1

Bolted Connections

1

Assoc. Prof. Akhrawat LenwariDepartment of Civil EngineeringFaculty of EngineeringChulalongkorn University

Outline

1. Introduction2. Bolted connection types3. Failure modes4. Tensile and shear strengths of bolts5. Combined tension and shear in bearing-type

connections6. Bearing strength at bolt holes7. High-strength bolts in slip-critical connections8. Combined tension and shear in slip-critical

connections9. Eccentric shear (elastic method)

2

2

1. Introduction• The primary connection methods for structural steel are

bolting and welding. • Connections made in a fabrication shop are called “shop

connections”.• Connections made in the field by the steel erector are called

“field connections”.• Bolting and welding may be used for shop connections and

field connections.• Field connections are typically bolted.• Bolting is generally a faster operation than welding. It does

not have the temperature and weather condition requirements that are associated with welding. Unexpected weather changes may delay welding operations.

3

ASTM Bolt Types

4

• A307 – Low carbon steel (not require pretensioning force)• High-Strength Bolts (see Research Council of Structural Connections)

Group A-ASTM F3125 Grades A325, F1852 and ASTM A354 Grade BCGroup B-ASTM F3125 Grades A490, F2280 and ASTM A354 Grade BGroup C-ASTM F3043 and F3111

3

Common Bolt Sizes

5

• A325 and A490 bolts are available in diameters ranging from 1/2” to 1-1/2”.

• The most common sizes are 3/4”, 7/8”, and 1”.• High-strength bolts are commonly available in incremental

lengths up to 8” (AISC).

Washers

• Hardened steel washers are used in many structural connections to spread pressure from the bolt tightening process over a larger area.

• Flat washers are most commonly used.• Tapered washers (above left) are used when the surface

being bolted has a sloped surface, such as the flange of a channel or an S shape.

6

4

Parts of the Bolt Assembly

• Grip is the distance from behind the bolt head to the back of the nut or washer. It is the sum of the thicknesses of all the parts being joined exclusive of washers.

• Thread length is the threaded portion of the bolt.• Bolt length is the distance from behind the bolt head to

the end of the bolt.

7

HeadShank

Washer

NutWasher Face

Grip

Thread

Length

2. Bolted Connection Types

There two basic types of bolted connections:• Bearing type

The load is transferred between members by bearing on the bolts.

• Slip-critical typeFormerly known as friction-type connection. The load is transferred between members by friction in the joint. Slip is not allowed at service load.

8

5

3. Failure Modes

• Bolts are subjected to tension, shear, or both.• The material the bolt bears against is also subject to bearing failure

(above right).

9

Bearing Fracture

Bearing Yield

Bearing-Type Connections

• In a bearing joint the connected elements are assumed to slip into bearing against the body of the bolt.

• If the joint is designed as a bearing joint the load is transferred through bearing whether the bolt is installed snug-tight or pretensioned.

10

6

• The “shear plane” is the plane between two or more pieces under load where the pieces tend to move parallel from each other, but in opposite directions.

• The threads of a bolt may either be included in the shear plane or excluded from the shear plane.

• The capacity of a bolt is greater with the threads excluded from the shear plane.

• The most commonly used bolt is an ASTM A325 3/4” bolt with the threads included in the shear plane.

11

Threads Included In The Shear Plane

Threads Excluded From The Shear Plane

Threads in the Shear Plane

Slip-Critical Connections

• In a slip-critical joint the bolts must be pretensioned to cause a clamping force between the connected elements. The minimum pretension force (about 70% the tensile strength of bolt) is specified in Table 1.

• This force develops frictional resistance between the connected elements.• The frictional resistance allows the joint to withstand loading without slipping

into bearing against the body of the bolt, although the bolts must still be designed for bearing.

• The faying surfaces in slip-critical joints require special preparation.

12

7

Table 1. Minimum Bolt Pretension (kN)

13

Slip-Critical Connections

Slip-critical joints are only required in the following applications involving shear or combined shear and tension• Joints that are subject to fatigue load with reversal of the

loading direction (not applicable to wind bracing).• Joints that utilize oversized holes.• Joints that utilize slotted holes, except those with applied

load approximately perpendicular to the direction of the long dimension of the slot.

• Joints in which slip at the faying surfaces would be detrimental to the performance of the structure.

14

8

Snug-tight Installation

Snug-tight is the tightness attained with a few hits of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the connected plies into firm contact.

