Chapter 10 Structural Connections

494

10Structural ConnectionsIntroductionIn Chapter 4, an examination was made of the load pathsthrough a structural assemblage, beginning with the roof,floors, walls, and columns and eventually tracing theloads to the foundation. Forces and support reactionswere obtained for each structural element through the useof FBDs and the equations of equilibrium (Chapter 3).Later, the construction of load, shear, and moment dia-grams (Chapter 7) enabled us to determine the criticalshears and moments used in the design of bending mem-bers, which include joists, rafters, beams, and girders.Beam and girder reactions were then used in the design oftimber and steel columns (Chapter 9).

Individual structural members were designed as isolatedelements, assuming that the transfer of loads from onemember to the next was assured. In reality, however, allstructural assemblages are interconnected, and individualmembers work in combination with other membersthrough physical connections. Structural members must bejoined in such a way that permits the safe transfer of loadsfrom one member to another. Many of the structural fail-ures in buildings and bridges occur in the connection ofmembers and not in the members themselves. The tech-niques used by carpenters, welders, and iron workers inmaking a specific joint can create situations that are notamenable to mathematical computations. An improperlyinstalled bolt, undersized or improper strength rating of thebolts, bolt holes that are too large, welds made withimproper penetration of the weld material, improper tight-ening of a bolt, and so on can all lead to unsatisfactorystructural attachments. To preclude such bad jointsor atleast limit the number of them that occurvarious stan-dard codes are in effect. Timber design, including its con-nections, is overseen by the AITC and the NDS by theAmerican Forest and Paper Association. In steel design, theAISCs Manual of Steel Construction and the specifications ofthe American Welding Society (AWS) are the most compre-hensive and most widely used in industry.

Figure 10.1 Sir Humphrey Davy(17781829).

The colleague and, in some ways, the rival ofThomas Young, Davy was appointed Professorof Chemistry at the Royal Institute at the age of24 in the same year that Young was appointed.Davy, in contrast to Young, exhibited adynamic and captivating public personality inhis lectures, which garnered both money andpublicity for the Institute. Davy was extremelyprolific in his work, isolating himself andconducting extensive research. He discoveredthe exhilarating and anesthetic effects ofnitrous oxide (laughing gas) as well aspioneering in the field of electrical engineering.One of the major contributions to the modernworld of construction was his development ofarc welding in the early 1800s. Davy remainedat the Institute for the duration of hisprofessional career and prospered. Heeventually was knighted and became SirHumphrey. He also served as president of theRoyal Society.

Figure 10.2 Roof truss with a tension spliceaxial shear.

Str uctur a l Connect ions 495

10.1 STEEL BOLTED CONNECTIONSBolts used in building connections are generally subjectedto forces that cause tension, shear, or a combination of thetwo. Typical connections subjecting fasteners to axialshear are splices used in trusses, beam-lap splices, andgussets (Figure 10.2). Brackets and web shear splices aretypical eccentric shear connections (Figures 10.3 and 10.4).Bolts in tension are common in hanger connections(Figure 10.5). Typical beam-to-column moment connec-tions are examples of combined tension and shear forces(Figure 10.6).

Figure 10.3 Bracket connection, mill buildingeccentricshear.

Figure 10.4 Beam spliceeccentric shear.

496 Chapter 10

Figure 10.5 Hanger connectionbolts intension.

Figure 10.6 Bolts in shear and tension.

(a) Bolted tee-stubconnectionmoment.

(b) Typical moment connectionwith bolts and welds.

(c) Knee brace connection intension and shear.

The connection shown in Figure 10.6(a) is no longer rec-ommended because of its difficulty in fabrication and ex-pense. Welding has simplified the moment connectionconsiderably, as shown in Figure 10.6(b). Many momentconnections today utilize a combination of shop weldingwith field bolting to simplify the erection process as wellas keep cost down.


Failure of Bolted ConnectionsThe choice of suitable design stresses for bolted joints isnot a simple task. Consideration must be given both to thepossible modes of failure of a given joint and to the behav-ior of materials under such loading. In addition, the meth-ods of fabricating a particular joint may induce latent localstresses or physical conditions that could cause the joint tofail. However, the design of a bolted joint in many cases isa comparatively simple matter, and the computations aregenerally not very involved.

In designing a bolted joint properly, one must anticipateand control the maximum stresses developed at criticalsections. Because it is to be expected that failure will occurat one of these critical sections, any knowledge as to wherethese sections may be located provides information for asuccessful design. Several common critical stress condi-tions develop in all joints, each of which is capable of pro-ducing failure. Shown in Figures 10.7 through 10.12 arethe five basic types of failure:

1. Shear of the bolt (Figures 10.7 and 10.8).2. Bearing failure (crushing) of the connected mem-

bers against the bolt (Figure 10.9).3. Tension failure of the connected member material

(Figure 10.10).4. End tear-out of the connected member (Figure

10.11).5. Block shear, a condition that can occur when the

top flange of the beam is coped to intersect witha girder (Figure 10.12).

Bolted joints are usually categorized by the type and com-plexity of the joint. The most commonly encounteredbolted joints are the lap joint and the butt joint, shownschematically in Figures 10.7 and 10.8 (and in Figures 5.11and 5.12 on pages 256 and 257).

Shear failureThe resisting shear stress developed is only an averageshear stress. Shear stresses are not uniform on the boltcross-section but are assumed so for design.

Single shear:

where:average shear stress (psi or ksi)load on the plate or connection (# of kips)cross-sectional area of bolt Allowable shear stress of bolt (psi or ksi)number of boltsn =

Fv =BA in.2A =

P =fv =

fv =P

n * A Fv

Figure 10.7 Shear failuresingle shear (oneshear surface).

(ave)(ave)

498 Chapter 10

Double shear:

(ave)

whereaverage shear stress (ksi or psi)loads on the plate or connection (# or kips)two cross-sectional areas per bolt Allowable shear stress (ksi or psi)number of bolts

Bearing failure (Figure 10.9)Bearing failure involves the connecting members and notthe bolt itself.

The contact surface of the bolt pushing on the plate istaken as

wherebearing area of plate plate thickness (in.)bolt diameter (in.)number of bolts in bearingn =

d =t =

A in.2 BAp =

Ap = n * t * d

n =Fv =

BA in.22A =P =fv =

fv =P

2A * n Fv

Figure 10.9 Bearing failure.

Figure 10.8 Double sheartwo shear planesto resist.


The average bearing stress developed between the bolt andplate can be expressed as

(ave)

whereaverage bearing stress (psi or ksi)

bearing area applied load (# or kips)allowable bearing stress (psi or ksi)

ultimate stress (psi or ksi)

Tensile failureThe shaded area in Figure 10.10 represents the amount ofmaterial left across the tear to resist the applied load afterdeducting the void left by the bolt hole:

wherenet plate area resisting tension plate width (in.)

diameter of a standard size bolt hole.

plate thickness (in.)number of bolts in a row

The average net tension in the plate across the first line ofbolts is expressed as

(net)ft =

PAnet

Ft

n =t =

AD = bolt diameter + 116 BD =b =

A in.2 BAnet =Anet = 1b - n * D2t

Fu =Fp = 1.2FuFp =P =

A in.2 BAp =fp =

fp =PAp

Fp

Figure 10.10 Tensile failure.

500 Chapter 10

whereaverage tension stress across the net area of themember (psi or ksi)applied load (# or kips)allowable tensile stress across the net area

ultimate tensile stress (psi or ksi)

End tear-out (Figure 10.11) and block shear (Figure 10.12) arepossible failure modes when high, allowable fastener val-ues are used in conjunction with relatively thin material.However, failure is less likely to occur because of thedesign specifications requiring ample edge and end dis-tances and minimum bolt spacing.

Figure 10.13 illustrates, for preliminary design purposes,the minimum pitch and edge distances specified by theAISC to avoid these failure modes. The AISC specifies anabsolute minimum pitch and gage (center-to-center) spac-ing of times the bolt diameter (d), with 3d being thepreferred spacing. In some cases, a dimension of threeinches is used for all sizes of bolts up to one inch in diam-eter. Shear and bearing failures, the result of excessivestresses on the bolts, are avoided by providing a sufficientnumber of fasteners to keep the stresses within the allow-able limits.

