Yu W., LaBoube R.A. Cold-Formed Steel Design

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SCREW CONNECTIONS 295

(1) Flat Steel Washer beneath

Hex Head Screw Head

(3) Domed Washer (Non-Solid) beneath Screw Head

(2) Flat Steel Washer beneath Hex Washer Head

Screw Head (HWH has Integral Solid Washer)

Figure 8.37 Screw pull-over with washer.

1.345

the safety factor or resistance factor shall be permitted to be

determined in accordance with Section F1 and shall be taken

as 1.25 ≤ 3.0 (ASD), φ/1.25 ≥ 0.5 (LRFD),or φ/1.25 ≥ 0.4

(LSD).

E4.5 Combined Shear and Pull-Over

E4.5.1 ASD Method

For screw connections subjected to a combination of shear and

tension forces, the following requirements shall be met:

+ 0.71

nov

≤

1.10



(8.68)

In addition, Q and T shall not exceed the corresponding

allowable strength determined by Sections E4.3 and E4.4,

respectively,

where Q = required allowable shear strength of connection

T = required allowable tension strength of connection

= nominal shear strength of connection

= 2.7t

nov

= nominal pull-out strength of connection

= 2.7t

= larger of screw head diameter or washer diameter

 = 2.35

E4.5.2 LRFD or LSD Methods

For screw connections subjected to a combination of shear and

tension forces, the following requirements shall be met:

P ns

+ 0.71

P nov

≤ 1.10ϕ (8.69)

In addition, Q and T shall not exceed the corresponding

allowable strength determined by Sections E4.3 and E4.4,

respectively,

where Q = required shear strength [factored shear force] of

connection

T = required tension strength [factored tensile force]

of connection

= nominal shear strength [resistance] of connection

= 2.7t

nov

= nominal pull-out strength [resistance] of

connection

= 2.7t

= larger of screw head diameter or washer diameter

φ = 0.65 (LRFD)

= 0.55 (LSD)

296 8 CONNECTIONS

Equations (8.68) and (8.69) shall be valid for connections that

meet the following limits:

1. 0.0285 in. (0.724 mm) ≤ t

≤ 0.0445 in. (1.13 mm)

2. No. 12 and No. 14 self-drilling screws with or without

washers

3. d

≤ 0.75 in. (19.1 mm)

4. F

≤ 70 ksi (483 MPa, or 4920 kg/cm

)

5. t

≥ 2.5

For eccentrically loaded connections that produce a nonuni-

form pull-over force on the fastener, the nominal pull-over

strength [resistance] shall be taken as 50 percent of P

nov

When using the above design provisions, the AISI

Commentary recommends that at least two screws should

be used to connect individual elements.

1.346

This provides

redundancy against undertorquing, overtorquing, and so on

and limits lap shear connection distortion of ﬂat unformed

members such as straps. Table 8.6 lists the nominal diam-

eters for the common number designations for screws.

Screw connections loaded in shear can fail either in one

mode or in a combination of several modes. The failure

modes include shearing of the screw, edge tearing, tilting

and subsequent pull-out of the screw, and bearing failure of

the joined materials. Tilting of the screw followed by thread

tearing out of the lower sheet reduces the connection shear

capacity from that of the typical bearing strength of the

connection as shown in Fig. 8.34.

1.346

With regard to the tilting and bearing failure modes, two

cases are considered in the speciﬁcation, depending on the

ratio of thicknesses of the connected members. If the head

of the screw is in contact with the thinner material as shown

Table 8.6 Nominal Body Diameter for Screws

1.310

Nominal Diameter for

Number

Screws

Designation in. mm

0 0.060 1.52

1 0.073 1.85

2 0.086 2.18

3 0.099 2.51

4 0.112 2.84

5 0.125 3.18

6 0.138 3.51

7 0.151 3.84

8 0.164 4.17

10 0.190 4.83

12 0.216 5.49

0.250 6.35

in Fig. 8.35, tilting is not a design consideration when

≥ 2.5. However, when both members are the same

thickness, or when the thicker member is in contact with

the screw head as shown in Fig. 8.36, tilting must also be

considered when t

≤ 1.0. Use linear interpolation for

1.0 <t

< 2.5.