15

Pretensioning Methods

16

• Installation beyond snug-tight is called pretensioning.• Pretensioning methods include▫ Turn-of-nut method▫ Calibrated wrench method▫ Use of tension-controlled bolts▫ Use of direct tension indicators

9

Turn-of-Nut Installation

Turn-of-nut method is a procedure whereby the specified pretension in high-strength bolts is controlled by rotating the fastener component a predetermined amount after the bolt has been snug tightened. The specified rotation varies by diameter and length (between 1/3 and 1 turn).

17

Calibrated Wrench Installation

• Calibrated wrench pretensioning uses an impact wrench (above left) to tighten the bolt to a specified tension.

• A Skidmore-Wilhelm calibration device (above right) is used to calibrate the impact wrench to the torque level which will achieve the specified tension.

• A sample of bolts representative of those to be used in the connections are tested to verify that the correct tension will be achieved.

18

10

ASTM F1852 Installation

• F1852 bolts are twist-off-type tension-control bolts.

• These bolts must be pretensioned with a twist-off-type tension-control bolt installation wrench that has two coaxial chucks.

• The inner chuck engages the splined end of the bolt.

• The outer chuck engages the nut.• The two chucks turn opposite to one another to

tighten the bolt.• The splined end of the F1852 bolt shears off at a

specified tension.

19

ASTM F959 Direct Tension Indicators

• Another way to try to ensure proper pretensioning of a bolt is through the use of direct tension indicators (DTIs).

• These washers have protrusions that must bear against the unturned element.

• As the bolt is tightened the clamping force flattens the protrusions and reduces the gap.

• The gap is measured with a feeler gage.• When the gap reaches the specified size the bolt is properly pretensioned.

20

Feeler Gages

DTI’s

11

Bolting Cost Considerations

• Snug-tightened joints are normally the most economical bolted joints.

• For pretensioned joints, F1852’s and DTI’s are popular and can be economical.

• Slip-critical joints are the most costly joints, and should only be specified when necessary.

21

Table 2. Nominal Bolt Hole Dimensions

• Bolts are installed in one of four types of holes (see Table 2).• Standard holes can be used anywhere.• Oversized holes may only be used in slip-critical connections.• Short-slotted holes are used with the slot perpendicular to the direction of stress• Long-slotted holes are primarily used when connecting to existing structures.

22

12

Table 3. Minimum Edge Distance from Center of Standard Hole to Edge of Connected Part

23

ดูหนังสือ หน้าท ี271

24

Minimum edge distances for oversized or slotted holes:

The distance from the center of hole to an edge of a connected part= value from Table 3 + C2

13

Maximum Edge Distance

• The maximum distance from the center of any bolt to the nearest edge of parts in contact shall be 12 times the thickness of the connected part under consideration, but shall not exceed 6 in. (150 mm).

25

Minimum and Maximum Spacings

Minimum Spacing

• The distance between centers of standard, oversized or slotted holes shall not be less than 2 2/3 times the nominal diameter, d, of the fastener; a distance of 3d is preferred.

Maximum SpacingThe longitudinal spacing of fasteners between elements consisting of a plate and a shape or two plates in continuous contact shall be as follows:(a) For painted members or unpainted members not subject

to corrosion, the spacing shall not exceed 24 times the thickness of the thinner part or 12 in. (305 mm).

(b) For unpainted members of weathering steel subject to atmospheric corrosion, the spacing shall not exceed 14 times the thickness of the thinner part or 7 in. (180 mm).

26

Note: requirements of (a) and (b) do not apply to elements consisting of two shapes in continuous contact.

14

4. Tensile and Shear Strengths of Bolts• Tensile Strength of a Bolt

• Shear Strength of a Bolt

where Ab = nominal unthreaded body area of bolt Fnt = nominal tensile stress (Table 4)Fnv = nominal shear stress (Table 4)

27

n nt bR F A=

n nv bR F A=

= 2.00 = 0.75

(1)

(2)

Table 4. Nominal Strength of Bolts

28

(N-type)

(X-type)

(N-type)

(X-type)

15

29

Table 4. Nominal Strength of Bolts (cont’d)

(N-type)

(X-type)

(N-type)

(X-type)

5. Combined Tension and Shear in Bearing-Type Connections

• Combined Tension and Shear Strength of a Bolt

whereFnt′ = nominal tensile stress modified to include the effects

of shear stress.