223

Fu =Ft = 0.5FuFt =P =

ft =

Figure 10.11 End tear-out.

Figure 10.13 Minimum pitch and edge distance.

Figure 10.12 Block sheartension failure at the endconnections along the perimeter of a group of bolts.


Design Stresses for BoltsBolting has replaced riveting as a means of structurallyconnecting members, because it is quieter, simpler, andcan be performed more quickly with smaller crews, result-ing in lower labor costs. Possibly the most common con-struction methodology today is to utilize shop welding incombination with field bolting.

Three primary types of bolts are used in steel constructiontoday. Common bolts, also known as rough bolts or unfin-ished bolts, are designated as American Society of TestingMaterials (ASTM) A307 and are suitable for use in light-steel-frame structures where vibration and impact do nothave to be considered. Also, unfinished bolts are not per-mitted under some building codes in the construction ofbuildings exceeding certain height limits. Where theseconditions exist, nuts may become loose, impairing thestrength of the connection.

The two other types of bolts, designated as ASTM A325and A490, are high-strength bolts are the most widely usedfasteners for structural steel connections made in the field(Figure 10.14). High-strength bolts are identified on thetop of the head with the legend A325 or A490 and themanufacturers mark. Bolts are available in a variety ofsizes, from to in diameter. However, the most typi-cally used sizes in building construction are and Larger-diameter bolts generally require special equipmentas well as increased spacing and edge distances for properbolt placement. Allowable AISC shear capacities are listedin Table 10.1 on page 504, and AISC allowable bearing val-ues are contained in Table 10.2 on page 505.

Figure 10.15 shows two plates held together by a high-strength bolt with a nut and two washers. When a high-strength bolt is tightened, a very high tensile force Tdevelops in the bolt, thus tightly clamping the connectedparts together. It is the resulting friction force S that resiststhe applied load; unlike unfinished bolts, there is no actualshear or bearing stress. When the joint load P exceeds themagnitude S, slip occurs. If there is slip between the plates,the edges of the plates are brought in contact with theshank of the bolt, thereby producing bearing and shearingstresses in the bolt. If there is no slip, the load P is transmit-ted from one plate to the other by the frictional resistance.

Theoretically, the bolts shank and the surfaces of theplates through which it passes are not in contact at all, be-cause the holes are punched slightly larger than the bolt.Even when a connection of this type is subjected to vibra-tion, the high residual tensile stress prevents loosening.

Mechanically fastened connections that transmit load bymeans of shear in their fasteners are categorized as eitherslip-critical (SC) or bearing-type (N or X). Slip-critical connec-tions, also formerly referred to as friction-type connections, de-pend upon sufficiently high clamping force to prevent slipof the connected parts under anticipated service conditions.

78.

34

11258

Figure 10.15 High-strength bolt in singleshear.

Figure 10.14 ASTM A490 tension-controlbolt with the fluted tip that twists off at theproper tightness. Available in A325 and A490for high-strength bolting. Image reprintedwith permission, courtesy of the LeJeune BoltCompany.

502 Chapter 10

A325-SC ksi (slip-critical connection, standard round hole).Fv = 17

A325-N ksi (bearing-type connection with threads included in shear plane).Fv = 21

A325-X ksi (bearing-type connection with threads excluded from shear plane).Fv = 30

A490-SC ksi (slip-critical connection, standard round hole).Fv = 22

A490-N ksi (bearing-type connection with threads included in shear plane).Fv = 28

A490-X ksi (bearing-type connection with threads excluded from shear plane).Fv = 40

Bearing-type connections are based on the contact (bearing)between the bolt(s) and the sides of the holes to transfer theload from one connected element to another.

Before tension is applied to high-strength bolts throughtightening, the joint surfaces adjacent to bolt heads, nuts,or washers must be free of scale, burrs, dirt, and other for-eign material.

Tightening of high-strength bolts in slip-critical connec-tions is accomplished by special torque or impactwrenches to a tension equal to 70% of the specified mini-mum tensile strength of the bolt. The Research Council onStructural Connections provides four methods of control-ling bolt tension: (a) the calibrated wrench, (b) the twist-off-type tension-control bolt, (c) the direct tensionindicator, and (d) the turn-of-nut method, which requiresspecified additional nut rotation after the bolts are snugtight. Snug tight is defined as the full effort of an ironworker with an ordinary spud wrench that brings the con-nected plies into firm contact. When bolts are not fully ten-sioned, the connections are designed as bearing-typeconnections, where a small amount of movement (slip-page) is expected (Figures 10.16 and 10.17).

The high clamping force produced by the properly tight-ened A325 and A490 bolts is sufficient to assure that slip-page between the connected parts will not occur at fullallowable stress in slip-critical connections and probablywill not occur at working loads in bearing-type connec-tions. The allowable stress in bearing-type connections isbased on a factor of safety of 2.0 or more, which over along period of observation has been found to be adequate.This safety factor is higher than that used for the design ofthe connected member.

The allowable shear stresses for A325 and A490 bolts usedin standard round holes are as follows:

See Table 10.1 for the allowable AISC shear stress of otherbolts.

Figure 10.17 Snugging the bolts and nutswith a tension-control wrench. Imagereprinted with permission. Courtesy of theLeJeune Bolt Company.

Figure 10.16 Tension-control wrench. Imagereprinted with permission. Courtesy of theLeJeune Bolt Company.


The efficiency of threaded fasteners in resisting shear inbearing-type connections is reduced when the bolt threadsextend into the shear plane(s) between the connectedplates (Figures 10.18 and 10.19). Allowable shear stressvalues remain the same for slip-critical connections thathave bolt threads included in the shear planes.

ASTM A325 and A490 high-strength bolts are purpose-fully proportioned and produced so that the threads canbe excluded from the shear planes when it is desirable.

Several types of holes are typically used for bolted connec-tions (Figure 10.20): (a) standard round holes, (b) oversizedholes, (c) short-slotted holes, and (d) long-slotted holes.

Standard round holes are made larger than the diameterof the bolt. In steel-to-steel structural connections, stan-dard round holes can be used in many joint applicationsand, in some cases, are preferred. For example, standardround holes are commonly used in girder and beam con-nections to columns as a means of controlling the center-to-center dimension between columns and to facilitate thealignment of the column to a plumb position. Oversizedand slotted holes are typically used in the field to reducethe fit-up and assembly time during erection.

Oversized holes have nominal diameters up to largerthan bolts and less in diameter, while bolts with a diameter will have a hole larger. Oversize holes are per-mitted in slip-critical connections only.

Short-slotted holes are wider than the bolt diameter andhave a length that does not exceed the oversize hole di-mensions by more than This type of hole can be usedin either bearing-type or slip-critical connections, but ifused in bearing-type connections, the slots must be per-pendicular to the direction of load.

Long-slotted holes are wider than the bolt diameter and have a length not exceeding times the bolt diameter.This type of connection may be used in slip-critical con-nections without regard for the load direction. However,in bearing-type connections, the load must be perpendicu-lar to the slot direction. Slotted holes are particularly use-ful where some amount of field adjustment is necessary.Long-slotted holes can only be used in one of the con-nected members at a joint; the other member must use astandard round hole or be welded.

Allowable shear stress values for bolts in slip-critical con-nections remain unchanged for the oversized and short-slotted holes. However, allowable shear stress valuesdecrease when using long-slotted holes for loads appliedperpendicular to the slot, and an even larger reductionoccurs for loads applied parallel to the slot.

212

116

116.

116

14

178

316

116

Figure 10.20 Typically used hole types.

Figure 10.19 Shear plane passing throughthreads.

Figure 10.18 Threads excluded from theshear plane.

504 Chapter 10

Copyright American Institute of Steel Construction, Inc. Reprinted withpermission. All rights reserved.