Screw connections subjected to tension can fail by either

pulling out of the screw from the plate (pull-out) or pulling

of material over the screw head and the washer (pull-over)

or by tension fracture of the screw. For the failure mode

of pull-out, Eq. (8.65) was derived on the basis of the

modiﬁed European Recommendations and the results of

a large number of tests. For the limit state of pull-over,

Eq. (8.66) was derived on the basis of the modiﬁed British

Standard and the results of a series of tests. The statistic

data on these tests are presented by Pekoz in Ref. 8.54.

8.5.2 Additional Information on Screw Connections

The North American Standard for Cold-Formed Steel

Framing—General Provisions

8.111

stipulates that a prop-

erly installed screw shall extend through the connection

a minimum of three exposed threads. Also, the guidance

is provided for remediation if a screw is stripped during

installation or if the screws are spaced closer than the AISI

North American speciﬁcation requirements.

Lease and Easterling

8.113

determined that the design

provisions in Section E4 of the Speciﬁcation are valid for

applications that incorporate 6

in. (162 mm) or less of

compressible insulation.

During recent years, research work on screw connections

has been conducted by Xu,

8.76

Daudet and LaBoube,

8.77

Serrette and Lopez,

8.78

Rogers and Hancock,

2.57,2.59

Kreiner

and Ellifritt,

8.80

Anderson and Kelley,

8.81

Sokol, LaBoube,

and Yu,

8.82

Zwick and LaBoube,

8.114

Carr, Mansour,

and Mills,

8.115

Fulop and Dubina,

8.116

Mahendran and

Maharachchi,

8.117,8.118

and other researchers.

8.6 OTHER FASTENERS

The 2007 edition of the AISI North American Speciﬁcation

provides design provisions only for welded connections

(Section 8.3), bolted connections (Section 8.4), and screw

connections (Section 8.5). There are a number of other

types of fasteners which are used in cold-formed steel

construction. The following provides a brief discussion on

other fasteners.

8.6.1 Rivets

Blind rivets and tubular rivets are often used in cold-formed

steel construction. They are used to simplify assembly,

improve appearance, and reduce the cost of connection.

OTHER FASTENERS 297

Figure 8.38 Types of blind rivets and methods of setting

8.3

:(a) pull-stem rivets; (b) explosive

rivets; (c) drive-pin rivets.

8.6.1.1 Blind Rivets

8.3

Based on the method of setting,

blind rivets can be classiﬁed into pull-stem rivets, explosive

rivets, and drive-pin rivets:

1. Pull-Stem Rivets. As shown in Fig. 8.38, pull-stem

rivets can be subdivided into three types:

a. Self-Plugging Rivets. The stem is pulled into but

not through the rivet body and the projecting end

is removed in a separate operation.

b. Pull-Through Rivets. A mandrel or stem is pulled

completely out, leaving a hollow rivet.

c. Crimped-Mandrel Rivets. A part of the mandrel

remains as a plug in the rivet body.

2. Explosive Rivets. Explosive rivets have a chemical

charge in the body. The blind end is expanded by

applying heat to the rivet head.

3. Drive-Pin Rivets. Drive-pin rivets are two-piece rivets

consisting of a rivet body and a separate pin installed

from the head side of the rivet. The pin, which can

be driven into the rivet body by a hammer, ﬂares out

the slotted ends on the blind side.

In the design of a joint using blind rivets, the following

general recommendations may be used:

1. Edge Distance. The average edge distance is two

times the diameter of the rivet. For lightly loaded

joints, the distance can be decreased to one and a half

diameters; for heavily loaded joints, an edge distance

of three diameters may be needed.

2. Spacing. The spacing of rivets should be three times

the diameter of the rivet. It may be desirable to

298 8 CONNECTIONS

die

steel sheets to be

connected

punch

shearing of metal

lateral deformation of

steel as die spreads

finished press join

Figure 8.39 Sequence of forming press joint.

