DESIGN OF JOINTS

Bolted Connections

Introduction

In a steel structure there are different types of structural members. Each member has to be

properly attached to the adjacent parts of the structure. This attachment is achieved using

a joint or connection. There are different types of connections. For example in a steel

frame buildings the main classes of connection are (1) Connection where a change of

direction occurs, e.g. beam-to-column connections, beam-to-beam connections and

connections between different members in trusses (2) Connection which ensure

manageable sizes of steel members for transportation and erection e.g. columns are

normally spliced every two or three storeys.(3) Connections provided where there is a

change of component e.g. column bases, and connections beams or columns with walls,

floors and roofs. Figure 1 shows the different types of connections in steel frame

buildings.

Figure 1 Connections in Multi-storey Frame

Difference between joints and connections as per EC3-1-8

Though “connection” and “joints” are regarded as the same, EC3-1-8 defines them

slightly differently as follows. A connection is the location at which two or more

elements meet. For design purposes it is the assembly of the basic components required

to represent the behaviour during the transfer of relevant internal forces and moments at

the connection. The joint is the zone where two or more members are connected.

Example: A beam – to – column joint consists of a web panel and either one connection

(single sided joint configuration) or two connections (double sided joint configuration).

General requirements of a joint:

Clause 2.2 (1) of EC3-1-8 states that all joints shall have design resistance such that the

structure satisfies all the design requirements of EC3-1-8 and EC3-1-1. The general

recommended partial safety factors and values recommended by UK NAD Mγ to be used

in the design are given in Table 1.

Table 1 Recommended partial safety factors for joints

Partial factor

Type Symbols General

value

UK NAD value

Resistance of members

and cross sections 0Mγ 1.0

1Mγ 1.0

2Mγ 1.0

Resistance of bolts

Resistance of rivets

Resistance of welds

Resistance of plates in

bearing

2Mγ 1.25

Slip resistance at the

ultimate limit state

(category C)

3Mγ 1.25

Slip resistance at the

serviceability limit state

(category B)

serM ,3γ 1.1

Bearing resistance of an

injection bolt 4Mγ 1.0

Resistance of joints in

hollow section lattice

girder

5Mγ 1.0

Preload of high strength

bolts 7Mγ 1.1

Resistance of concrete cγ

The design of joints subject to fatigue is dealt in EN 1993-1-9. The forces and moments

acting on a joint at the ultimate limit state is to be determined using the principles of

EC3-1-1.

Requirements of a connection

The principal structural requirement of a connection is that it be capable of safely

transferring load from the supported members to the supporting member. This

implies that three properties of the connection need to be considered: strength, stiffness,

deformation capacity. Beam-to-column connections can be classified by their stiffness

as nominally pinned, semi-rigid or rigid. For their capability to transfer moments, they

can be classified as nominally pinned, partial-strength and full-strength connections. The

analysis of connections implies the assumption of a realistic internal distribution of forces

that are in equilibrium with the external forces, where each element is capable of

transferring the assumed force and the deformations are within the deformation capacity

of the elements (Clause 2.5 EC3-1-8). In the analysis of connections, a number of

basic load transfers mechanisms can generally be identified.

Bolting and welding are the main methods for making structural steel connections.

Riveting is not used nowadays. The advantages of bolted connections are very rapid field

erection requiring less labour. Even though the cost of high strength bolt is several times

that of a rivet; its overall cost is much lesser than that of a riveted connection due to

reduced labour, equipment cost and lower number of bolts required to carry the same

load.

The principles of design using EC3 are basically same as that by BS 5950. The results of

design are seen to be slightly more conservative in EC3 compared to BS 5950 largely

because of the larger material safety factor for connection, γM=1.25.

Clause 2.6 recommends the type of joints to be used for a joint loaded in shear and

subject to impact, vibration or load reversal. For a joint in shear subject to impact or

significant vibration, welding, bolts with locking devices, preloaded bolts, injection bolts,

rivets or other types of bolt which prevent movement of the connected parts should be

used. Slip is not acceptable for joints subject to reversal of shear load. In this case,

preloaded bolts in category B or C connections, fit bolts, rivets or welding is to be used.

For connections for wind and stability bracings, bolts in category A connections are to be

used.

Effect of joint eccentricity:

Clause 2.7 gives guidance on how the effect of joint eccentricity is considered in different

types of structures. Joints and members should be designed for resulting moments and

forces due to eccentricity at the joints.

The effect of joint eccentricity can be neglected where it can be shown that it is not

necessary to consider the joint eccentricity. For example clause 5.1.5 states that the

distribution of axial forces in a lattice girder is determined on the assumption of pinned

joints. For more details clause 5.1.5 should be referred.

