10_COLD_FORMED_STEEL DESIGN AND CONSTRUCTION

Cold-Formed Sections In 1939, the American Iron and Steel Institute AISIstarted sponsoring studies, which still continue,under the direction of structural specialists asso-ciated with th

10 Don S Wolford Wei-Wen Yu

Consulting EngineerMiddletown, Ohio

England in 1784 by Henry Cort led to the

first cold-formed-steel structural

appli-cation, light-gage corrugated steel sheets

for building sheathing Continuous hot-rolling

mills, developed in America in 1923 by John Tytus,

led to the present fabricating industry based on

coiled strip steel This is now available in widths up

to 90 in and in coil weights up to 40 tons, hot- or

cold-rolled

Formable, weldable, flat-rolled steel is

avail-able in a variety of strengths and in black,

galvanized, or aluminum-coated Thus,

fabrica-tors can choose from an assortment of raw

materials for producing cold-formed-steel

pro-ducts (In cold forming, bending operations are

done at room temperature.) Large quantities of

cold-formed sections are most economically

pro-duced on multistand roll-forming machines from

slit coils of strip steel Small quantities can still be

produced to advantage in presses and bending

brakes from sheared blanks of sheet and strip

steel Innumerable cold-formed-steel products are

now made for building, drainage, road, and

construction uses Design and application of such

lightweight-steel products are the principal cern of this section

Shapes are MadeCold-formed shapes are relatively thin sectionsmade by bending sheet or strip steel in roll-formingmachines, press brakes, or bending brakes Because

of the relative ease and simplicity of the bendingoperation and the comparatively low cost offorming rolls and dies, the cold-forming processalso lends itself well to the manufacture of specialshapes for specific architectural purposes and formaximum section stiffness

Door and window frames, partitions, wallstuds, floor joists, sheathing, and moldings aremade by cold forming There are no standard series

of cold-formed structural sections, like those forhot-rolled structural shapes, although some dimen-sional requirements are specified in the AmericanIron and Steel Institute (AISI) Standards for cold-formed steel framing

Source: Standard Handbook for Civil Engineers

Cold-formed shapes cost a little more per pound

than hot-rolled sections They are nevertheless

more economical under light loading

Shapes

Cold-formed shapes are made from sheet or strip

steel, usually from 0.020 to 0.125 in thick In

hot-rolled steel usually costs less to use Cold-hot-rolled

steel is used in the thinner gages or where the

surface finish, mechanical properties, or more

uniform thickness resulting from cold reducing

are desired (The commercial distinction between

steel plates, sheets, and strip is principally a matter

of thickness and width of material.)

Cold-formed shapes may be either black

(uncoated) or galvanized Despite its higher cost,

galvanized material is preferable where exposure

conditions warrant paying for increased corrosion

protection Uncoated material to be used for

structural purposes generally conforms to one of

the standard ASTM Specifications for

structural-quality sheet and strip (A1008, A1011 and others)

ASTM A653 covers structural-quality galvanized

sheets Steel with a hot-dipped aluminized coating

(A792 and A875) is also available

The choice of grade of material usually depends

on the severity of the forming operation required

to make the desired shape Low-carbon steel has

wide usage Most shapes used for structural

purpo-ses in buildings are made from material with yield

points in the range of 33 to 50 ksi under ASTM

Specifications A1008 and A1011 Steel conforming

generally to ASTM A606, “High-Strength,

Low-Alloy, Hot-Rolled and Cold-Rolled Steel Sheet and

Strip with Improved Corrosion Resistance,” A1008,

‘‘Steel, Sheet, Cold-Rolled, Carbon, Structural,

Low-Alloy with Improved Formability,’’ or A1011,

‘‘Steel, Sheet and Strip, Hot-Rolled, Carbon,

Structural, Strength Low-Alloy and

High-Strength Low-Alloy with Improved Formability,’’

is often used to achieve lighter weight by designing

at yield points from 45 to 70 ksi, although higher

yield points are also being used

Sheet and strip for cold-formed shapes are

usually ordered and furnished in decimal or

millimetre thicknesses (The former practice of

specifying thickness based on weight and gagenumber is no longer appropriate.)