8.65

decrease or increase the spacing depending upon the

nature of the load.

3. Tension and Bearing Stresses. The tension stress on

the net section and the bearing stress may be deter-

mined by the method used for bolted connections.

4. Shear Stress. The shear stress on rivets should be

obtained from the manufacturer.

8.6.1.2 Tubular Rivets

8.3

Tubular rivets are also often

used to fasten sheetmetal. The strength in shear or compres-

sion is comparable to that of solid rivets. Nominal body

diameters range from 0.032 to 0.310 in. (0.8 to 7.9 mm).

The corresponding minimum lengths range from

in. (0.8 to 6.4 mm). When tubular rivets are used to join

heavy- and thin-gage stock, the rivet head should be on the

side of the thin sheet.

8.6.2 Press Joints and Rosette Joints

8.6.2.1 Press Joints Press joining is a relatively

new technique for joining cold-formed steel sections.

It has many advantages over conventional connection

techniques.

8.64,8.65

The joint is formed using the parent

metal of the sections to be connected. The tools used to

form a press joint consist of a male and female punch and

die. Figure 8.39 shows the sequence of forming a press

joint.

Press joining can be used for fabrication of beams, studs,

trusses, and other structural systems. The structural strength

and behavior of press joints and fabricated components and

systems have been studied recently by Pedreschi, Sinha,

Davies, and Lennon at Edinburgh University.

8.64,8.65,8.84–8.86

8.6.2.2 Rosette Joints Rosette joining (Fig. 8.40) is

also a new automated approach for fabricating cold-formed

steel components such as stud wall panels and roof

trusses.

8.87,8.88

It is formed in pairs between prefabricated

holes in one jointed part and collared holes in the other

part. The joining process is shown in Fig. 8.41.

Figure 8.40 Rosette joint.

8.87

Figure 8.41 Rosette-joining process.

8.87

During recent years, the strength and behavior of the

Rosette joints and the fabricated thin-walled sections

have been investigated by Makelainen, Kesti, Kaitila, and

Sahramaa at the Helsinki University of Technology.

8.87

The

tests were compared with the values calculated according

to the 1996 edition of the AISI Speciﬁcation supported

by a distortional buckling analysis on the basis of the

Australian/New Zealand Standard.

8.6.3 Power-Actuated Fasteners

Power-actuated fastening is used by virtually all building

trades. It is used to attach electrical boxes and conduit,

I- OR BOX-SHAPED COMPRESSION MEMBERS MADE BY CONNECTING TWO C-SECTIONS 299

to attach lath, to hang sprinklers and ductwork, to build

concrete formwork, and to attach ﬂoor and roof decking.

The most common application in the cold-formed steel

framing industry is the attachment of a bottom wall track

to concrete or structural steel supports. Power-actuated

fastening was ﬁrst developed in the 1920s and has been

in widespread use in the United States for decades.

Power-actuated fastening systems are produced by a

number of manufacturers. Although some systems offer

special features, all consist of three components: fastener,

powder load, and power-actuated tool. The CFSEI Tech

Note

8.119

provides more information.

8.7 RUPTURE FAILURE OF CONNECTIONS

In the design of connections, consideration should also be

given to the rupture strength of the connection along a plane

through the fasteners. In 1999, the AISI Speciﬁcation was

revised to include the following provisions in Section E5

of the supplement for rupture strength.

1.333

E5 Rupture

E5.1 Shear Rupture

At beam-end connections, where one or more ﬂanges are coped

and failure might occur along a plane through the fasteners, the

nominal shear strength, V

, shall be calculated as follows:

= 0.6F

(8.70)

 = 2.0 (ASD)

φ = 0.75 (LRFD)

where A

= (h

− n

)t (8.71)

= coped ﬂat web depth

= number of holes in the critical plane

= hole diameter

= tensile strength of the connected part as

speciﬁedinSectionA3.1orA3.3.2

t = thickness of coped web

E5.2 Tension Rupture

The nominal tensile rupture strength along a path in the

affected elements of connected members shall be determined

by Section E2.7 or E3.2 for welded or bolted connections,

respectively.