In the case of joints of angles or tees attached by a single line of bolts or two line of bolts,

eccentricity should be considered. This is done by considering the relative positions of

the centroidal axis of the member and the setting out lines of the bolts in the plane of the

connection. For single angle in tension connected by bolts on one leg the simplified

design procedure provided in clause 3.10.3 can be used.

For design of angles used as web members in compression in triangulated and lattice

structures, the effect of eccentricity is neglected and effect of end fixities considered by

calculating an effective slenderness ratio.

Connections with bolts, rivets and pins

Two types/classes of bolts are provided for use by EC3-1-8:2005 in Table 3.1 namely

ordinary bolts and high strength bolts. The rules in EC3-1-8 are valid for 7 bolt classes

4.6, 4.8, 5.6, 5.8, 6.8, 8.8 and 10.9. Ordinary bolts or ordinary grade bolts are put in bolt

classes 4.6, 4.8, 5.6, 5.8 and 6.8 whereas High strength grip bolts or high strength bolts

are put in classes 8.8and 10.9. They can be used for preloaded bolts which are

characterized by a slip-type resistance in shear.

EC3-1-8 provides the yield strength fyb and fub for the bolts classes which may be used as

characteristic values in design (Table 2).

Table 2 Bolt class and corresponding yield strength and ultimate tensile strength

class 4.6 4.8 5.6 5.8 6.8 8.8 10.9

ybf N/mm2 240 320 300 400 480 640 900

ubf N/mm2 400 400 500 500 600 800 1000

Nominal diameters of bolts is given in mm. Bolts are designated as M12, M16, M20,

M22, M24, M27, M30 etc where 12, 16, etc are diameters in mm (Table 3). Bolt holes

are made by drilling or punching. Bolt holes are usually drilled. Punching may be full

size or sometimes the holes are punched under size and then reamed to the required size.

Using gas cutting for forming holes is not recommended since it is inaccurate and the

heat may affect the local properties of steel. Punching of holes saves time and is less

costly but results in distortion near the holes which causes reduced toughness and

ductility leading to brittle fracture. Bolt holes are larger than the bolt diameter to take

care of drilling tolerances. The drilling tolerances are given in EN 1090-2 Requirements

for the execution of steel structures and is summarised in Table 3:

Table 3 Drilling tolerances for bolt holes

Nominal SIZE bolt diameter in mm Nominal clearance

M12,M14 12, 13 1 mm

M16 to M24 16, 18, 20, 22, 24 2 mm

M27 , M30 and above 27, 30 3 mm

The minimum and maximum spacing and end and edge distances for bolts are provided

in table 3.3 of EC 3-1-8 (Table 4) and Figure 2. For maximum values, structures made

from steels conforming to “EN 10025 and not EN 10025-5” and structures conforming to

“EN 10025-5” are identified. In the former category, guidelines for steel are exposed to

weather and corrosive influences or not are provided. For the latter, generally steel used

is unprotected. Unlike BS 5950, EC3 has assumed that plates are machine cut. In the

table, d0 is the hole diameter and t is the thickness of the thinner outer connected plate.

The requirements for structures subject to fatigue are different and are provided in EC3-

1-9 Design of Steel Structures: Fatigue. In the table 4 for more details of the terms 3e ,

4e and other terms, reference to Figure 3.1 of the code is to be made.

Table 4 Minimum and maximum spacing, end and edge distances

Distances and

spacing

Minimum Maximum

Conform to EN 10025 Conform EN

10025-5

End distance

1e 02.1 d mmt 404 + The larger of 8t

or 125 mm

Edge distance

2e

Distances 3e in

slotted holes

05.1 d

Distances 4e in

slotted holes

Spacing 1p 02.2 d The smaller of

t14 or 200mm

The smaller of

14t or 200mm

The larger of

min14t or 175

mm

Spacing 0,1p

Spacing ip ,1 The smaller of

t28 or 400mm

Spacing 2p 04.2 d The smaller of

t14 or 200mm

The smaller of

14t or 200mm

The larger of

min14t or 175

mm

Figure 7.2Symbols for spacing of fasteners (excerpted from EC 3-3-8)

Design Resistance of Individual Fasteners

The design resistance of individual fasteners subject to shear and or tension is given in

Table 3.4 of EC3-1-8. Some important aspects are discussed below.

The design shear resistance RdvF , is to be used where the bolts are used in holes with

nominal clearances not exceeding those for normal holes given in Table 3. M12 and M14

bolts may also be used in 2 mm clearance holes provided that the design resistance of the

bolt group based on bearing is greater or equal to the design resistance of the bolt group

based on bolt shear. The code also states that in addition for class 4.8, 5.8, 6.8, 8.8 and

10.9 bolts the shear resistance RdvF , is taken as 0.85 times the value obtained using

Table 3.4.