For the use of steel plates for cold-formedshapes, see the AISI Specification

ShapesSome cold-formed shapes used for structural pur-poses are similar in general configuration to hot-rolled structural shapes Channels (C-sections),angles, and Z’s can be roll-formed in a singleoperation from one piece of material I sections areusually made by welding two channels back toback, or by welding two angles to a channel Allsuch sections may be made with either plainflanges, as in Fig 10.1a to d, j, and m, or with flangesstiffened by lips at outer edges, as in Fig 10.1e to h,

k, and n

In addition to these sections, the flexibility ofthe forming process makes it relatively easy toobtain hat-shaped sections, open box sections, orinverted-U sections (Fig 10.1o, p, and q) Thesesections are very stiff in a lateral direction.The thickness of cold-formed shapes can beassumed to be uniform throughout in computingweights and section properties The fact that cold-formed sections have corners rounded on both theinside and outside of the bend has only a slight effect

on the section properties, and so computations may

be based on sharp corners without serious error.Cracking at 908 bends can be reduced by use ofinside bend radii not smaller than values recom-mended for specific grades of the steels mentioned

in Art 10.2 For instance, A1008, SS Grade 33 steel,for which a minimum yield point of 33 ksi isspecified, should be bent around a die with a

See ASTM Specification grade for appropriate bendradius that can safely be used in making right anglebends

Cold-Formed Sections

In 1939, the American Iron and Steel Institute (AISI)started sponsoring studies, which still continue,under the direction of structural specialists asso-ciated with the AISI Committees of Sheet and Strip

Steel Producers, that have yielded the AISI

Specification for the Design of Cold-Formed Steel

Structural Members (American Iron and Steel

Institute, 1140 Connecticut Ave., N.W.,

Washing-ton, DC 20036.) The specification, which has been

revised and amended repeatedly since its initial

publication in 1946, has been adopted by the major

building codes of the United States

Structural behavior of cold-formed shapes

con-forms to classic principles of structural mechanics,

as does the structural behavior of hot-rolled shapes

and sections of built-up plates However, local

buckling of thin, wide elements, especially in

cold-formed sections, must be prevented with special

design procedures Shear lag in wide elements

remote from webs that causes nonuniform stress

distribution and torsional instability that causes

twisting in columns and beam of open sections also

need special design treatment

Uniform thickness of cold-formed sections and

the relative remoteness from the neutral axis of

their thin, wide flange elements make possible the

assumption that, in computation of section

proper-ties, section components may be treated as line

elements (See “Section 3 of Part I of the AISI

Cold-Formed Steel Design Manual,” 2002.)

(Wei-Wen Yu, “Cold-Formed Steel Design,”

John Wiley & Sons, Inc., New York.)

Design Method (ASD) is used currently instructural design of cold-formed steel structuralmembers and described in the rest of this sectionusing US customary units In addition, the Loadand Resistance Factor Design Method (LRFD) canalso be used for design Both methods are included

in the 2001 edition of the AISI “North AmericanSpecification for the Design of Cold-Formed SteelStructural Members.” However, these two methodscannot be mixed in designing the various cold-formed steel components of a structure

In the allowable strength design method, the quired strengths (bending moments, shear forces,axial loads, etc.) in structural members are computed

re-by structural analysis for the working or serviceloads using the load combinations given in the AISISpecification These required strengths are not toexceed the allowable design strengths as follows:

V

Specification

V ¼ safety factor specified in the AISISpecification

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION

Rn/V ¼ allowable design strength

Unlike the allowable strength design method,

the LRFD method uses multiple load factors and

resistance factors to provide a refinement in the

design that can account for different degrees of the

uncertainties and variabilities of analysis, design,

loading, material properties and fabrication In this

method, the required strengths are not to exceed

the design strengths as follows:

The load factors and load combinations are also

specified in the AISI North American Specification

for the design of different type of cold-formed steel

structural members and connections For design

examples, see AISI “Cold-Formed Steel Design

Manual,” 2002 edition

The ASD and LRFD methods discussed above

are used in the United States and Mexico The AISI

North American Specification also includes the

Limit States Design Method (LSD) for use in

Canada The methodology for the LSD method is

the same as the LRFD method, except that the load

factors, load combinations, and some resistance

factors are different The North American

Specifi-cation includes Appendixes A, B, and C, which are

applicable in the United States, Canada, and

Mexico, respectively

Flat Compression

Elements

For buckling of flat compression elements in beams

and columns, the flat-width ratio w/t is an

impor-tant factor It is the ratio of width w of a single flat

element, exclusive of edge fillets, to the thickness t

of the element (Fig 10.2)

