Hancock G.J., Murray Th.M., Ellifritt D.S. Cold-Formed Steel Structures to the AISI Specification

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framework. Farm buildings, storage sheds, and grain bins

are typical applications, although such construction has

historically been used for industrial buildings as well.

Residential framing: Lipped and unlipped channels,

made to the same dimensions as nominal 24s, and some-

times known as ``steel lumber,'' are typically used in the

walls of residential buildings. Larger channel sections are

used as ¯oor and ceiling joists, and roof trusses are

commonly made of small channel sections screwed or

bolted together. Some examples are shown in Figure 1.7b

for a simple roof truss and in Figure 1.7a for wall framing.

Steel ¯oor and roof deck: Formed steel deck is laid

across steel beams to provide a safe working platform and a

form for concrete. It is normally designated as a wide rib,

intermediate rib, or narrow rib deck. Some deck types have

a ¯at sheet attached to the bottom of the ribs which creates

hollow cells providing raceways for electrical and other

cables. This bottom sheet may be perforated for an acous-

FIGURE 1.3 Sliding clip and ®xed clip.

Chapter 1

tical ceiling. Typical examples of each of these decks are

shown in Figure 1.2.

Some decks have embossments in the sloping sides of

the rib that engage the concrete slab as a kind of shear key

and permit the deck to act compositely with the concrete.

FIGURE 1.4 Storage rack sections: (a) plan view of uprights and

bracing; (b) pallet beam sections.

Introduction

It is not common to use a concrete slab on roofs, so roof

deck is generally narrow rib so that it can provide support

for insulating board. A typical narrow rib roof deck is

shown in Figure 1.2.

Utility poles: All of the foregoing may lead the reader

to think that ``cold-formed'' means ``thin sheet.'' It is often

surprising to one not familiar with the subject that cold-

formed structures may be fabricated from plates up to 1 in.

thick. (In fact the 1996 AISI Speci®cation declares that it

covers all steel up to an inch thick.) A good example is a

type of vertical tapered pole that may be used for street

lights or traf®c signals. It is dif®cult to make a 90



bend in a

FIGURE 1.5 Steel storage rack.

Chapter 1

1 in.-thick plate, and even if it could be done it would

probably crack the plate, but it is feasible to make a 60



bend. Such poles are generally octagonal in cross section

and are made in halves that are then welded together.

Because they are tapered, they can be stacked to attain

greater height. An example of this kind of cold-formed

structure can be seen in Figure 1.8.

Automotive applications: All major structural

elements can be used, but normally hat sections or box

sections are used. Several publications on this subject are

available from the American Iron and Steel Institute.

Grain storage bins or silos: Grain bins usually consist

of curved corrugated sheets stiffened by hat or channel

FIGURE 1.6 Plane truss frames: (a) tubular truss; (b) channel

section truss.

Introduction

sections. While the AISI Speci®cation for cold-formed struc-

tures does not deal speci®cally with such structures, the

same principles can be applied. A good reference on this

kind of structure is Gaylord and Gaylord (Ref. 1.8).

Cold-formed tubular members: Hollow structural

sections (HSS) may be made by cold-roll-forming to produce

FIGURE 1.7 Residual construction: (a) wall framing; (b) roof truss.

Chapter 1

a round, which is then closed by electric resistance welding

(ERW). The round shape can then be used as is or further

formed into a square or rectangle. Examples are shown in

Figure 1.9.

1.6 MANUFACTURING PROCESSES

Cold-formed members are usually manufactured by one of

two processes: (1) roll forming and (2) brake forming.

1.6.1 Roll Forming

Roll forming consists of feeding a continuous steel strip

through a series of opposing rolls to progressively deform

the steel plastically to form the desired shape. Each pair of

rolls produces a ®xed amount of deformation in a sequence

FIGURE 1.8 Cold-formed steel utility poles.

FIGURE 1.9 Typical tubular sections.

Introduction

of the type shown in Figure 1.10. In this example, a Z

section is formed by ®rst developing the bends to form the

lip stiffeners and then producing the bends to form the

¯anges. Each pair of opposing rolls is called a stage, as

shown in Figure 1.11a. In general, the more complex the

cross-sectional shape, the greater the number of stages that

are required. In the case of cold-formed rectangular hollow

sections, the rolls initially form the section into a circular

section and a weld is applied between the opposing edges of

the strip before ®nal rolling (called sizing) into a square or

rectangle.