Clause 3.6.1 (10) states that in single lap joint with only one bolt row, the bolts are to be

provided washers under both the head and the nut. The expression for design bearing

resistance RdbF , for each bolt is limited to

2

,

5.1

M

u

Rdb

dtfF

γ≤

Also the code cautions from using single bolts in single lap joints.

Types of bolted/ riveted joints based on geometry

Bolted or riveted joints may be classified as lap joint and butt joint. When two members

are overlapped and connected, the joint is called as a lap joint (Figure 3a). Figure 3b

shows a single bolted lap joint and figure 3c shows a double bolted lap joint. Figure 3d

shows how the centre of gravity of loads in the two members are not in one line. This

produces a couple which causes undesirable bending moments in the joint. To minimise

this atleast two rows of bolts must be provided.

(a) 3D view of single riveted lap joint (b) Top view and sectional elevation

of single riveted lap joint

(c) Top view and sectional elevation

of double riveted lap joint

(d) Eccentricity of loads in lap joint

(e) (f) Rivet in single shear in a lap

joint

Figure 3 Bolted Lap Joint

When two members are placed end to end and connected using cover plates, it is called a

butt joint. When plate is provided on one side only, it is called single cover butt joint

(Figure 4a). If the cover plates are provided on both sides, it is called double cover butt

joint (Figure 4d). Figure 7.4b shows single bolted single cover butt joint. Figure 4c shows

double bolted single cover butt joint. Figure 4e shows single bolted double cover butt

joint. Figure 4f shows double bolted double cover butt joint.

(a) Single cover butt joint (b) Single bolted single cover butt

joint

(c) Double bolted single cover butt

joint

(d) Double cover butt joint

(e) Single bolted double cover butt

joint

(f) Double bolted double cover butt

joint

(g) (h) Bolt in double shear in butt joint

Figure 4 Double cover butt joints

Double cover butt joints are preferred due to two reasons. The shear force transmitted by

the members’ acts on two planes whereas in lap joint it acts on only one plane. Therefore

the shear carrying capacity in double cover butt joint is twice that in a lap joint. Also in

double cover butt joint, there is no eccentricity of force and thus bending is eliminated.

Clause 3.6.1 (12) gives reduction factors to be used when packing plates are used bolted

joints.

Types of joints based on force transfer

Simple joints can be classified based on the nature of force transfer as follows

• Direct shear joints

• Direct tension joints

• Eccentric connections: There are two principal types of eccentric loaded connections

namely:

• Bolt group in direct shear and torsion

• Bolt group in shear and tension

These are explained below.

Direct shear joints

In direct shear joints, the bolts are arranged to act in single or double shear. A bolt may

be considered as a simple pin inserted in holes drilled in two or more steel plates or

sections to prevent relative movement. Figure 5 shows a joint in which the bolt is in

single shear.

Figure 7.5

Shearing strength

If the loads are large enough, the bolt may fail by shearing as shown in Figure 6:

Figure 6 The area resisting this failure is the circular area of the bolt shank. The shear resistance

per plane is calculated using equation given in table 3.4 of BS EN 1993 -1-8:

2

,

M

ubv

Rdv

AfF

γ

α=

where the value of vα is given in table 5

Table 5

Condition Value of vα

When the shear plane passes through the threaded

portion of the bolt (A is the tensile area of the bolt)

For classes 4.6, 5.6, 8.8

αv =0.6

For classes 4.8, 5.8, 6.8 and 10.9

αv =0.5

When the shear plane passes through the

unthreaded portion of the bolt (A is the gross cross

section of the bolt)

αv =0.6

The shear type of failure assumes that fairly thick plates are used. Where it can be shown

that the threads do not occur in the shear plane As may be taken as the shank area, A.

These areas are based on BS 3692:2001 and BS 4190:2001. Table 6 shows the gross

areas and tensile areas of standard diameter bolts.

Table 6

Bolt diameter

mm

Gross area

mm2

Tensile stress area

At mm2

8 50 36

10 78 58

12 113 84

16 201 157

18 254 192

20 314 245

22 380 303

24 452 353

27 573 459

30 707 561

36 1017 817

Bearing strength

When relatively large diameter bolts are used to connect two thin steel plates, then failure

will take place by tearing of plates by bolt. This type of failure is known as bearing

failure. The area of contact of bolt with the plates on one side is actually semi-cylindrical

(figure 8), but since the variation of stress around the perimeter of hole is indeterminate,

the strength of bolt in bearing is determined using equation in table 3.4 of EC3 -1-8

2

1

,

M

ub

Rdb

dtfakF

γ=

Where ba is the smallest of dα ;

u

ub

f

for 1.0.

bα is evaluated in the direction of load transfer for end bolts and inner bolts using

following expressions.