Flat compression elements of cold-formed

structural members are classified as stiffened and

unstiffened Stiffened compression elements haveboth edges parallel to the direction of stressstiffened by a web, flange, or stiffening lip Unstiff-ened compression elements have only one edgeparallel to the direction of stress stiffened If thesections in Fig 10.1a to n are used as compressionmembers, the webs are considered stiffenedcompression elements But the wide, lipless flangeelements and the lips that stiffen the outer edges ofthe flanges are unstiffened elements Any sectioncomposed of a number of plane elements can bebroken down into a combination of stiffened andunstiffened elements

The cold-formed structural cross sectionsshown in Fig 10.3 illustrate how effective portions

of stiffened compression elements are considered

to be divided into two parts located next to the twoedge stiffeners of that element In beams, a stiffenermay be a web, another stiffened element, or a lip

In computing net section properties, only theeffective portions of elements are considered andthe ineffective portions are disregarded For beams,flange elements subjected to uniform compressionmay not be fully effective Accordingly, sectionproperties, such as moments of inertia and sectionmoduli, should be reduced from those for a fullyeffective section (Effective widths of webs can bedetermined using Section B2.3 of the AISI NorthAmerican Specification.) Effective areas of columncross sections needed for determination of columnloads from Eq (10.21) of Art 10.12 are based on fullcross-sectional areas less all ineffective portions

the critical load for an elastic prismatic bar

loaded as a column from

steel

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION

This equation is the basis for designing long

columns of prismatic cross section subject to elastic

buckling It might be regarded as the precursor of

formulas used in the design of thin rectangular

plates in compression

Bryan, in 1891, proposed for design of a thin

rectangular plate compressed between two

oppo-site edges with the other two edges supported:

edge-sup-port restraint

n ¼ Poisson’s ratio

Until the 1986 edition, all AISI Specifications

based strength of thin, flat elements stiffened along

one edge on buckling stress rather than effective width

as used for thin, flat elements stiffened along both

edges Although efforts were made by researchers

to unify element design using a single concept,

unification did not actually occur until Pekoz, in

1986, presented his unified approach using

effec-tive width as the basis of design for both stiffened

and unstiffened elements and even for web

elements subjected to stress gradients

Conse-quently, the AISI Specification uses the following

equations to determine the effective width of

uniformly compressed stiffened and unstiffened

elements based on a slenderness factor l:

l ¼

ffiffiffiffiffif

¼ 0.43 for unstiffened elements

of the section, computed on the basis of

the design width, ksi

radii, in

The effective width is given by

By definition, unstiffened cold-formed elementshave only one edge in the compression-stressdirection supported by a web or stiffened element,while the other edge has no auxiliary support (Fig.10.1a) The coefficient k in Eq (10.3) is 0.43 for such

an element When the ratio of flat width to

, an unstiffened elementwith unit stress f is fully effective; that is, theeffective width b equals flat width w Generally,however, Eq (10.3) becomes

r

t

ffiffif

p

(10:7)

on the basis of effective widths, Eq.(10.3)

When l is substituted in Eq (10.6), the b/w ratio rresults The lower portion of Fig 10.5 shows curvesfor determining the effective-width ratio b/t forunstiffened elements for w/t between 0 and 60,with f between 15 and 90 ksi

In beam-deflection determinations requiring theuse of the moment of inertia of the cross section, f isthe allowable stress used to calculate the effectivewidth of an unstiffened element in a cold-formed-steel beam However, in beam-strength determi-nations requiring use of the section modulus of thecross section, f is the unit compression stress to beused in Eq (10.7) to calculate the effective width ofthe unstiffened element and provide an adequatemargin of safety In determining safe column loads,effective width for the unstiffened element must

be determined for a nominal column bucklingstress to ensure adequate margin of safety for suchelements

(“Cold-Formed Steel Design Manual,” American

Iron and Steel Institute, Washington, D.C.)