Table 10.1

BOLTS, THREADED PARTS AND RIVETSShear

Allowable load in kips

TABLE SHEAR

ASTMDesig-nation

A307

A325

A490

A502-1

A502-2A502-3A36(Fu=58 ksi)

A572, Gr. 50(Fu=65 ksi)

A588(Fu=70 ksi)

Conn-ectionType'

SC*Class

A

N

X

SC'Class

A

N

X

N

X

N

X

N

X

HoleType"

STDNSLSTD

OVS,SSLLSL

STD,NSLSTD,NSLSTD

OVS,SSLLSL

STD,NSLSTD,NSLSTD

STD

STD

STD

STD

STD

STD

STD

Fvksi

10.0

17.0

15.0

12.0

21.0

30.0

21.0

18.0

15.0

28.0

40.0

17.5

22.0

9.9

12.8

11.1

14.3

11.9

15.4

Load-ing'

SDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSDSD

Nominal Diameter d, in.Va | % | % | 1 | 1'A | 1'/. I 1% | 114

Area (Based on Nominal Diameter) in.2.30683.16.15.22

10.44.609.203.687.366.4

12.99.2

18.46.44

12.95.52

11.04.609.208.6

17.212.324.55.4

10.76.7

13.53.06.13.97.93.46.84.48.83.77.34.79.4

.44184.48.87.51

15.06.63

13.35.30

10.69.3

18.613.326.59.28

18.67.95

15.96.63

13.312.424.717.735.37.7

15.59.7

19.44.48.75.7

11.34.99.86.3

12.65.3

10.56.8

13.6

.60136.0

12.010.220.49.02

18.07.22

14.412.625.318.036.112.625.310.821.6

9.0218.016.833.724.148.110.521.013.226.56.0

11.97.7

15.46.7

13.38.6

17.27.2

14.39.3

18.5

.78547.9

15.713.426.711.823.6

9.4218.816.533.023.647.116.533.014.128.311.823.622.044.031.462.813.727.517.334.67.8

15.610.120.18.7

17.411.222.59.3

18.712.124.2

.99409.9

19.916.933.814.929.811.923.920.941.729.859.620.941.717.935.814.929.827.855.739.879.517.434.821.943.7

9.819.712.725.411.022.114.228.411.823.715.330.6

1.22712.324.520.941.718.436.814.729.425.851.536.873.625.851.522.144.218.436.834.468.749.198.221.542.927.054.012.124.315.731.413.627.217.535.114.629.218.937.8

1.48514.829.725.250.522.344.617.835.631.262.444.589.131.262.426.753.522.344.641.683.259.4

119.026.052.032.765.314.729.419.038.016.533.021.242.517.735.322.945.7

1.76717.735.330.060.126.553.021.242.437.174.253.0

106.037.174.231.863.626.553.049.599.070.7

141.030.961.838.977.717.535.022.645.219.639.225.350.521.042.127.254.4

"SC = Slip critical connection.N: Bearing-type connection with threads included in shear plane.X: Bearing-type connection with threads excluded from shear plane.

bSTD: Standard round holes (d + Vie in.) OVS: Oversize round holesLSI: Long-slotted holes normal to load direction SSL: Short-slotted holesNSL: Long-or short-slotted hole normal to load direction

(required in bearing-type connection).CS: Single shear D: Double shear.For threaded parts of materials not listed, use Fy = 0.1 7FU when threads are included in a shearplane, and Fv = 0.22FU when threads are excluded from a shear plane.To fully pretension bolts 1Vi-in. dia. and greater, special impact wrenches may be required.When bearing-type connections used to splice tension members have a fastener pattern whose length,measured parallel to the line of force, exceeds 50 in., tabulated values shall be reduced by 20%. SeeAISC ASD Commentary Sect. J3.4.



Table 10.2

BOLTS AND THREADED PARTSBearing

Allowable loads in kipsTABLE BEARING

Slip-critical and Bearing-type ConnectionsMate-

rialThirlf1 IIIUKness

'/83/ie

1/45/16

%7/16

1/29/165/8

"/16

3/4

'3/16%

'5/16

1

Fu = 58 ksiBolt dia.

3/4

6.59.8

13.116.319.622.8

26.129.432.6

52.2

7/8

7.611.4

15.219.022.826.6

30.534.338.141.9

45.7

60.9

1

8.713.1

17.421.826.130.5

34.839.243.547.9

52.256.660.9

69.6


%

7.311.0

14.618.321.925.6

29.332.9

58.5

7/8

8.512.8

17.121.325.629.9

34.138.442.746.9

68.3

1

9.814.6

19.524.429.334.1

39.043.948.853.6

58.5

78.0


3/4

7.911.8

15.819.723.627.6

31.5

63.0

%

9.213.8

18.423.027.632.2

36.841.345.9

73.5

1

10.515.8

21.026.331.536.8

42.047.352.557.8

84.0


3/4

11.316.9

22.528.133.8

90.0

%

13.119.7

26.332.839.445.9

105.0

1

15.022.5

30.037.545.052.5

60.0

120.0

Notes:This table is applicable to all mechanical fasteners in both slip-critical and bearing-type con-nections utilizing standard holes. Standard holes shall have a diameter nominally Vie-in.larger than the nominal bolt diameter (d + Vie, in.).Tabulated bearing values are based on Fp = 1 .2 Fu.Fu = specified minimum tensile strength of the connected part.In connections transmitting axial force whose length between extreme fasteners measuredparallel to the line of force exceeds 50 in., tabulated values shall be reduced 20%.Connections using high-strength bolts in slotted holes with the load applied in a directionother than approximately normal (between 80 and 1 00 degrees) to the axis of the hole andconnections with bolts in oversize holes shall be designed for resistance against slip at work-ing load in accordance with AISC ASD Specification Sect. J3.8.Tabulated values apply when the distance / parallel to the line of force from the center of thebolt to the edge of the connected part is not less than 1 1/2 d and the distance from the centerof a bolt to the center of an adjacent bolt is not less than 3d. See AISC ASD CommentaryJ3.8.Under certain conditions, values greater than the tabulated values may be justified underSpecification Sect. J3.7.Values are limited to the double-shear bearing capacity of A490-X bolts.Values for decimal thicknesses may be obtained by multiplying the decimal value of the un-listed thickness by the value given for a 1-in. thickness.

506 Chapter 10

Example Problems

10.1 Determine the allowable load capacity of the con-nection shown in Figure 10.21 if unfinished boltsare used with standard-size round holes. Assume theplates are A36 steel.

Solution:

Three possible failure modes could occur in this typicalbutt splice condition. Shear, bearing, and net tension willbe checked to determine the critical condition that governsthe connection capacity.

ShearDouble Shear

Take a section cut through the connection at the buttsplice, and draw an FBD for one-half of the assembly. Thebolts pass through three plates and are thus subjected todouble shear. The general equation for determining theshear capacity of this connection is

where

(see Table 10.1)

double shear

Another way of obtaining the same result but minimizingsome of the computations is to use Table 10.1, where theactual load capacities of commonly used bolt sizes andgrades are given for both single and double shear.

Bearing

Unfinished bolts are checked for bearing in which, assum-ing standard round holes, the allowable stress is taken as

where

The center plate is critical.

Fp = 1.2 * 158 ksi2 = 69.6 ksiFu = 58 ksi for A36 steel

Fp = 1.2Fu

Pv = 2 bolts * 12 k>bolt = 24 k

Pv = Fv * Av = 10 k>in.2 * 24.1 in.2 = 24.1 kAv = 2 * 2 * J * a

78b 2

4 K = 2.41 in.2Av = 2 bolts * 2 * a * d

2

4b

Fv = 10 ksi

Pv = Fv * Av

78 A307

Figure 10.21 Typical butt splice.

Bearing stress on plate.

Bolts in double shear.

(in double shear)

Tension stress on the gross area of bar.

Net tension stress.


Or, using the AISC allowable bearing in Table 10.2,

Remember that bearing failure is in the plate material be-ing connected and not in the connector (bolt).

Net TensionAt Connection

Net tension results in the tearing of the plate due to insuf-ficient material (cross-section) to resist the tension stress.The number and placement of bolts in a row across theconnection greatly influences the susceptibility of theplate to net tension failure.

Tension on the gross area of the plate (in a region beyondthe connection):

and

Because the shear check resulted in the smallest allowablevalue, it governs the capacity of the connection.

Pallowable = 24 k

Pt = 1.31 in.2 * 22 k>in.2 = 28.2 kFt = 0.6Fy = 0.6136 ksi2 = 22 ksi

Pt = Ft * Agross

Agross = A38 B * A3 12 B = 1.31 in.2

Pt = 0.96 in.2 * 29 k>in.2 = 27.8 kAnet = A38 B * A3 12 - 1516 B = 0.96 in.2Ft = 0.5Fu = 0.5158 ksi229 ksi

Pp = 2 bolts * 22.8 k>bolt = 45.6 k

Pp = 2 bolts * A38 * 78 B * 69.6 k>in.2 = 45.7 k

(gros)

508 Chapter 10

Section cut AA.