For end bolts:

0

1

3d

ed =α

For inner bolts: 4

1

3 0

1−=

d

pdα

1k is evaluated perpendicular to the direction of load transfer separately for edge bolts

and inner bolts by expressions:

For edge bolts: 1k is the smallest of 7.18.20

2−

d

eor 2.5

For inner bolts: 1k is the smallest of 7.14.10

2−

d

por 2.5

Note that the value of k1 depends on the end distance and the value of αb depends on

edge distances. The value of fu is the lesser of the bolt grade or the adjacent plate

grade.

Figure 7

When the plates are pulled, the left plate moves to left, causing the bolt to press against

the plate as shown. Similarly for right plate.

Figure 8

Where thicknesses of plates connected are not equal, the thickness of the thinner plate is

used.

Principal provisions of positioning of holes for bolts is given in section 3.5 of BSEN

1993-1-8:2005

A shear joint can fail in 5 modes namely

(1) Mode 1: by shear on the bolt shank

(2) Mode 2: By bearing on the member or bolt

(3) Mode 3: By shear at the end of the member

(4) Mode 4: By tension in the member and

(5) Mode 5: By block shear.

Failure in shear joints for the different modes can be prevented by the following methods.

Mode 1 and 2 failure can be prevented by providing sufficient number of bolts of suitable

diameter. Failure by mode 3 can be prevented by providing sufficient end distance. Mode

4 failure can be prevented by design tension members for its effective area. Mode 5

failure is observed in a shear joint involving a group of bolts as shown in figure 9. To

prevent such failure check the effective shear area as per the code provisions.

Design for block tearing/ block shear

Block tearing or block shear is a potential failure mode in bolted connections for tension

members, coped beams and gusset plates. Typically this failure mode is characterised by

tearing out of a block of steel with a combination of tension and shear failures through

the bolt holes. Clause 3.10.2 provides the guidelines for design for block tearing. Block

tearing consists of failure in shear at the row of bolts along the shear face of the hole

group accompanied by tensile rupture along the line of bolt holes on the tension face of

the bolt group. Figure 9 shows different cases of block tearing gusset plates (Figure 9d

and e), beam web shear connections with and without coped I-beams(Figure 9a,b and c)

and angles connected by one leg(Figure 9f).

Figure 7.9a,b and c shows bolt group subject to eccentric loading. The design block

shearing resistance RdeffV ,2, is given by

02

,2,3

105

M

nvy

M

ntu

Rdeff

AfAfV

γγ+=

Figure 7.9d and e shows symmetric bolt group subject to concentric loading. The design

block shearing resistance RdeffV ,1, is given by

02

,1,3

1

M

nvy

M

ntu

Rdeff

AfAfV

γγ+=

Note that in the above expressions, for tension rupture net area is taken whereas for shear

yielding the gross area is used.

(a) I-beam with no coping (b) I-beam with single coping

(c) I-beam with double coping (d) Plate under concentric loading

(e) Plate under concentric loading (f) Angle section

Figure 9 Block tearing

Direct Tension joints

The code has two categories for connections loaded in tension as per clause 3.4.2 BS EN

1993-1-8:

Category D: Non – preloaded: (clause 3.4.2(1) a)

Bolt classes 4.6 to 10.9 should be used. No preloading is required. Where connections are

subject to variations in tensile loading, this type of connection is not to be used. However,

they can be used in connections subject to normal wind loads.

Category E: Preloaded: (clause 3.4.2(1) b)

Bolt classes 8.8 and 10.9 are used with controlled tightening as per reference Standards

1.2.7: Group & to be applied.

The design tension resistance is given by the expression:

2

2

,

M

sub

Rdt

AfkF

γ=

Where 2k =0.63 for countersunk bolt; otherwise 2k =0.9

Table 3.2 provides the design checks to be carried out for tension connections. The

design punching shear resistance is given in Table 3.4:

2

,

6.0

M

upm

Rdp

ftdB

γ

π=

Figure 10

Eccentric connections

There are two principal types of eccentric loaded connections namely:

• Bolt group in direct shear and torsion

• Bolt group in shear and tension

Bolt group in direct shear and torsion

Case 1:

Figure 11 shows a bracket subject to a load P at a distance e from the centroid of the

bracket connection. The bolt group is subject two actions namely a direct shearing action

as well as a moment.

Figure 7.11

∑=

××=

noofbolts

n

b

z

zePP

1

2

1

Direct shearing action:

Figure 12 shows the effect of the direct shearing action. The effect of the action on each

bolt is determined by the equation:

n

PPDS =

Figure 12