Subject to Local

Buckling

By definition, stiffened cold-formed elements have

one edge in the compression-stress direction

sup-ported by a web or stiffened element and the other

edge is also supported by a qualified stiffener (Fig

10.4b) The coefficient k in Eq (10.3) is 4.00 for such

an element When the ratio of flat width to

, the stiffened

ksi, in the compression element of the structuralsection computed on the basis of effective widths,

r

t

ffiffif

p

(10:8)

inter-mediate stiffeners, beam webs, and edge stiffeners

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION

If l is substituted in Eq (10.6), the b/w ratio r

10.5 shows curves for determining the

effective-width ratio b/t for stiffened elements w/t between 0

and 500 with f between 10 and 90 ksi

In beam-deflection determinations requiring the

use of the moment of inertia of the cross section, f is

the allowable stress used to calculate the effective

width of a stiffened element in a

cold-formed-steel member loaded as a beam However, in

beam-strength determinations requiring the use of

the section modulus of the cross section, f is the unit

compression stress to be used in Eq (10.8) to

calculate the width of a stiffened element in a

cold-formed-steel beam In determination of safe

column loads, effective width for a stiffened

element should be determined for a nominal

column buckling stress to ensure an adequate

margin of safety for such elements

4:00=0:43p

¼3:05 times as great for unstiffened elements as for

stiffened elements at applicable combinations of

stress f and width-thickness ratio w/t This

emphasizes the greater effective width and

economy of stiffened elements

uni-formly compressed stiffened elements with a singleintermediate stiffener, as shown in Fig 10.4c, the

com-pression flange can be determined by the followinglocal buckling coefficient k:

unstiffened and stiffened elements

In the above equations,

about its own centroidal axis parallel to the

Pekoz’s unified approach using effective widths

(Art 10.5) also applies to stiffened elements

sub-jected to stress gradients in compression, such as in

Fig 10.4d calculated on the basis of the effective

com-pression portion of the web calculated on

the basis of effective section

Uniformly Compressed Elements with

under-stand the capabilities of edge stiffeners (depicted in

Fig 10.4e for a slanted lip) However, due to the

complexity of this subject, the following tation is confined primarily to simple lip stiffeners.Two ranges of w=t values are considered relative

presen-to a parameter 0.328 S The limit value of w=t forfull effectiveness of the flat width without auxiliarysupport is

0:328 S ¼ ð0:328Þð1:28Þ

ffiffiffiEf

s

¼ 0:420

ffiffiffiEf

sð10:13Þ

the basis of effective widths, ksi

and no edge support is needed

edge support is needed with the required moment

w=t

u ¼ angle between normals to stiffened ment and its lip (908 for a right-anglelip) (Fig 10.4e)

ele-The effective width, b, of the compression flangecan be determined from Eqs (10.3) to (10.6) with kcalculated from the following equations for single

ð10:16aÞFor 0:25 , D=w
0:8;

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION

The values of b1and b2; as shown in Fig 10.4e,

can be computed as follows:

The effective width b depends on the actual

stress f, which, in turn, is determined by reduced

section properties that are a function of effective

width Employment of successive approximations

consequently may be necessary in using these

equations This can be avoided and the correct

values of b/t obtained directly from the formulas

when f is known or is held to a specified

maximum value This is true, though, only when

the neutral axis of the section is closer to the

tension flange than to the compression flange, so

that compression controls The latter condition

holds for symmetrical channels, Z’s, and I

sections used as flexural members about their

major axis, such as Fig 10.1e, f, k, and n For

wide, inverted, pan-shaped sections, such as deck

and panel sections, a somewhat more accurate

determination, using successive approximations,

is necessary

For computation of moment of inertia for

deflection or stiffness calculations, properties of

the full unreduced section can be used without

significant error when w/t of the compression

elements does not exceed 60 For greater accuracy,

use Eqs (10.7) and (10.8) to obtain effective widths

determination, consider the hat section in Fig 10.6

The section is to be made of steel with a specified

as a simply supported beam with the top flange in

compression Safe load-carrying capacity is to be

computed Because the compression and tension

compute b/t

The top flange is a stiffened compression element

) and Eq (10.8) applies

For this value of w/t and f ¼ 33 ksi, Eq (10.8) or

Fig 10.5 gives b/t as 41 Thus, only 85% of the

top-flange flat width can be considered effective in this

case The neutral axis of the section will lie below the

horizontal center line, and compression will control

made at the start, controls maximum stress, and b/t

can be determined directly from Eq (10.8), withoutsuccessive approximations

For a wide hat section in which the horizontalcentroidal axis is nearer the compression than thetension flange, the stress in the tension flangecontrols So determination of unit stress andeffective width of the compression flange requiressuccessive approximations

(“Cold-Formed Steel Design Manual,” ican Iron and Steel Institute, Washington, D.C.,

to permit buckles to develop in the sheet and takeadvantage of what is known as the postbucklingstrength of the section The effective-width for-mulas [Eqs (10.3), (10.6), (10.7), and (10.8)] arebased on this practice of permitting some incipientbuckling to occur at the allowable stress To avoidintolerable deformations, however, overall flat-width ratios, disregarding intermediate stiffenersand based on the actual thickness of the element,should not exceed the following:

