can be installed with little or no interruption to the building’s occupants. The preferred material for the roof panel is galvanized or aluminum-zinc coated sheet steel.

The second type is architectural standing seam roof panels. They are generally used on roofs with slopes of 3 to 12 or greater, and provide water protection as well as aesthetic value. These panels usually require some form of decking for support. The preferred materials for architectural standing seam roof panels are prepainted galvanized and prepainted aluminum-zinc coated sheet steel. The architectural possibilities provided by the variety of colors and patterns are limitless.

Cold-formed steel panels are also used for enclosing the building to keep out the weather and keep in the heat. There are now available cold-formed steel panels with insulation already incorporated in the construction of the panel. Some of these configurations make use of foamed in place rigid urethanes sandwiched in between the steel. Others use different types of insulation, the more conventional being fiberglass installed in the panels.

There are wide varieties of panel systems. The simple, single element is available primarily as a ribbed sheet; formed, coated and otherwise modified to fulfill special functions that may be required.

Panels for walls may be as straightforward as a painted ribbed panel or as interesting as a porcelain or metal and glass curtain wall system. In many metal buildings, the wall panel is similar to the ribbed roof panel, with either wide or narrow ribs, and is painted or aluminum coated for long service life. The same tongue and groove lap or standing seam joints is employed here.

Metal stud framing is widely used as the structural support for the exterior brick veneer and exterior insulation finishing system (EIFS).

Cold-formed steel framing is also used for interior walls and partitions that provide flexibility in layout of interior spaces as well as the freedom to relocate and redesign.

The main advantages of these systems are strength, durability, noncombustibility, and a wide choice of coatings, including porcelain, vinyls and paints.

Cold-formed, corrugated steel pipe is used for everything from storm sewers to utility tunnels, and simple tunnels under roadways to huge bridge structures. The corrugated steel pipe can be installed in a cut, covered with filled and topped out with a roadway to form an inexpensive and durable bridge.

Cold-formed steel is also used for a variety of building products such as doors, windows. In highway construction, cold-formed steel is also used for guard rails, median and glare barriers, permanent bridge deck forms, signs and other components.

Cold-formed steel offers many advantages, including: ease of prefabrication and mass production, uniformity of quality, low weight, economy of transportation and handling, and quick and simple erection or installation.

The obvious benefit of permanent metal decks forms over the conventional wood form system is a significantly reduced construction cost and schedule since the metal forms remain in place as opposed to the wood forms which must be stripped. In addition, permanent forms provide a safe working platform for the subsequent operations of shear stud placement and re-bar installation.

The main forms of connections of cold-formed steel in construction are mechanical fasteners, welding and seams of various designs.

Seams are used on roof decks, corrugated steel pipe and in other products. Possible seam designs include the single lock seam, grooved single lock seam and the batten seam. Others include the double lock standing seams, the standing seam and double lock seam.

With mechanical fasteners, and particularly bolted connections, the ratio of bolt diameter to plate thickness is usually much larger in cold-formed steel construction than in heavy construction. This means that larger numbers of smaller diameter mechanical fasteners should be used in joining cold-formed sheet steel sections, although the general rules governing their placement are not unlike those of hot rolled structural steel. These general rules, however, are strongly subject to modification for behavior in particular cases. Sometimes it is necessary to conduct tests to determine the best type and placement of fasteners, although for commercially available products, this information is supplied by the manufacturer. Bolts are usually hex head and used with washers.

Self-tapping and self-drilling screws are important fasteners for cold-formed steel. They come in a variety of lengths, diameters, strengths and coatings, and their use should be in accordance with the manufacturer’s suggestions.

Self-drilling screws drill and tap their own hole and are used with mechanical drivers. Because these types of screws require no pre-drilled hole, they are useful where lineup of the parts being joined is accomplished through other means before joining.

These kinds of fasteners are used with steel framing systems for joining framing members to one another and for fastening sheathing material to the framing structure. For the latter job, there are many specialized types of screws, nails and staples. They are designed to anchor into both the framework and the sheathing. Special coatings such as cadmium, zinc and other metals provide longer fastener life.

Welding is widely used for joining cold-formed sheet steel parts. The two types of welds used are fusion and resistance. Fusion welding includes a group of weld types such as fillet welds, butt welds, plug welds and puddle welds, in which sheets are welded together by bringing them to the molten state at the surface to be joined by means of deposition of weld metal. Fusion welds are often used for erection, joining one cold-formed steel member to another or to hot-rolled framing members.

Resistance welding includes spot and projection welding. In this type of welding, coalescence is produced by the heat obtained from resistance to an electric current passed through the work parts, which are held together under pressure by electrodes. Resistance welding is used mostly for in-shop fabrication of cold-formed steel components.

Industry Standard Nomenclature for Steel Studs and Tracks

Although the steel stud thickness can be as thin as 18 mils (25 gauge) for interior construction applications, the minimum material thickness shall be 33 mils (20 gauge) for any structural applications. In addition, a galvanized finish with a minimum G60 coating is recommended for most structural applications not exposed to weather. A heavy protective coating G90 should be considered for structural applications exposed to weather.

To help designers and contractors identify the material thickness of cold-formed steel products in the field, stud manufacturers usually paint one end of studs and tracks according to the following color coding table:

Table 1. Steel Gauge Identification by Color

Special Considerations for Load-Bearing Studs

As more and more cold-formed steel studs are used in the load-bearing applications, a cold-formed steel designer needs to pay special attention to the unique characteristics of steel stud construction and to require better quality control during the construction. One of the most common problems with load-bearing stud construction is the gap at the top and bottom of steel studs. For lightly loaded studs, the connection between the stud and the track can be designed to carry the axial load without the direct bearing. For heavily loaded studs, load-bearing stud wall panels shall be fabricated in a plant where the gap between the stud and the track can be eliminated using a special pressing equipment.

When there is a significant axial load in the steel studs, horizontal cold-rolled steel channel bracing may not be sufficient to provide adequate bracing force to the heavy gauge steel studs. In this case, horizontal and diagonal steel strap bracing on each face of load-bearing stud walls should be considered. Another option is to introduce stand-alone box-sections so that each composite stud can function properly without relying on the horizontal bracing.

Unless adequate load distribution members are provided at each floor level, it is imperative for steel studs align vertically for direct load transfer from the top floor to the bottom floor. To allow certain construction tolerance, a designer may consider using heavy gauge tracks with 2- to 3-inch deep legs at the top and bottom of heavy load-bearing stud walls.

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