Figure 10.22 Butt splice connection.

10.2 The butt splice shown in Figure 10.22 uses twoplates to sandwich in the plates being

joined. Four A325-SC bolts are used on both sides ofthe splice. Assuming A36 steel and standard round holes,determine the allowable capacity of the connection.

Solution:

Shear, bearing, and net tension will be checked to deter-mine the critical condition that governs the capacity of theconnection.

Shear: Using the AISC allowable shear in Table 10.1,

Bearing: Use the AISC allowable bearing value found inTable 10.2.

The thinner material with the largest proportional loadgoverns; therefore, the center plate governs. Assume thebolts are at a spacing, center to center.

Tension: The center plate is critical, because its thickness isless than the combined thickness of the two outer plates.

where

The maximum connection capacity is governed by shear.

Pallow = 81.6 k

Pt = 29 k>in.2 * 3.06 in.2 = 88.7 kFt = 0.5 Fu = 0.5158 ksi2 = 29 ksi

Pt = Ft * Anet

= 1516

Hole diameter = 1bolt diameter2 + 116 = 78 + 116

Pb = 30.5 k>bolt * 4 bolts = 122 k3d

12

Pv = 20.4 k>bolt * 4 bolts = 81.6 k 1double shear2

78

8 * 128 *38

Figure 10.23 Typical truss connection with gusset plate.


10.3 A simple truss connection is accomplished usingA325-N bolts in standard round holes. Determine the sizeof the bolts required for the load condition shown inFigure 10.23.

Solution:

Each truss member will be examined individually to de-termine the minimum number of bolts required. Shearand bearing will be checked in each design. However, nonet tension computation will be made, because the doubleangles and gusset plate have large cross-sectional areas.

Diagonal Members A and B

Shear: Double shear, two bolts:

Using Table 10.1,

Bearing: The gusset plate is critical in bearing.

Bearing area:

d =0.216 in.2

38

.= 0.576 in.

Ap = d * t = 0.216 in.2

Abolt =15 k>bolt

Fp=

15 k

69.6 k>in.2 = 0.216 in.2

38

2 - 34 A325-N 1Pv = 2 * 18.6 k>bolt = 37.2 k2

30 k2 bolts

= 15 k>bolt

510 Chapter 10

Figure 10.24 A typical bolt pattern.

Two bolts are necessary for bearing.

Shear governs the design; therefore, use two bolts.

Horizontal Member C

The unbalanced load is 30 k in the horizontal direction.

The design load is the same as for members A and B;therefore, two A325-N bolts are required. However, itis useful in practice to provide an odd number of fastenerssuch that the intersection of the lines of force from the twodiagonals occur in the center of the horizontal membersbolt pattern. This tends to reduce the possibility of undueeccentricity at the connection. Also, it is advisable to main-tain the same bolt size throughout a connection to mini-mize errors resulting from bolt substitution.

Therefore, use three A325-N bolts.

10.4 For the three-row bolted butt joint shown inFigure 10.24, determine the load that can be carriedbased on shear, bearing, and tension. Assume A325-SCbolts in standard round holes.

Solution:

Shear: Six bolts in double shear (see Table 10.1):

Bearing: The center plate is critical in bearing. UsingTable 10.2,

Net Tension: The tension capability of the plate (center) willbe checked across the three rows of bolts. This particulartype of bolt arrangement is sometimes used to reduce thepossibility of net tension failure. The idea is to have the loadtransfer through the rows of bolts, diminishing the forceprogressively for each subsequent row of bolts.

Pp = 6 bolts * 26.1 k >bolt = 156.6 k

Pv = 6 bolts * 15 k >bolt = 90 k34

34

34

P = 30 k

34

58


Section 1

Hole diameter

(across one bolt)

Section 2

The bolt across section 1 reduces the total tensile load oc-curring at section 2. Therefore, the tensile capacity of sec-tion 2 will include the shear contribution of one bolt fromsection 1.

(1 bolt in shear)

Section 3

This section will include the shear contribution of bolts insections 1 and 2. The net area of the plate across the row ofbolts at section 3 is

Based on examining the conditions of shear, bearing, andnet tension across three different sections, the capacity ofthe connection is governed by shear:

Pallow = Pv = 90 k

Pt3 = 29 k>in.2 * 3.28 in.2 + 3115 k2 = 140.1 kAnet =

12

* 19 - 3 * 0.8132 = 3.28 in.2

Pt2 = 129 k>in.2 * 3.69 in.22 + 15 k = 92 k1across two bolts2Anet =

12

* 19 - 2 * 0.8132 = 3.69 in.2

Pt1 = Ft * Anet = 29 k>in.2 * 4.1 in.2Anet =

12

* 19 - 0.8132 = 4.1 in.2D = 34 +

116 = 0.8125

Ft = 29 ksi

512 Chapter 10

Problems

10.1 Determine the allowable load P permitted for thisdouble shear joint connection assuming A36 steel andA325-SC bolts in standard round holes.

10.2 The vertical steel bar shown is thick and must bedesigned to withstand a tensile load Two A325-Xbolts will be used. Assuming A36 material and standardround holes, calculate the following:

a. The required diameter d of the bolts.b. The required width W of the bar.

10.3 A connection of the type shown uses three A325-

X (STD) bolts in the upper connection and two A325-X(STD) bolts in the three inch bar. What is the maximumload P that this connection can support?

Note: This is an academic exercise. Generally, it is not advisableto use different-sized bolts in the same connection.

10.4 Determine the number of bolts necessary for eachmember framing into the truss joint shown. Bolts are A325-X (NSL), and members are A36 steel.

34

78

34

P = 28 k.38


10.5 Determine the capacity of this butt splice based onshear, bearing, and net tension. The plates are made of A36steel, and the four bolts on each side of the splice are A325-SC with standard round holes.

10.6 A suspension bridge over a river uses a system oflinked bars connected as shown for the main suspensionsystem. Assuming A36 steel and A490-X bolts, determinethe maximum load P that the system can carry. Check forshear, bearing, and net tension at the bolt and tension ofthe member.

Standard Framed Beam ConnectionsStandard AISC tables are available to cover the design of avast majority of typical structural connections where fillerbeams frame into girders or girders frame into columns(Figures 10.25 and 10.26). This type of standard shear con-nection consists of two clip angles placed back to back oneither side of the beam or girder web.

When a beam frames into a girder such that the upper sur-faces of the top flanges are at the same elevation, the termflush top is used. To accomplish this, it is necessary to cutaway a portion of the upper flange as shown in the illus-tration on the right in Figure 10.25. This is known ascoping, or blocking, and for economy, it should be avoidedwhenever possible.

Figure 10.25 Typical beam-girder shearconnection.

514 Chapter 10

Framed beam connections are generally designed forshear, bearing, and web tear-out or block shear (for beamswith the top flange coped). A sample AISC table is shownin Table 10.3 for use in designing slip-critical and bearing-type connections based on shear capacity for standard-size holes. Other AISC tables (not included in this text) areused to check the bearing and web tear-out capacities.

Figure 10.26 Standard framed beam-column connection.

Table 10.3 has provisions for bolt type, bolt size, hole type,number of bolts (using a three inch pitch dimension),angle thickness, and length. High-strength bolts, in eitherslip-critical or bearing-type connections, assume a doubleshear condition through the beam web and a single shearattachment to the column flange or girder web. Clip anglethickness and length are dependent on the fastener size,the magnitude of the applied load, and space limitationwithin the beams flanges. Angles must be able to fit be-tween the clearance of top and bottom flange fillets(Figure 10.27). Angle lengths are generally at least one-halfof the beams depth to provide some resistance to end ro-tation at the beams end.

Figure 10.27 Beam cross-section with clipangles.



Table 10.3

TABLE Bolt Shear3For A307 bolts in standard or slotted holes and for A325 and A490 bolts in slip-critical

connections with standard holes and

Bolt TypeFv , Ksi

Bolt r>iaIn.

d

Angle Thickness/

LIn.