10.9 Beam Design

Considerations

For the design of beams, considerations should be

given to (a) bending strength and deflection, (b)

web strength for shear, combined bending and

shear, web crippling, and combined bending and

web crippling, (c) bracing requirements, (d) shear

lag, and (e) flange curling

Based on the AISI ASD method, the required

bending moment computed from working loads

shall not exceed the allowable design moment

determined by dividing the nominal bending

moment by a factor of safety For laterally supported

beams, the nominal bending moment is based on

the nominal section strength calculated on the basis

of either (a) initiation of yielding in the effective

section or (b) the inelastic reserve capacity in

accordance with the AISI Specification The factor

of safety for bending is taken as 1.67

Cold-Formed Beams

In the relatively infrequent cases in which

cold-formed sections used as beams are not laterally

supported at frequent intervals, the strength must

be reduced to avoid failure from lateral instability

The amount of reduction depends on the shape and

proportions of the section and the spacing of lateral

supports This is not a difficult obstacle (For

de-tails, see the AISI “North American Specification

for the Design of Cold-Formed Steel Structural

Members,” 2001.)

Because of the torsional flexibility of cold-formedchannel and Z sections, their use as beams withoutlateral support is not recommended When one flange

is connected to a deck or sheathing material, thenominal flexural strength of the member can bedetermined in accordance with the AISI specification.When laterally unsupported beams must be used,

or where lateral buckling of a flexural member islikely to be a problem, consideration should be given

to the use of relatively bulky sections that have twowebs, such as hat or box sections (Fig 10.1o and p)

Strength and Web Crippling Strength in Webs

The shear force at any section should not exceed

p,

measured along the plane of the web, in

does not exceed 200

For design of reinforced webs, especially when h/texceeds 200, see AISI “North American Specifica-tion for the Design of Cold-Formed Steel StructuralMembers,” 2001

For a web consisting of two or more sheets, eachsheet should be considered a separate elementcarrying its share of the shear force

For beams with unreinforced webs, the moment

M, and the shear V, should satisfy the following

Stiffened compression element having one

longitudinal edge connected to a web or

Stiffened compression element having one

longitudinal edge connected to a web or

flange, the other stiffened by any other

Stiffened compression element with both

longitudinal edges connected to a web or

flange element, such as in a hat, U, or box

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION

cen-troidal axis, in-kips

alone exists, kips

For beams with reinforced webs, the interaction

equation for combined bending and shear is given

in the AISI North American Specification

In addition to the design for shear strength of

beam webs, consideration should also be given to

the web crippling strength and combined bending

and web crippling strength as necessary The web

crippling strength depends on several parameters

the plane of the web and the plane of the

bearing surface In the above ratios, N is the actual

bearing length and R is the inside bend radius

Other symbols were defined previously

The 2001 edition of the AISI North American

Specification includes the following equation for

determining the nominal web crippling strength of

webs without holes:

r 

ffiffiffiht

r 

ð10:19Þ

Specification for built-up sections, single web

channel and C-sections, single web Z-sections,

single hat sections, and multi-web deck sections

under different support and loading conditions

For beam webs with holes, the web crippling

strength should be multiplied by the reduction

provides interaction equations for combined

bend-ing and web cripplbend-ing strength

Compression Members

The following applies to members in which the

resultant of all loads acting on the member is an

axial load passing though the centroid of theeffective section calculated for the nominal buck-

and torsional-flexural buckling stressFigure 10 7 shows the ratio between the column

For the elastic flexural mode,

cross section, in

Moreover, non-compact angle sections should

be designed for the applied axial load P actingsimultaneously with a moment equal to PL/1000applied about the minor principal axis causingcompression in the tips of the angle legs

The slenderness ratio KL/r of all compressionmembers preferably should not exceed 200 exceptthat, during construction only, KL/r preferablyshould not exceed 300

For treatment of open cross sections which may

be subject to torsional-flexural buckling, refer to

AISI “North American Specification for the Design

of Cold-Formed Steel Structural Members,” 2001

Bending Stresses

Combined axial and bending stresses in

cold-formed sections can be handled in a similar way as

for structural steel The interaction criterion to be

used is given in the AISI “North American

Specification for the Design of Cold-Formed-Steel

Structural Members,” 2001

Cold-Formed Steel

Welding offers important advantages to fabricators

and erectors in joining metal structural

compo-nents Welded joints make possible continuous

structures, with economy and speed in fabrication;