= (LRFD)

= (ASD)

frv = required shear stress using LRFD or ASD load combinations (must be less than the available shear stress).

30

= n nt bR F A

ntnt nt rv nt

nv

FF F f F

F = 1.3

ntnt nt rv nt

nv

FF F f F

F = 1.3

= 2.00 = 0.75

(3)

16

31

ut

nt

R

R

uv

nv

R

R

2 2

= 1.0ut uv

nt nv

R R

R R

= 1.3ut uv

nt nv

R R

R R

When the required stress, f, in either shear or tension, is less than or equal to 30% of the corresponding available stress, the effects of combined stress need not be investigated.

6. Bearing Strength at Bolt Holes

32

PP

17

Available Bearing StrengthThe nominal bearing strength of the connected material, Rn, is determined as follows:(a) For a bolt in a connection with standard, oversized

and short-slotted holes, independent of the direction of loading, or a long-slotted hole with the slot parallel to the direction of the bearing force

(i) When deformation at the bolt hole at service load is a design consideration

(ii) When deformation at the bolt hole at service load is not a design consideration

33

= 1.2 2.4n c u uR l tF dtF

= 1.5 3.0n c u uR l tF dtF

(4)

(5)

(b) For a bolt in a connection with long-slotted holes with the slot perpendicular to the direction of force

whereFu = specified minimum tensile strength of the

connected materiald = nominal bolt diameterlc = clear distance, in the direction of the force, between

the edge of the hole and the edge of the adjacent hole or edge of the material

t = thickness of connected material

34

= 1.0 2.0n c u uR l tF dtF

= 2.00 = 0.75

(6)

18

The below figure illustrates the distance lc. When computing the bearing strength for bolt 1, lc is measured to the edge of bolt 2. The bearing strength for bolt 2 is calculated with lcmeasured to the edge of the connected part.

35

l l

l

• The effective strength of an individual bolt may be taken as the lesser of the bolt shear strength or the bearing strength at the bolt hole

• The strength of the bolt group is taken as the sum of the effective strengths of the individual bolts.

36

19

7. High-Strength Bolts in Slip-Critical Connections

Slip-critical connections shall be designed to prevent slipand for the limit states of bearing-type connections.The nominal slip resistance is

(a) For standard size and short-slotted holes perpendicular to the direction of the load

= 1.00 (LRFD) = 1.50 (ASD)(b) For oversized and short-slotted holes parallel to the

direction of the load= 0.85 (LRFD) = 1.76 (ASD)

(c) For long-slotted holes= 0.70 (LRFD) = 2.14 (ASD)

37

= n u f b sR D h T n (7)

whereμ = mean slip coefficient for Class A or B surfaces

= 0.30 for Class A surfaces= 0.50 for Class B surfaces

Du = 1.13, a multiplier that reflects the ratio of the mean installed bolt pretension to the specified minimum bolt pretension.

Tb = minimum bolt pretension given in Table 1.hf = factor for fillers; coefficient to reflect the reduction in

slip due to multiple fills.= 1.0 for single filler plate= 0.85 for multiple filler plates

ns = number of slip planes required to permit the connection to slip

38

20

• Filler plates are used in bolted steel connections where hot-rolled structural steel members of different depths are joined. They are commonly found in long span truss connections, steel girders splices, and column splices.

• The slip resistance of the joint may be reduced for the presence of fillers.

39

8. Combined Tension and Shear in Slip-Critical Connections

When a slip-critical connection is subjected to an applied tension that reduces the net clamping force, the available slip resistance per bolt shall be multiplied by the factor, ksc, as follows:

LRFD

ASD

40

( ) = = n sc n sc u f b sR k R k D h T n

= 1 usc

u b b

Tk

D T n

1.5= 1 a

scu b b

Tk

D T n

(8)

21

whereTa = required tension force using ASD load

combinationsTu = required tension force using LRFD load

combinationsnb = number of bolts carrying the applied tension

41

9. Eccentric Shear (Elastic Method)

42

• The elastic method uses basic mechanics and superposition to estimate the shear stress in each bolt.

• The load is moved (by components) to the center of gravity of the bolt group and a moment added to account for the eccentricity.

• The stresses for each case are then determined and vectorally added together.

22

Calculation of Shear Forces in Bolts

• Bolt shear force due to moment (M)

43

(9)

• Bolt shear force due to a concentric force (P)

• The total force due to M and P in a bolt is

44