E5.3 Block Shear Rupture

The nominal block shear rupture design strength, R

, shall be

determined as the lesser of the following equations:

= 0.6F

+ F

(8.72)

= 0.6F

+ F

(8.73)

Figure 8.42 Shear rupture of beam-end connection.

1.310,1.346

For bolted connections,

 = 2.22 (ASD)

φ = 0.65 (LRFD)

For welded connections,

 = 2.50 (ASD)

φ = 0.60 (LRFD)

where A

= gross area subject to shear

= net area subject to shear

= net area subject to tension

For the AISI design provisions, Eq. (8.70) deals with the

tearing failure mode along the perimeter of the holes as

shown in Fig. 8.42. This design criterion was developed

on the basis of the tests conducted by Birkemoe and

Gilmor.

8.89

Additional information can be found in Refs.

8.90 and 8.91.

In some connections, a block of material at the end of the

member may tear out, as shown in Fig. 8.43. The design

equations are based on the assumption that one of the failure

paths fractures and the other yields. In Eqs. (8.72) and

(8.73), the gross area is used for yielding and the net area

is used for fracture. The shear yield stress is taken as 0.6F

and the shear strength is taken as 0.6F

8.8 I- OR BOX-SHAPED COMPRESSION

MEMBERS MADE BY CONNECTING TWO

C-SECTIONS

I-sections fabricated by connecting two C-sections back

to back are often used as compression members in cold-

formed steel construction. In order to function as a single

compression member, the C-sections should be connected

at a close enough spacing to prevent buckling of individual

C-sections about their own axes parallel to the web at a

load equal to or smaller than the buckling load of the

entire section. For this reason, in previous editions Section

D1.1(a) of the AISI Speciﬁcation limited the maximum

longitudinal spacing of connections to

max

(8.74)

300 8 CONNECTIONS

Figure 8.43 Block shear rupture in tension.

1.310,1.346

where s

max

= maximum permissible longitudinal spacing

of connectors

L = unbraced length of compression member

= radius of gyration of I-section about the

axis perpendicular to the direction in which

buckling would occur for the given

conditions of end support and intermediate

bracing

= radius of gyration of one C-section about its

centroidal axis parallel to the web

This requirement ensured that the slenderness ratio of

the individual C-section between connectors is less than

or equal to one-half of the slenderness ratio of the entire

compression member in the case that any one of the

connectors may be loosened or ineffective.

Box-shaped sections made by connecting two C-sections

tip to tip are also often found in use in cold-formed steel

structures due to the relatively large torsional rigidities

and their favorable radius of gyration about both principal

axes. The foregoing requirement for maximum spacing

of connectors for I-shaped members was also applicable

to box-type compression members made by C-sections

tip to tip, even though it is not speciﬁed in the AISI

speciﬁcation.

1.310

In 2007 for two cross sections in contact Section D1.2 of

the Speciﬁcation stipulates the use of a modiﬁed slenderness

ratio as deﬁned in Section 5.10 in this volume when

computing the available axial strength of a compression

member. When using Section D1.2, the following fastener

requirements are to be satisﬁed:

1. The intermediate fastener or spot weld spacing, a,is

limited such that a/r

does not exceed one-half the

governing slenderness ratio of the built-up member.

2. The ends of a built-up compression member are

connected by a weld having a length not less than

the maximum width of the member or by connectors

spaced longitudinally not more than four diameters

apart from a distance equal to 1.5 times the maximum

width of the member.

3. The intermediate fastener(s) or weld(s) at any longi-

tudinal member tie location are capable of transmit-

ting a force in any direction of 2.5% of the nominal

axial strength (compressive resistance) of the built-up

member.

Example 8.6 In Example 5.2 fasteners were spaced 12 in.

on center to achieve the built-up member shown in Figure

5.25. Fastener requirement 1 above was evaluated in

I-BEAMS MADE BY CONNECTING TWO C-SECTIONS 301

Example 5.2. Evaluate fastener requirements 2 and 3 if the

fastener is a 1-in.- long ﬂare V-groove weld at each ﬂange

to web junction. Assume F

= 60 ksi.