29'/226'/223V4201/j17'/2

14'/211V6

8

516 Chapter 10


Table 10.3 Continued.

FRAMED BEAM CONNECTIONSBolted

TABLE Allowable loads in kips

TABLE Bolt Shear8For bolts in bearing-type connections with standard or slotted holes.

Bolt TypeF,, Ksi

Bolt niaIn.

[/

Angle Thickness/, In.

LIn.

29'/226'/223'/220V417V414'/2

111/2

8V45'/2

/.'In.31282522191613107

n

1098765432

A325-N21.0

%

Vie

%

%

1

%

18616714813011192.874.255.737.1

25322720217715212610175.8"50.5"

33029726423119816513299.066.0

A490-N

28.0

%

%

7/e

'/2

1

%

24722319817314812499.074.249.5

337303269236202168135101"67.3b

440b

396"352"308"264"220"176"132"88.0"

A325-X

30.0

3/4

%

%

%

1

5/a

26523921218615913310679.5"53.0"

36132528925321618014410872.2

C

c

C

c

28323618814194

A490-X

40.0

%

1/2

7/8

5/8

1

5/e

353318283247212177141106"70.7"

48143338533728924219214496

C

C

C

c

377314251188126

Tabulated load values are based on double shear of bolts unless noted. See RCSCSpecification for other surface conditions.

"Capacity shown is based on double shear of the bolts; however, for length L, net shear onthe angle thickness specified is critical. See Table II-C.

"Capacity is governed by net shear on angles for lengths L and U. See Table II-C.


Figure 10.28 Typical beam-column connection.

Example Problems

10.5 Using the AISC framed beam connection bolt shearin Table 10.3, determine the shear adequacy of the connec-tion shown in Figure 10.28. What thickness and anglelength are required?

End beam reaction = 60 k.

Figure 10.29 Typical beam-girder shearconnection.

Solution:

Enter a bolt diameter of (from Table 10.3), A325-N typefasteners, and bolts in double shear through thebeam web.

The shear

The connection is adequate in shear.

Angles are thick and have a length of Becausethe angles are less than the clear dimension T between theflange fillets, there should not be a problem of fit.

10.6 Determine the number of bolts required for the con-nection in Figure 10.29 based on shear if the end reactionis 120 k. What is the required angle thickness and length?Does the angle fit within the flanges?

Solution:

From Table 10.3, this connection requires five A325-Nbolts through the beam web and ten bolts through thegirder web.

Shear

Angles are thick and long.

Because is greater than the connec-tion angles should fit adequately between the flanges.

L = 14.5,T = 15.5

141238

capacity = 126 k 7 120 k.

78

78

L = 1112.5

16

allowable = 74.2 k 7 60 k.

n = 434

518 Chapter 10

Problems

10.7 A beam-to-girder connection is bolted using twoclip angles and five A490-X bolts as shown. The beam re-action is equal to 210 kips. Assuming A36 steel and threeinch bolt spacing, determine the bolt diameter required,the clip angle thickness, and the angle length.

10.8 A standard beam-column framed connection usesA36 steel with A325SC bolts at three inch spacing. Forthe connection shown, determine the following:

a. The maximum allowable shear capacity for theconnection.

b. The number of bolts required.c. The length L of the clip angle.

34


10.2 WELDED CONNECTIONSWelding, as ordinarily considered for structural use, may bedefined as a method of joining metals by fusion without theapplication of pressure. The metal at the joint, together withadditional metal supplied in the form of filler metal (froman electrode), is melted, forming a small pool or crater.Upon cooling, the weld and base metal form a continuousand almost homogeneous joint. Many welding processesare recognized by the AWS, but for structural steel used inbuilding construction, arc welding is the method generallyused. For this book, the term welding refers to arc welding,in which the fusion process occurs by the generation of heatfrom an electric arc. Arc welding was first made possible bythe discovery of the electric arc by Sir Humphrey Davy (seeFigure 10.1 on page 494) early in the 19th century. He alsodeveloped the methodology of starting and maintaining anelectric arc.

Electric arc welding requires a power source connected ina circuit that includes a ground cable to the piece beingwelded and, on the electrode cable, the electrode holderand electrode (Figure 10.30).

A sustained arc is formed between the work to be weldedand the electrode in a gap, completing the electrical cir-cuit. The resistance from the air or gas in the gap trans-forms the electrical energy into heat at extremely hightemperatures (approximately 6,500F at the electrode tip).Intense heat is generated by the arc in which the basemetal and electrode filler metal liquefy (at a temperaturein excess of 3,000F) into a pool (called a crater). As themolten metal cools and solidifies, the metals are joinedinto a metallurgically solid, homogeneous piece.

Shielded metal arc welding (Figure 10.31) is used to con-trol the oxidation of the molten pool to prevent porosity inthe metal (causing embrittlement) and to control the melt-ing of the rod for more effective penetrative power.

Shielding in manual arc welding is generally accomplishedthrough the use of a chemical coating on the electrode,while automatic welding frequently uses a powdered fluxto submerge the arc and protect the molten metal from air.

In recent years, great advances have been made in auto-matic and semiautomatic welding processes so that man-ual welding today is generally limited to short welds andfield welding (welding done at the site). A very commonpractice for making a structural connection is to shopweld a connecting deviceclip angles, bearing plates, andso onto one member and then fasten through bolting toa connecting member in the field.

In some instances, such as in moment-resisting connec-tions, fully welded joints are a reasonable option. Becausewelded members can be attached together for momentcapability without using connecting plates or angles(Figures 10.32 and 10.33), the welded connection is usuallysimpler, is more compact, and requires a smaller crew.

Figure 10.30 The welding circuit.

Figure 10.31 Shielded metal arc welding.

Figure 10.32 Bolted tee-stub momentconnectionnot recommended for newconstruction.

520 Chapter 10

Holes for bolts are avoided; therefore, the gross sectionrather than the net section is used to determine the cross-sectional area of members in tension.

On occasion, problems will arise in the application ofwelding to structural connections. The selection of a wrongelectrode, use of an improper amperage/voltage setting onthe welding machine, too rapid a cooling rate of the weld,and development of internal stresses from differential cool-ing are some of the factors affecting proper weldments. Inthe past, these considerations were primarily the concernof the welder. However, with the inception of better meth-ods and standardization, much of this responsibility hasnow been shifted from the welder to the AWS weldingcode. Seriously flawed work has been substantially elimi-nated by requiring each welder to pass rigid qualificationtests and submit his or her work to the careful scrutiny of atrained inspector. To further test the safety of weldedjoints, ultrasonic testing and magnetic particle inspectionare occasionally used to locate internal flaws.

Designers of structural connections, whether bolted orwelded, should always be aware of the actual conditionsduring the erection procedure to facilitate that process andto provide for an economical solution. A wide variety ofconnection types and combinations are possible, and anexperienced designer is most desirable in determining apractical and economical connection.

Types of Welded JointsThere are various types of welded joints in common use. Theselection of the appropriate type is a function of the magni-tude of the load at the joint, the direction of the applied load,the configuration of the joint, the difficulty of the joint prepa-ration, and the cost of erection. Fillet welds (Figure 10.34)and groove (butt) welds (Figure 10.35) are the two mostcommonly used weld types in building construction.

Figure 10.34 Typical fillet welds.

Figure 10.33 Typical welded momentconnection.

Figure 10.35 Typical groove (butt) welds.


On occasion, plug and slot welds (Figure 10.36) are usedfor special circumstances. The following discussion will belimited to load-carrying fillet and groove welds. Commonsymbols used for designating the type of weld is shown inTable 10.4. Appropriate weld symbols are indicated for fil-let and groove welds in Figures 10.34 and 10.35.

Fillet welds and groove welds differ primarily from themanner in which the stress transfer takes place. Groovewelds are normally in direct tension or compression(Figure 10.37), whereas fillet welds are generally subjectedto shear as well as tension or compression (Figure 10.38).

Figure 10.36 Plug and slot welds.

Figure 10.37(a) Full-penetration groove welddevelops the full tensile capability of the plate.

Figure 10.37(b) Full- or partial-penetration groove weldsdevelop full compressive capability of the section.