100% joint efficiencies are possible

Conversion to welding of joints initially signed for mechanical fasteners is poor practice.Joints should be specifically designed for weld-ing, to take full advantage of possible savings.Important considerations include the following:The overall assembly should be weldable, weldsshould be located so that notch effects areminimized, the final appearance of the structureshould not suffer from unsightly welds, andwelding should not be expected to correct poorfit-up

de-Steels bearing protective coatings requirespecial consideration Surfaces precoated withpaint or plastic are usually damaged by welding.And coatings may adversely affect weld quality.Metallically coated steels, such as galvanized (zinc-coated), aluminized, and terne-coated (lead-tinalloy), are now successfully welded using pro-cedures tailored for the steel and its coating.Generally, steel to be welded should be cleanand free of oil, grease, paints, scale, and so on Paintshould be applied only after the welding operation

Society, 550 N.W LeJeune Rd., Miami, FL 33135

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION

www.aws.org; O W Blodgett, “Design of

Weld-ments,” James F Lincoln Arc Welding Foundation,

Cleveland, OH 44117 www.weldinginnovation

com.)

Cold-Formed Steel

Arc welding may be done in the shop and in the

field The basic sheet-steel weld types are shown in

Fig 10.8 Factors favoring arc welding are

porta-bility and versatility of equipment and freedom in

joint design (See also Art 10.14.) Only one side of a

joint need be accessible, and overlap of parts is not

required if joint fit-up is good

Distortion is a problem with lightweight steel

weldments, but it can be minimized by avoiding

overwelding Weld sizes should be matched to

service requirements

Always design joints to minimize shrinking,

warping, and twisting Jigs and fixtures for holding

lightweight work during welding should be used

to control distortion Directions and amounts of

distortion can be predicted and sometimes

counter-acted by preangling the parts Discrete selection of

welding sequence can also be used to control

distortion

Groove welds (made by butting the sheet edgestogether) can be designed for 100% joint efficiency.Calculations of design stress is usually unnecessary

if the weld penetrates 100% of the section.Stresses in fillet welds should be considered asshear on the throat for any direction of the appliedstress The dimension of the throat is calculated as0.707 times the length of the shorter leg of the weld

fillet and plug welds should be proportionedaccording to the AISI specification For theallowable strength design method, the factors ofsafety for various weld types are given in the AISINorth American Specification

Shielded-metal-arc welding, also called ual stick electrode, is the most common arc weldingprocess because of its versatility, but it calls forskilled operators The welds can be made in anyposition Vertical and overhead welding should beavoided when possible

to feed a continuous spool of bare or flux-coredwire into the arc A shielding gas such as argon orcarbon dioxide is used to protect the arc zone fromthe contaminating effects of the atmosphere Theprocess is relatively fast, and close control can bemaintained over the deposit The process is not

arc seam weld (oblong puddle weld); (d) fillet welds; (e) flare-bevel-groove weld; ( f) flare-V-groove weld

applicable to materials below 1⁄32in thick but is

extensively used for thicker steels

main-taining an arc between a nonconsumable tungsten

electrode and the work Filler metal may or may

not be added Close control over the weld can be

maintained This process is not widely used for

high-production fabrication, except in specialized

applications, because of higher cost

One form of arc spot welding is an adaption of

gas-metal-arc welding wherein a special welding

torch and automatic timer are employed The

welding torch is positioned on the work and a weld

is deposited by burning through the top

com-ponent of the lap joint The filler wire provides

sufficient metal to fill the hole, thereby fusing

together the two parts Access to only one side of

the joint is necessary Field welding by unskilled

operators often makes this process desirable

Another form of arc spot welding utilizes

gas-tungsten arc welding The heat of the arc melts a

spot through one of the sheets and partly through

the second When the arc is cut off, the pieces fuse

No filler metal is added Design of arc-welded

joints of sheet steel is fully treated in the American

Welding Society “Structural Welding Code-Sheet

Steel,” AWS D1.3, www.aws.org Allowable

maxi-mum-load capacities of arc-welded joints of sheet

steel, including cold-formed members 0.180 in or

less thick, are determined in the following ways

maximum load for a groove weld in a butt joint,

welded from one or both sides, is determined by

the base steel with the lower strength in the

connection, provided that an effective throat equal

to or greater than the thickness of the material is

consistently obtained

welding sheet steel to thicker supporting members

in the flat position Arc spot welds (puddle welds)