SOLUTION

1. The maximum width of the member is the web depth

8 in. Therefore at each end of the member 8 in. of

weld is required.

2. The intermediate 1-in. groove welds must be capable

of transmitting a force of 2.5% of the nominal axial

strength, P

. From Example 5.2, P

= 46.07 kips.

Thus, the required transmitting force is 0.025 × 46.07

kips = 1.15 kips. The welds must be capable of trans-

mitting this force in any direction or this implies

that the welds must be evaluated for both the trans-

verse loading and longitudinal loading in accordance

with Section E2.5 of the AISI North American speci-

ﬁcation:

a. Transverse loading (for 1 in. of weld length):

= 0.833tLF

= 0.833 × 0.075 ×1 × 45

= 2.81 kips

For the ASD method,



2.81

2.55

= 1.10 kips per weld × 2

= 2.20 kips > 1.15 kips

For the LRFD method,

φP

= 0.60 ×2.81 = 1.69 kips per weld × 2

= 3.37 kips > 1.15 kips

b. Longitudinal loading (for 1 in. of weld length):

R =

+ 0.075 = 0.1688 in. R<

in.

Therefore t

R = 0.0844 in. Because t ≤ t

2t,

= 0.75tLF

= 0.75 ×0.075 × 1 × 45

= 2.53 kips

For the ASD method,



2.53

2.80

= 0.90 kip per weld × 2

= 1.80 kips > 1.15 kips

For the LRFD method,

φP

= 0.55 ×2.53 = 1.39 kips per weld × 2

= 2.78 kips > 1.15 kips

8.9 I -BEAMS MADE BY CONNECTING TWO

C-SECTIONS

In cold-formed steel construction, I-beams are often fabri-

cated from two C-sections back to back by means of two

rows of connectors located close to both ﬂanges. For this

type of I-beam, Section D1.1 of the AISI North Amer-

ican Speciﬁcation includes the following limitations on the

maximum longitudinal spacing of connectors:

max

≤

2gT

(8.75)

where L = span of beam

g = vertical distance between rows of connectors

nearest top and bottom ﬂanges

= design strength (factored resistance) of

connectors in tension

q = design load (factored load) on the beam for

spacing of connectors (use nominal loads for

ASD, factored loads for LRFD)

m = distance from shear center of one C-section to

midplane of its web

For simple C-sections without stiffening lips at the outer

edges,

∗

m =

+ d/3

(8.76)

For C-sections with stiffening lips at the outer edges,

m =



d + 2D



d −



(8.77)

where w

= projection of ﬂanges from inside face of web

(for C-sections with ﬂanges of unequal

widths, w

shall be taken as the width of the

wider ﬂange)

d = depth of C-section or beam

t = thickness of C-section

D = overall depth of lip

= moment of inertia of one C-section about its

centroid axis normal to the web

The maximum spacing of connectors required by Eq.

(8.75) is based on the fact that the shear center is neither

coincident with nor located in the plane of the web and

that when a load Q is applied in the plane of the web, it

produces a twisting moment Qm about its shear center, as

shown in Fig. 8.44. The tensile force of the top connector

can then be computed from the equality of the twisting

moment Qm andtheresistingmomentT

g,thatis,

Qm = T

g (8.78)

∗

See Appendix B for the location of the shear center.

302 8 CONNECTIONS

Figure 8.44 Tensile force developed in the top connector for

C-section.

(8.79)

Considering that q is the intensity of the load and that

s is the spacing of connectors, then the applied load is Q

= qs/2. The maximum spacing s

max

in Eq. (8.75) can easily

be obtained by substituting the above value of Q into Eq.

(8.79).

The determination of the load intensity q is based upon

the type of loading applied to the beam:

1. For a uniformly distributed load,

q = 3w



(8.80)

considering the fact of possible uneven loads.