1.6.2 Brake Forming

Brake forming involves producing one complete fold at a

time along the full length of the section, using a machine

called a press brake, such as the one shown in Figure 1.11b.

For sections with several folds, it is necessary to move the

steel plate in the press and to repeat the braking operation

several times. The completed section is then removed from

the press and a new piece of plate is inserted for manufac-

ture of the next section.

FIGURE 1.10 Typical roll-forming sequence for a Z-section.

Chapter 1

Roll forming is the more popular process for producing

large quantities of a given shape. The initial tooling costs

are high, but the subsequent labor content is low. Press

braking is normally used for low-volume production where

a variety of shapes are required and the roll-forming tool-

ing costs cannot be justi®ed. Press braking has the further

limitation that it is dif®cult to produce continuous lengths

exceeding about 20 ft.

FIGURE 1.11 Cold-forming tools: (a) roll-forming tools; (b) brake

press dies.

Introduction

A signi®cant limitation of roll forming is the time it

takes to change rolls for a different size section. Conse-

quently, adjustable rolls are often used which allow a rapid

change to a different section width or depth. Roll forming

may produce a different set of residual stresses in the

section when compared with braking, so the section

strength may be different in cases where buckling and

yielding interact. Also, corner radii tend to be much

larger in roll-formed sections, and this can affect structural

actions such as web crippling.

1.7 GENERAL APPROACH TO THE DESIGN

OF COLD-FORMED SECTIONS

1.7.1 Special Problems

The use of thinner material and cold-forming processes

result in special design problems not normally encountered

in hot-rolled construction. For this reason, the AISI Cold-

Formed Speci®cation has been produced to give designers

guidance on the different modes of buckling and deforma-

tion encountered in cold-formed steel structures. In addi-

tion, welding and bolting practices in thinner sections are

also different, requiring design provisions unique to thin

sheets. A brief summary of some of these design provisions

follows.

1.7.2 Local Buckling and Post-Local Buckling

of Thin Plate Elements

The thicknesses of individual plate elements of cold-formed

sections are normally small compared to their widths, so

local buckling may occur before section yielding. However,

the presence of local buckling of an element does not

necessarily mean that its load capacity has been reached.

If such an element is stiffened by other elements on its

edges, it possesses still greater strength, called ``postbuck-

ling strength.'' Local buckling is expected in most cold-

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formed sections and often ensures greater economy than a

heavier section that does not buckle locally.

1.7.3 Effective Width Concept

In the ®rst speci®cation published by the American Iron

and Steel Institute in 1946, steel designers were introduced

to the concept of ``effective width'' of stiffened elements of a

cold-formed section for the ®rst time. The notion that a ¯at

plate could buckle and still have strength leftÐpostbuck-

ling strength as it was calledÐwas a new concept in steel

speci®cations. To have this postbuckling strength required

that the plate be supported along its edges or ``stiffened'' by

some element that was attached at an angle, usually a right

angle. These stiffening elements are achieved in cold-

formed steel by bending the sheet.

Because cold-formed members typically have very

high width-to-thickness ratios, they tend to buckle elasti-

cally under low compressive stress. However, the stiffened

edges of the plate remain stable and a certain width of the

plate close to the corners is still ``effective'' in resisting

further compressive load. The problem is to determine how

much of the original width of the plate is still effective. This

is called ``effective width,'' and formulas for calculating it

were developed under the leadership of Dr. George Winter

at Cornell University in the early 1940s. These effective

width formulas appeared in the ®rst Cold-Formed Speci®-

cation in 1946 and remained unchanged until 1986.

Plates that had a stiffening element on only one edge

were called ``unstiffened.'' These did not require calculation

of an effective width, but were designed on the basis of a

reduced stress.

Until 1986, there were only stiffened and unstiffened

elements. Examples of each are shown in Figure 1.12a. An

element was deemed stiffened if it had an adequate stif-

fener on both edges of the element. The stiffener could be

``edge'' or ``intermediate,'' as shown in Figure 1.12b. The

Introduction