may not be made on steel where the thinnest

connected part is over 0.15 in thick, nor through a

combination of steel sheets having a total thickness

of over 0.15 in Arc spot welds should be specified

between two or more sheets and a supporting

member should not exceed the smaller of the

values calculated from Eq (10.25) or, as priate, Eqs (10.26), (10.27), (10.28)

p:

ffiffiffiffiffiE

coatings), of all the sheets involved inshear transfer through the spot weld

middepth of the shear transfer zone

¼ d 2 t for a single sheet or multiple sheets(not more than four lapped sheets over asupporting member)

spot weld

¼ 0.7d 2 1.5t but not more than 0.55d

electrode classification

speci-fied, ksiThe distance measured in the line of force fromthe centerline of a weld to the nearest edge of anadjacent weld or to the end of the connected parttoward which the force is directed should not be

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION

In addition, the distance from the centerline of

any weld to the end or boundary of the connected

member may not be less than 1.5d In no case may

the clear distance between welds and the end of the

member be less than d

weld between sheet and supporting member

should be computed as the smaller of either:

If it can be shown by measurement that a given

weld procedure will consistently give a larger

as applicable, this larger diameter may be used, if

the welding procedure required for making those

should not exceed the values calculated from either

circular ends, in (For computation

pur-poses, L should not exceed 3d)

¼ d 2 t for a single sheet or double sheets

fused surfaces, in

¼ 0.7d 2 1.5t

arc spot welds Also, minimum edge distance is thesame as that defined for arc spot welds If it can

be shown by measurement that a given weldprocedure will consistently give a larger effective

this value may be used, if the welding procedurerequired for making the welds that were measured

is followed

of joints in any position, either sheet to sheet orsheet to thicker steel member The nominal shear

should not exceed the following:

For Longitudinal LoadingFor L/t , 25:

a fillet weld in lap and T joints should not exceed

welding of joints in any position, either:

flare-bevel-groove welds

governed by the thickness, t, in, of the sheet steeladjacent to the weld

For flare-bevel-groove welds, the transverse

load should not exceed

For flare-V-groove welds, when the effective

thickness t of the sheets being joined but less than

2t, or if the lip height is less than the weld length L,

in, the longitudinal loading should not exceed

is equal to or greater than L,

Resistance welding comprises a group of welding

processes wherein coalescence is produced by the

heat obtained from resistance of the work to flow of

electric current in a circuit of which the work is a

part and by the application of pressure Because of

the size of the equipment required, resistance

welding is essentially a shop process Speed and

low cost are factors favoring its selection

Almost all resistance-welding processes require

a lap-type joint The amount of contacting overlap

Access to both sides of the joint is normally

required Adequate clearance for electrodes and

welder arms must be provided

resistance-welding process The work is held under pressure

between two electrodes through which an electric

current passes A weld is formed at the interface

between the pieces being joined and consists of a

cast-steel nugget The nugget has a diameter about

equal to that of the electrode face and should

penetrate about 60 to 80% of each sheet thickness

For structural design purposes, spot welding

can be treated the same way as rivets, except that

no reduction in net section due to holes need be

made Table 10.1 gives the essential information

for uncoated material based on “Recommended

Practices for Resistance Welding,” American

Welding Society Note that the thickest material

be resistance-welded by projection or by pulsationmethods if high-capacity spot welders for material

which the effects of current and pressure areintensified by concentrating them in small areas ofprojections embossed in the sheet to be welded.Thus, satisfactory resistance welds can be made onthicker material using spot welders ordinarilylimited to thinner stocks

Pulsation welding, or multiple-impulse ing, is the making of spot welds with more thanone impulse of current, a maneuver that makessome spot welders useful for thicker materials Thetrade-offs influencing choice between projectionwelding and impulse welding involve the workbeing produced, volume of output, and equipmentavailable

weld-The spot welding of higher-strength steels thanthose contemplated under Table 10.1 may requirespecial welding conditions to develop the highershear strengths of which the higher-strength steelsare capable All steels used for spot welding should

be free of scale; therefore, either hot-rolled andpickled or cold-rolled steels are usually specified.Steels containing more than 0.15% carbon are not asreadily spot welded as lower-carbon steels, unlessspecial techniques are used to ensure ductile welds.However, high-carbon steels such as ASTM A653,