2. For concentrated load or reaction,

q =

(8.81)

where w



= uniformly distributed load based on nominal

loads for ASD, factored loads for LRFD

P = concentrated load or reaction based on

nominal loads for ASD, factored loads for

LRFD

N = length of bearing

If the length of bearing of a concentrated load or reaction

is smaller than the spacing of the connectors (N < s), the

required design strength of the connectors closest to the

load or reaction is

(8.82)

where P

is a concentrated load (factored load) or reaction

based on nominal loads for ASD, factored loads for LRFD.

It should be noted that the required maximum spacing

of connectors, s

max

, depends upon the intensity of the load

applied at the connection. If a uniform spacing of connec-

tors is used over the entire length of the beam, it should be

determined at the point of maximum load intensity. If this

procedure results in uneconomically close spacing, either

one of the following methods may be adopted

1.314

1. The connector spacing may be varied along the

beam length according to the variation of the load

intensity.

2. Reinforcing cover plates may be welded to the ﬂanges

at points where concentrated loads occur. The strength

in shear of the connectors joining these plates to the

ﬂanges shall then be used for T

, and the depth of the

beam can be used as g.

In addition to the above considerations on the required

strength of connectors, the spacing of connectors should

not be so great as to cause excessive distortion between

connectors by separation along the top of ﬂange. In view

of the fact that C-sections are connected back to back

and are continuously in contact along the bottom ﬂange,

a maximum spacing of L/3 may be used. Considering

the possibility that one connector may be defective, a

maximum spacing of s

max

= L/6 is required in the AISI

North American Speciﬁcation.

Example 8.7 Determine the maximum longitudinal

spacing of welds for joining two 6 × 2

× 0.105-in.

channels tip to tip to make a box-shaped section (Fig. 8.45)

for use as a simply supported column member. Assume

that the column length is 10 ft.

SOLUTION. Using the method described in Chapters

3–5, the radius of gyration of the single-channel section

(6 × 2

× 0.105 in.) about the y axis is

= 0.900 in.

Figure 8.45 Example 8.6.

I-BEAMS MADE BY CONNECTING TWO C-SECTIONS 303

The radii of gyration of the box-shaped section (6 × 5

× 0.105 in.) about the x and y axes are

= 2.35 in. r

= 1.95 in.

Since r

¡ r

, the governing radius of gyration for the box-

shaped section is r = 1.95 in. Based on Eq. (8.63), the

maximum longitudinal spacing of welds is

max

(10 × 12)(0.900)

2 × 1.95

= 27.7in.

Use 27 in. as the maximum spacing of welds.

Example 8.8 Use the ASD and LRFD methods to deter-

mine the maximum longitudinal spacing of

-in. A307 bolts

joining two 6 × 1

× 0.105-in. C-sections to form an I-

section used as a beam. Assume that the span length of the

beam is 12 ft, the applied uniform load is 0.4 kip/ft, and

the length of the bearing is 3.5 in. (Fig. 8.46). Assume that

the dead load–live load ratio is

SOLUTION

A. ASD Method

1. Spacing of Bolts between End Supports. The

maximum permissible longitudinal spacing of

-in.

bolts can be determined by Eq. (8.75) as follows:

max

(12 × 12) = 24 in.

and

max

≤

2gT

Use

g = d −2(t +R) −2(

)

= 6.0 −2(0.105 + 0.1875) − 0.5 = 4.915 in.

From Table 8.5,

gross area × nominal tensile stress of bolts



0.049 × 40.5

2.25

= 0.88 kip

Figure 8.46 Example 8.7.

From Eq. (8.76),

m =

(1.49 −0.105)

2(1.49 −0.105) +

= 0.402 in.

From Eq. (8.80),

q =

(3 × 0.40) = 0.10 kip/in.

Then based on Eq. (8.75),

max

≤

2(4.915)(0.88)

0.402(0.1)

= 215 in.

Since the maximum longitudinal spacing determined

by L/6 will govern the design, use 24 in. as the

maximum spacing of bolts between end supports.