The strength of a full-penetration groove weld is propor-tional to its cross-sectional area and the strength of thefiller metal. Because the filler metal from the electrode ex-ceeds the strength of typical A36 steel base metal, thegroove weld is stronger than the base material in shear,tension, and compression. The strength for full-penetrationgroove welds is conservatively assumed to be equal to thatof the base material. In other words, a groove weld of thesame cross-section as the connected members is assumedto be 100% efficient in transferring stress. If a groove weldwere made with an incomplete penetration, its strengthwould have to be reduced in accordance with the weldingcode used.

Groove welds are generally used for structural assembliesin which full-strength welds are mandatory. They requirerelatively large amounts of weld metal and can sometimesexperience problems during the welding process. Groovewelds also require the cutting of structural members tomore or less exact lengths for the ends to butt, and theynecessitate extensive edge preparation. As a result, groovewelds are more expensive to produce than fillet welds.

Figure 10.38(a) Fillet welds resist in shear.

Figure 10.38(b) Fillet weld resists tensionthrough shear across throat.

522 Chapter 10


Table 10.4

BASIC WELD

BACK FILLETPLUG

ORSLOT

SYMBOLSGroove or Butt

SQUARE V BEVEL

SUPPLEMENTARY WELD

BACKING SPACER WELD ALLAROUND FIELD WELD

U j FLARE V FLAREBEVEL

SYMBOLSCONTOUR

FLUSH CONVEX For other basic andsupplementary weldsymbols, see

AWS A2.4-86

STANDARD LOCATION OF ELEMENTS OF A WELDING SYMBOLFinish symt

Contour syn

Root openmof fill ing forand slot weEffective thrDepth of presize or strencertain welcReference 1

Speaftcatioor other re

Tail (omittecreference is

Basic weldor detail refNote:

Size, vence line. 1

ThepeArrow

must be shFlago

with the aSymbc

symbol or th fors

n. process

whennot used)

ymbol?rence

reld symbol, leNeither orientarpendicular leand Other Sidiown on both tf field-weld syrrow.Is apply betwtjtherwise dimesymbols do niaterial (such iadopted this cjn the far sidee far side.

ngthlion9 ofti we16 Ambc>enjnsio>texsstionveas

angle ol countersink(or plug welds

Length of weld

Pitch (c t o e spacing)oi welds

Field weld symbol

Weld all-around symbol

Arrow connecting reference lineo arrow side member of jointor arrow side of joint

of weld and spacing must read in that order from left to right along the refer-3f reference line nor location of the arrow alters this rule.V V. V. l^ weld symbols must be at left.ds are of the same size unless otherwise shown. Dimensions of fillet weldsrow Side and the Other Side Symbol.1 shall be placed above and at right angle to reference line of junctionbrupt changes in direction of welding unless governed by the "all around"ned.Dlicitly provide for the case that frequently occurs in structural work, whereffeners) occurs on the far side of a web or gusset plate. The fabricating in-ntion: that when the billing of the detail material discloses the existence ofell as on the near side, the welding shown for the near side shall be dupli-

WELDED JOINTSStandard symbols


The fillet weld is one of the most commonly used welds. Itis the weld by which steel fabricators join plate material tomake built-up beams and girders and, more frequently, tojoin beams to columns or to girders. Even though groovewelds possess greater strength than fillet welds, moststructural connections are joined by fillet welding. Filletwelds allow for greater fit-up tolerances and generallyrequire no edge preparation before welding. The ultimatestrength of a fillet weld is dependent upon the direction ofthe applied load, which is parallel (longitudinal) or trans-verse to the weld.

Experiments have shown that the ends of a fillet weld,lying parallel to the line of action of the load, carry higherunit stresses than the midportion of the weld, as illus-trated in Figure 10.39. Also, when an end weld is com-bined with longitudinal welds, the unit stress in the end(transverse) weld will be approximately 30% higher thanthose in the side (longitudinal) welds; however, this fact isnot recognized by most design specifications.

In fillet welds, with a theoretically triangular cross-section, the critical stress is assumed to be acting on theminimum throat area, regardless of the direction of theapplied load. The throat of a fillet weld (Figure 10.40) ismeasured from the root (inside vertex of the triangle) tothe theoretical face of the weld. The throat is equal to theproduct of the theoretical throat T and the weld length.Shear, bending, and axial forces all cause shear stresses(across the throat) in fillet welds.

Fillet welds are generally specified with equal legs, andthe length of these legs is conveniently used to representthe size of the weld. The effective throat T thickness of anequal-leg 45 fillet weld is considered as

The compatible and most commonly used electrodes forwelding A36 steel are the E60XX and E70XX, where the pre-fix E denotes electrode and the first two digits indicate theultimate tensile strength in thousands of pounds per squareinch. For example, an E70XX electrode has an ultimate ten-sile capacity of 70 ksi. The next-to-last digit indicates theweld position (Figure 10.41) in which the electrode is capa-ble of making satisfactory welds. For example,

T = 0.707 * weld size

Figure 10.39 Longitudinal and transversestresses in fillet welds.

Figure 10.40 Parts of a fillet weld.

E701X All positionsE702X Flat position and horizontal fillets

Fillet weld strength is based on the allowable shear stressfor the weld metal across the effective throat area. The AISCspecifications limit the allowable shear stress on the effec-tive area to 30% of the nominal tensile strength of the weldmetal. Therefore, for A36 steel and E60XX and E70XXelectrodes,

Fv = 0.30 * 70 ksi = 21 ksi 1E70XX2Fv = 0.30 * 60 ksi = 18 ksi 1E60XX2

524 Chapter 10

Figure 10.41 Types of welds based on weldposition.

Weld strengths per inch of weld for any size equal-legweld can be found by multiplying the weld size by 0.707times the allowable shear stress.

For a weld:

weld strength per area

Table 10.5 is included for quicker computations involvingfillet welds.

1At2 * allowable shear stress 1Fv2inch = throat

Athroat = T * 11

Throat area: Athroat = T * length of weld

Throat dimension: T = 0.707 * weld size

In addition to fillet weld strength based on the size andlength of the weld, other welding code provisions areaddressed fully in the AISCs Manual of Steel ConstructionAllowable Stress Design and the structural code of the AWS.Some of the other code items are the following:

The maximum size of a fillet weld applied to asquare edge of a plate or section or more inthickness should be less than the nominalthickness of the edge. Along edges of materialless than thick, the maximum size may beequal to the thickness of the material.

14

116

14

Table 10.5 Allowable strength of filletwelds per inch of weld

Weld Size (in.) E70XX (k/in.)

316

2.78

14

3.71

516

4.64

38

5.57

716

6.49

12

7.42

58

9.27

34

11.13


The minimum size of a fillet weld is dependent onthe thicker of two members being welded but can-not exceed the thickness of the thinner member. Theminimum size of fillet welds is for material witha thickness of or less, for a material thickness over to for material thickness over to and for material thickness over

The minimum effective length of fillet welds shouldbe four times the nominal size, or else the weld sizeis to be taken as of its effective length.

If two or more welds are parallel to each other, thelength must be at least equal to the perpendiculardistance between them (Figure 10.42).

The minimum length of intermittent fillet weldsshould be not less than four times the weld size,with a minimum of

Side or end fillet welds terminating at the ends orsides should be returned, if practical, around thecorners for a distance not less than two times thenominal weld size (Figure 10.43). Added strengthis given to fillet welds with end returns.

Cost concerns also dictate to a large extent the size of thefillet weld to be used. Weld metal volume has a direct cor-relation to the labor costs involved in depositing the weld.The most economical weld minimizes weld metal volumeand, at the same time, reduces the heat input and the asso-ciated shrinkage and distortion of the joint. Minimizingthe weld metal also minimizes the potential for welddefects. Fillet weld sizes should be kept to or less,because the weld is the largest weld size that can bedeposited in one pass with the shielded metal arc processin the horizontal and flat positions. Larger fillet weldsgenerally require two or more passes.

In practice, it is also advisable to maintain the same-sizefillet throughout the connection. A change of fillet sizenecessitates a change of welding rods and, therefore, slowsthe work and may cause errors.

516

516

112.

14

34.

516

34,

12

12,

14

14

316

14

18

Figure 10.42 Minimum length for parallelwelds.

Figure 10.43 End returns for fillet welds.