SS Grade 50 (formerly, Grade D), which can have acarbon content as high as 0.40% by heat analysis,are not recommended for resistance welding De-signers should resort to other means of joining suchsteels

Maintenance of sufficient overlaps in detailingspot-welded joints is important to ensure consistentweld strengths and minimum distortions at joints.Minimum weld spacings specified in Table 10.1should be observed, or shunting to previously madeadjacent welds may reduce the electric current to alevel below that needed for welds being made Also,the joint design should provide sufficient clearancebetween electrodes and work to prevent short-circuiting of current needed to make satisfactoryspot welds For design purposes, the AISI NorthAmerican Specification provides design equationsand a factor of safety on the basis of “Recommended

Low-Carbon Steel,” American Welding Society,

550 N.W LeJeune Rd., Miami, FL 33135, www.aws.org

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION

10.17 Bolting of

Cold-Formed-Steel Members

Bolting is convenient in cold-formed-steel

con-struction Bolts, nuts, and washers should

gener-ally conform to the requirements of the ASTM

specifications listed in Table 10.2

Maximum sizes permitted for bolt holes are

given in Table 10.3 Holes for bolts may be standard

or oversized round or slotted Standard holes

should be used in bolted connections when

possible The length of slotted holes should benormal to the direction of shear load Washersshould be installed over oversized or slottedholes

the line of force from the center of a standard hole

to the nearest edge of an adjacent hole or to the end

of the connected part toward which the force is

Min WeldSpacing

c to c, in

ApproxDia ofFusedZone, in

Min ShearStrength perWeld, lb

Dia ofProjection

In addition, the minimum distance between centers

of bolt holes should provide sufficient clearance forbolt heads, nuts, washers, and the wrench but notless than three times the nominal bolt diameter d.The distance from the center of any standard hole

to the end or boundary of the connecting member

Specification

member should be determined from Section C2

of the AISI North American Specification Forfracture in the effective net section of flat sheetconnections having washers provided under the

High-Pressure and High-Temperature Service

Joints

Alloy Steel Bolts, Studs, and Other

Exter-nally Threaded Fasteners (for diameter of

in)

for General Use

Tens-ion Indicators for Use with Structural

Oversized Hole Dia,

d, in

Short-Slotted HoleDimensions, in

Long-Slotted HoleDimensions, in

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION

¼ 2.22 for single shear and 2.00 for

double shear

holes in cross section, in

ksi

When washers are not provided under the bolt

head and nut, see AISI Specification The

Specifica-tion also provides the design informaSpecifica-tion for flat

sheet connections having staggered hole patterns

and structural members such as angles and

channels

10.4a

Table 10.4b

nominal shear and tension for various grades of

bolts The bolt force resulting in shear, tension, or

combination of shear and tension should not

Eq (10.48)

Factors of safety given in Tables 10.5 and 10.6should be used to compute allowable loads onbolted joints

Table 10.6 lists nominal tension stresses for boltssubject to the combination of shear and tension

bolts, with washers under both bolt head and nut.Determine the allowable load based on the ASDmethod

A Based on Tensile Strength of Steel Sheets

transverse row A force T=2 is applied to each boltand the total force T has to be carried by the netsection of each sheet through the bolts So, in Eq

Thickness of

Connected Part,

t, in

Ratio of FastenerDiameter toMemberThickness,

Single Shear and Outside Sheets of DoubleShear Connection with Washersunder Both Bolt Head and Nut

1:00Single Shear and Outside Sheets of Double

Shear Connection without Washersunder Both Bolt Head and Nut, Or withonly One Washer

0.75

Inside Sheet of Double Shear Connectionwith or without Washers

1:33

Table 10.5 Nominal Tensile and Shear Strength for Bolts

Description of Bolts

Factor ofSafetyV

NominalStress

Factor ofSafetyV

NominalStress

A325 bolt, when threads are

not excluded from shear

planes

A325 bolts, when threads are

excluded from shear planes

threads are not excluded

from shear planes

when threads are not

excluded from shear planes

when threads are excluded

from shear planes

A490 Bolts, when threads are

not excluded from shear

planes

A490 Bolts, when threads are

excluded from shear planes

from Shear Planes

Threads Excludedfrom Shear Planes

The shear stress, f v , shall also satisfy Table 10.5.

COLD-FORMED-STEEL DESIGN AND CONSTRUCTION