2. Spacing of Bolts at End Supports. The maximum

spacing of bolts at the end supports can be computed

as follows:

max

≤

2gT

in which

q =

6 × 0.4

3.5

= 0.686kip/in.

and g, T

,andm are the same as those used in item

1 above. Then

max

≤

2(4.915)(0.88)

0.402(0.686)

= 31.4in.

Use s

max

= 24 in. Since N<s

max

, from Eq. (8.82)

the required design strength of bolts closest to the

reaction is

0.4(6)(0.402)

2(4.915)

= 0.098 kip < 0.88 kip

(furnished design strength of bolts) OK

B. LRFD Method

1. Spacing of Bolts between End Supports. From item

A.1, the maximum spacing of bolts is

max

L = 24in.

and

max

≤

2gT

where g = 4.915 in. From Table 8.5,

= φT

= (0.75)(A

)

= (0.75)(0.049 ×40.5) = 1.49 kips

From Eq. (8.76),

m = 0.402 in.

304 8 CONNECTIONS

From Eq. (8.80),

q =

3(1.2w



+ 1.6w



)

3(1.2 × 0.1 +1.6 × 0.3)

= 0.15kip/in.

Based on Eq. (8.75),

max

≤

2(4.915)(1.49)

(0.402)(0.15)

= 242.9in.

Use s

max

= 24 in.

2. Spacing of Bolts at End Supports. The maximum

spacing of bolts at end supports can be computed as

follows:

max

≤

2gT

in which

q =

P = 6(1.2 ×0.1 +1.6 × 0.3) = 3.6kips

q =

3.6

3.5

= 1.029kips/in.

max

≤

2(4.915)(1.49)

(0.402)(1.029)

= 35.4in.

Use s

max

= 24 in. Since N<s

max

, from Eq. (8.82),

the required design strength of bolts closest to the

reaction is

3.6(0.402)

2(4.915)

= 0.147 kip

<1.49 kip (furnished design strength) OK

8.10 SPACING OF CONNECTIONS IN

COMPRESSION ELEMENTS

When compression elements are joined to other sections by

connections such as shown in Fig. 8.47, the connectors must

be spaced close enough to provide structural integrity of the

composite section. If the connectors are properly spaced,

the portion of the compression elements between rows

of connections can be designed as stiffened compression

elements.

In the design of connections in compression elements,

consideration should be given to:

1. Required shear strength

2. Column buckling behavior of compression elements

between connections

3. Possible buckling of unstiffened elements between the

center of the connection lines and the free edge

Figure 8.47 Spacing of connectors in composite section.

1.161

For this reason, Section D1.3 of the AISI North American

Speciﬁcation contains the following design criteria: The

spacing s in the line of stress of welds, rivets, or bolts

connecting a cover plate, sheet, or a nonintegral stiffener in

compression to another element shall not exceed:

1. that which is required to transmit the shear between

the connected parts on the basis of the design strength

per connection speciﬁed elsewhere herein; nor

2. 1.16t

√

(E/f

), where t is the thickness of the cover

plate or sheet, and f

is the stress at design load in

the cover plate or sheet; nor

3. three times the ﬂat width, w, of the narrowest unstiff-

ened compression element tributary to the connec-

tions, but need not be less than 1.11t



(E/F

) if

w/t < 0.50



(E/F

), or 1.33t



(E/F

) if w/t ≥

0.50



(E/F

), unless closer spacing is required by

1 or 2 above.

In the case of intermittent ﬁllet welds parallel to the

direction of stress, the spacing shall be taken as the clear

distance between welds plus

in. (12.7 mm). In all other

cases, the spacing shall be taken as the center-to-center

distance between connections.

Exception: The requirements of this section of the Spec-

iﬁcation do not apply to cover sheets which act only as

sheathing material and are not considered as load-carrying

elements.

According to item 1, the spacing of connectors for the

required shear strength is

s =

total shear strengths of connectors ×I

where s = spacing of connectors, in.

I = moment of inertia of section, in.

V = total shear force, kips

Q = static moment of compression element being

connected about neutral axis, in.