COLD FORMED STEEL SHAPES BASICS AND TUTORIALS

COLD FORMED STEEL SHAPES BASIC INFORMATION
What Are Cold Formed Steel Shapes?


A wide variety of shapes can be produced by cold-forming and manufacturers have developed a wide range of products to meet specific applications. Figure 3.11 shows the common shapes of typical cold-formed steel framing members. Figure 3.12 shows common shapes for profiled sheets and trays used for roofing and wall cladding and for load bearing deck panels.

For common applications, such as structural studs, industry organizations, such as the Steel Framing Alliance (SFA) and the Steel Stud Manufacturers Association (SSMA) have developed standard shapes and nomenclature to promote uniformity of product availability across the industry. Figure 3.11 shows the generic shapes covered by the Universal Designator System.

The designator consists of four sequential codes. The first code is a three or four-digit number indicating the member web depth in 1/100 inches. The second is a single letter indicating the type of member, as follows:


framing member with stiffening lips
L = Angle or L-header
F = Furring channels
U = Cold-rolled channel
T = Track section


The third is a three-digit numeral indication flange width in 1/100 inches followed by a dash. The fourth is a two or three-digit numeral indicating the base steel thickness in 1/1000 inch (mils). As an example, the designator system for a 6'', C-shape with 1-5/8'' (1.62'') flanges and made with 0.054'' thick steel is 600S162-54.


Special Design Considerations for Cold-Formed Steel
Structural design of cold-formed members is in many respects more challenging than the design of hot rolled, relatively thick, structural members. A primary difference is cold-formed members are more susceptible to buckling due to their limited thickness.

The fact that the yield strength of the steel is increased in the cold-forming process creates a dilemma for the designer. Ignoring the increased strength is conservative, but results in larger members, hence more costly, than is needed if the increased yield strength is considered.

Corrosion creates a greater percent loss of cross section than is the case for thick members. All cold-formed steel members are coated to protect steel from corrosion during the storage and transportation phases of construction as well as for the life of the product.

Because of its effectiveness, hot-dipped zinc galvanizing is most commonly used. Structural and non-structural framing members are required to have a minimum metallic coating that complies with ASTM A1003/A1003M, as follows:
■ structural members – G60 and
■ non-structural members G40 or equivalent minimum.

To prevent galvanic corrosion special care is needed to isolate the cold-formed members from dissimilar metals, such as copper. The design, manufacture and use of cold-formed steel framing is governed by standards that are developed and maintained by the American Iron and Steel Institute along with organizations such as ASTM, and referenced in the building codes.

BOLTS AND WELDS : STRUCTURAL FASTENERS BASIC AND TUTORIALS

STRUCTURAL FASTENERS BASIC: BOLTS AND WELDS
What Are Structural Fasteners?


Steel sections can be fastened together by rivets, bolts, and welds. While rivets were used quite extensively in the past, their use in modern steel construction has become almost obsolete. Bolts have essentially replaced rivets as the primary means to connect nonwelded structural components.

Bolts
Four basic types of bolts are commonly in use. They are designated by ASTM as A307, A325, A490, and A449. A307 bolts are called unfinished or ordinary bolts. They are made from low carbon steel.

Two grades (A and B) are available. They are available in diameters from 1/4 in. to 4 in. in 1/8 in. increments. They are used primarily for low-stress connections and for secondary members. A325 and A490 bolts are called high-strength bolts. A325 bolts are made from a heat treated medium carbon steel.

They are available in three types: Type1—bolts made of medium carbon steel; Type 2—bolts made of low carbon martensite steel; and Type 3—bolts having atmospheric corrosion resistance and weathering characteristics comparable to A242 and A588 steel. A490 bolts are made from quenched and tempered alloy steel and thus have a higher strength than A325 bolts.

Like A325 bolts, three types (Types 1 to 3) are available. Both A325 and A490 bolts are available in diameters from 1/2 in. to 1-1/2 in. in 1/8 in. increments. They are used for general construction purposes.

A449 bolts are made from quenched and tempered steel. They are available in diameters from 1/4 in. to 3 in. A449 bolts are used when diameters over 1-1/2 in. are needed. They are also used for anchor bolts and threaded rod.

High-strength bolts can be tightened to two conditions of tightness: snug-tight and fully tight. Snug-tight conditions can be attained by a few impacts of an impact wrench, or the full effort of a worker using an ordinary spud wrench.