526 Chapter 10

Figure 10.45 Parallel fillet welds.

Figure 10.44 Plate fillet welded on three sides.

Example Problems

10.7 Determine the capacity of the connection in Figure 10.44 assuming A36 steel with E70XX electrodes.

Solution:

Capacity of weld:

For a fillet weld,

Weld

Weld

Capacity of plate:

allow

The weld size used is obviously too strong. To what size,then, can the weld be reduced so that the weld strength ismore compatible to the plate capacity? To make the weldcapacity plate capacity,

From Table 10.5, use weld .

Minimum size based on a thick plate.

10.8 Determine the size and length of longitudinal filletwelds that will develop the strength of the smaller plate(Figure 10.45). Assume A36 steel with E70XX electrodes.

Solution:

Maximum weld size (limited by the plate thickness):

Note: This is good, because this weld size can be deposited inone pass.

total weld length required =33 k

4.64 k>in. = 7.1 in.Allowable weld strength: S = 4.64 k>in.

weld size = 38 -1

16 =5

16

plate capacity = 4 * 38 * 22 k>in.2 = 33 k

38fillet =

316

1S = 2.78 k>in.2316weld capacity per inch =

49.5 k22 in.

= 2.25 k>in.22 * 1weld capacity per in.2 = 49.5 k

L

plate capacity governs, Pallow = 49.5 k

plate capacity = 38 * 6 * 22 k>in.2 = 49.5 kFt = 0.6Fy = 22 ksi

capacity = 22 * 3.98 k>in. = 87.6 klength = 22

S = 3.98 k>in.516

Figure 10.46 Transverse full-penetrationgroove weld.


Rounding upward to the nearest use a total of of weld or on each side.

Note: The AWS specifies that the weld length on each side of theplate for parallel welds should not be less than the perpendiculardistance between the welds. This requirement is to ensure fulldevelopment of the plate capacity.

Use minimum of weld length on each side ofthe plate.

A smaller weld size can be tried because more weld lengthis required.

Try:

weld with

total weld length

Use weld on each side.

10.9 Determine the capacity of the full-penetrationgroove weld shown in Figure 10.46. Assume A36 steelwith E70XX electrodes.

Solution:

Full-penetration groove welds carry the full capacity ofthe plate.

Pt = 38 * 4 * 22 k>in.2 = 33 k

14 * 4

12

required =33 k

3.71 k>in. = 8.9 in.S = 3.71 k>in.14

4

516 * 3

58

516

7 1414,

528 Chapter 10

Figure 10.47 Angle welded to a gusset plate.

Eccentricity in Welded JointsOne of the most common examples of eccentrically loadedwelded joints is that of a structural angle welded to a gus-set plate as shown in Figure 10.47. The load P in the angleis presumed to act along its centroidal axis. Consequently,because an angle is an asymmetrical cross-section, thewelds marked and are made unequal in length sothat their stresses will be proportional in accordance withthe distributed area of the angle.

L2L1

Writing the equations of equilibrium for the applied forceand weld resistance and

or

If the strength of the weld (Table 10.5) is defined as S,

or

and if the weld size is constant,

where

L1 =L2ba

S1 = S2

S1L1a = S2L2b

3M0 = 04 R1 * a = R2 * b

S1L1 + S2L2 = P

3Fx = 04 R1 + R2 = PR2,R1


Example Problem

10.10 An angle (A36 steel) is attached to alarge gusset plate by fillet welds s shown in Figure 10.48.Determine the lengths and to support a tensile load of30 k. Assume the angle to be subjected to repeated stress vari-ations (minimize eccentricity).

L2L1

14

L3 * 2 * 516

Figure 10.48 Welded connection of angle iron and gusset plate.

Substituting:

For fillet welds (E70 electrode),

Weld strength:

Use: weld.

Use: weld.

Check the tensile capacity of the angle:

OKPt = A * Ft = 1.46 in.2 * 22 k>in.2 = 32.1 k 7 30 k

14 * 5

38

R2 = S * L2 = 3.71 k>in. 1L2214 * 2

34

R1 = S * L1 = 3.71 k>in. 1L12S = 4.64 k>in.

14

R2 = 19.8 k and R1 = 10.2 k

0.515R2 + R2 = 30 k

R1 =1.02R2

1.98= 0.515R2

3M0 = 04R111.982 = R211.0223Fx = 04R1 + R2 = 30 k

FDB of the angle iron welds.

530 Chapter 10

Problems

Assume in each problem that the base metal is A36 steeland that the electrodes are E70XX.

10.9 Determine the maximum load-carrying capacity ofthis lap joint.

10.10 Determine the shear capacity of the fillet weldshown.

10.11 What length L is required to develop the full capac-ity of the plate?

10.12 Determine the capacity of the fillet weld connec-tion shown. What would the capacity of the connection beif a full-penetration groove weld were used instead?

10.13 Compute the length and size of the fillet weldneeded to develop the full tensile strength of the angle.Use a full-transverse fillet weld on the end and balancedwelds on the sides (for minimizing eccentricity).

14


10.3 COMMON FRAMING DETAILSIN STEEL

Structural analysis theory assumes that connectionsbetween beams and columns or between columns andfoundations are rigid (fixed), allowing no relative rotationbetween connected elements or true pins, hinged with nomoment resistance. In reality, structural design deals withconnections that fit neither of these assumptions fully.Most connections exhibit some degree of moment resis-tance and a varying degree of rotational joint resistance.

The true behavior of the connection (i.e., moment androtational characteristics) affects the strength and stabilityof the individual connected elements and the stability ofthe entire building structure. Proper detailing of the con-nections to provide a continuous load path through theinterconnected elements is essential in assuring that theresisting forces will develop in the physical connection toprovide overall structural stability. Consideration of a lat-eral resisting strategy for a structural framework shouldoccur in the early planning stages of a project. Selection ofa lateral resisting system greatly influences the design ofindividual members and their connections.

The three basic lateral resisting systems for steel-framedstructures, discussed in Chapter 4, are the rigid frame(Figure 10.49), the braced frame (Figure 10.50), and theshearwall system. Rigid frame systems require the use ofrigid, moment resisting connections, while braced framesare generally designed as pin connections. Material andlabor costs are generally much lower in pin connections ascompared to rigid, moment connections.

Examples of Connection DetailsIt is particularly useful, when studying structural steelframing, to examine some of the standard or typical detailsof connections used in the preparation of structural draw-ings. The accompanying drawings in Figures 10.51 throughFigure 10.55 show standard details, in a very basic andgeneral form, that are frequently employed in detail draw-ings of steel buildings. Selected details are referenced to thehypothetical structure shown in Figure 10.51. The examplestructure assumes a lateral strategy that utilizes a rigidframe system working in conjunction with a centrallylocated, concrete shear core. Lateral stability is achievedthrough the use of a braced frame-shearwall system in theperpendicular direction. To illustrate the various types ofconnections employed with the different types of bracing,a composite braced frame has been created. This is not atype of braced frame system that one would normally seein an actual building frame.

Figure 10.49 Exposed rigid frame for lateralstability.

Figure 10.50 Concentric bracing in lowertwo floors and eccentric bracing in uppertwo stories.

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Figure 10.51 Steel-framed building utilizing a rigid frame with coreshearwalls in the longitudinal direction and a braced frame with coreshearwalls in the transverse direction.


Rigid FramesRigid frames are constructed with beam and columnsrigidly attached using moment-resisting connections. Therigid frame derives its strength to resist gravity and lat-eral loads from the moment interaction between beamsand columns (as shown in Figure 10.52). Beam-columnconnections maintain their relative 90 orientation to eachother under load, even if the connection assembly rotatesas a unit.

Figure 10.52 Rigid frame with beam-column interaction.

Rigid beam/column connection and rigid bases.

All members share in resisting the lateral force

through bending.

Rigid frame - beam/column and support

Most frames assume pin connections at the base

since footings are susceptible to some degree of

rotation.

Two-hinged frame - pinned base

Three-hinged frame

Three-hinged frame; one hinge at the beams

mid-span

Three-hinged frame pitched roof

Rigid frame with a pitched roof. Large bending

moments develop at the haunch (pitched-rafter

and column joint).

Figure 10.53 Examples of single-bay, single-story rigid frames.

Columns, beams, girders, and joints are responsible fortransferring the horizontal, vertical, and rotational (mo-ment) forces throughout the rigid frame. Because bendingmoments are shared by both beams and columns, membersizes are generally heavier than would be found in bracedframe systems. Stability of the structure is maintained bythe usually high stiffness required in the column andbeam or girder. Therefore, members are typically orientedto take advantage of its strong axis.

Beam-column connections often consist of a shear connec-tion for gravity loads acting in combination with field-welded beam flanges for moment resistance (Figure 10.53).

Compared with braced frames or shearwall structures,rigid frames have the advantage of providing unob-structed space. However, under high seismic loading, thelarge deformations encountered may cause distress to thearchitectural finishes. Rigid frame systems are effective forlow- to medium-rise buildings.

1Ix2

534 Chapter 10

Figure 10.54 Common rigid connection details for beam-column, column splice,and column-foundation.


Figure 10.55(a) Diagonal eccentricbrace.

Figure 10.55(b) Eccentric K-brace.

Braced FramesBraced frames found in use today include X-bracing[Figure 4.51(a)], K-bracing (Vee or inverted Vee) [Figure4.51(c)], and eccentric bracing [Figure 4.51(b)]. Two basiccategories of braced frames are the concentrically and ec-centrically braced frames. Diagonal bracing [Figure4.50(b)], X-bracing, and K- or Vee-bracing are classified asconcentrically braced. The members of a concentricallybraced frame act as a vertical truss system, and diagonalmembers are generally assumed to act primarily intension and sometimes compression. Diagonal tension X-bracing is typically analyzed as having only tensionforces. This design assumption utilizes only one-half ofthe members to resist lateral loads, while the adjacentmember within the same panel is assumed to be negligiblein resisting compressive stress. K-bracing is used in designcircumstances when access through the bracing plane isrequired. The inverted Vee (K-bracing) allows clearancefor doorways, corridors, and rooms.

Bracing members in eccentrically braced frames, asshown in Figure 10.55(a) and 10.55(b), are connected tothe beam so as to form a short link beam between thebrace and the column or between two opposing braces.Link beams act as a fuse to prevent other elements inthe frame from being overstressed. In low-to-moderateground shaking, an eccentrically braced frame performsas a braced frame rather than as a moment frame.Therefore, the structure experiences smaller lateral dis-placements, minor architectural damage, and no struc-tural damage. In major earthquakes, the link beam(Figures 10.56 and 10.57) is specifically designed to yield,thereby absorbing large quantities of seismic energy andpreventing buckling of the other bracing members.Braced frames are more cost effective when compared torigid frames.

Figure 10.56(a) Detail of aneccentrically braced frameconnection.

Intermediate stiffenersEnd stiffeners

Beam

Braces

Figure 10.56(b) Link beam detail.

Plastic hinges

Figure 10.57 Link beam rotation.

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Figure 10.58 Lateral resisting strategy using a concentrically and eccentricallybraced frame.

A summary of the various lateral resisting systems in steelis shown in Figure 10.58.


Summary Five basic types of failure that can cause a critical stress

condition at a joint are

Shear of the bolt. Bearing failure of the connected members against

the bolt. Tension failure of the connected member material. End tear-out of the connected member. Block shear.

There are three primary types of bolts used in steel con-struction currently:

ASTM A307 unfinished boltused in light steelframe structures where vibration and impact arenot critical.

ASTM A325 and A490 high-strength boltsthemost widely used fasteners of steel construction.

Mechanically fastened high-strength connections thattransmit load by means of shear in their fasteners arecategorized as either slip-critical (SC) or bearing-type(N or X).

Slip-critical connections depend upon sufficiently highclamping force to prevent slip of the connected partsunder service conditions.

Bearing-type connections are based on the contact(bearing) between the bolt(s) and the sides of the holesto transfer load from one connected member to another.

Standard AISC tables are used to cover the design of avast majority of typical structural connections wherefiller beams frame into girders or girders into columns.

The various types of welded joints commonly use a fil-let weld or a groove (butt) weld.

Fillet welds resist load in shear. The critical shear stressis assumed to be acting on the minimum throat area ofthe weld. The throat of a fillet weld is measured fromthe root to the theoretical face of the weld.

The strength of a full-penetration groove weld is pro-portional to its cross-sectional area and the strength ofthe filler metal. In general, a groove weld is assumedequal to the strength of the base material being welded.

CoverTitle PageCopyright PageForewordPrefaceACKNOWLEDGMENTSDefinition of TermsContentsCHAPTER 1 INTRODUCTION1.1 Definition of Structure1.2 Structural Design1.3 Parallels in Nature1.4 Loads on Structures1.5 Basic Functional Requirements1.6 Architectural Issues

CHAPTER 2 STATICS2.1 Characteristics of a Force2.2 Vector Addition2.3 Force Systems2.4 Equilibrium Equations: Two-Dimensional2.5 Free-Body Diagrams of Rigid Bodies2.6 Statical Indeterminacy and Improper Constraints

CHAPTER 3 ANALYSIS OF SELECTED DETERMINATE STRUCTURAL SYSTEMS3.1 Equilibrium of a Particle3.2 Equilibrium of Rigid Bodies3.3 Plane Trusses3.4 Pinned Frames (Multiforce Members)3.5 Three-Hinged Arches3.6 Retaining Walls

CHAPTER 4 LOAD TRACING4.1 Load Tracing4.2 Lateral Stability Load Tracing

CHAPTER 5 STRENGTH OF MATERIALS5.1 Stress and Strain5.2 Elasticity, Strength, and Deformation5.3 Other Material Properties5.4 Thermal Effects5.5 Statically Indeterminate Members (Axially Loaded)

CHAPTER 6 CROSS-SECTIONAL PROPERTIES OF STRUCTURAL MEMBERS6.1 Center of GravityCentroids6.2 Moment of Inertia of an Area6.3 Moment of Inertia of Composite Areas6.4 Radius of Gyration

CHAPTER 7 BENDING AND SHEAR IN SIMPLE BEAMS7.1 Classification of Beams and Loads7.2 Shear and Bending Moment7.3 Equilibrium Method for Shear and Moment Diagrams7.4 Relationship Between Load, Transverse Shear, and Bending Moment7.5 Semigraphical Method for Load, Shear, and Moment Diagrams

CHAPTER 8 BENDING AND SHEAR STRESSES IN BEAMS8.1 Flexural Strain8.2 Flexural (Bending) Stress Equation8.3 Shearing StressLongitudinal and Transverse8.4 Development of the General Shear Stress Equation8.5 Deflection in Beams8.6 Lateral Buckling in Beams8.7 Introduction to Load Resistance Factor Design (LRFD)

CHAPTER 9 COLUMN ANALYSIS AND DESIGN9.1 Short and Long ColumnsModes of Failure9.2 End Support Conditions and Lateral Bracing9.3 Axially Loaded Steel Columns9.4 Axially Loaded Wood Columns9.5 Columns Subjected to Combined Loading or Eccentricity

CHAPTER 10 STRUCTURAL CONNECTIONS10.1 Steel Bolted Connections10.2 Welded Connections10.3 Common Framing Details in Steel

CHAPTER 11 STRUCTURE, CONSTRUCTION, AND ARCHITECTURE11.1 Initiation of ProjectPredesign11.2 Design Process11.3 Schematic Design11.4 Design Development and Construction Documents11.5 Integration of Building Systems11.6 Construction Sequence11.7 Conclusion

APPENDIX: TABLES FOR STRUCTURAL DESIGNLumber Section Properties(a) Dimensioned SizesRafters, Joists, and Studs(b) Beams and Columns

Allowable Stress Design for Shapes Used as BeamsStructural SteelWide-Flange ShapesStructural SteelAmerican Standard Shapes and ChannelsStructural SteelTubing (Square) and PipeStructural SteelAnglesDefinition of Metric (S.I.) Terms and Conversion TablesWide Flange Shapes (Abridged Listing)S.I. MetricElastic Section ModulusU.S. and S.I. MetricWestern Glue-Laminated SectionsU.S. and S.I. MetricPlastic Section ModulusSelected Beam Shapes

ANSWERS TO SELECTED PROBLEMSINDEXABCDEFGHIJKLMNOPRSTUVWXYZ

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