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UNIT II

BOLTED CONNECTIONS

1.0 INTRODUCTION

Connections form an important part of any structure and are designed more conservatively than

members. This is because, connections are more complex than members to analyse, and the

discrepancy between analysis and actual behaviour is large. Further, in case of overloading, we prefer

the failure confined to an individual member rather than in connections, which could affect manymembers.

Connections account for more than half the cost of structural steelwork and so their design and

detailing are of primary importance for the economy of the structure.

The type of connection designed has an influence on member design and so must be decided even

prior to the design of the structural system and design of members. For example, in the design of

bolted tension members, the net area is calculated assuming a suitable number and diameter of bolts

based on experience. Therefore, it is necessary to verify the net area after designing the connection

Similarly in the analysis of frames, the member forces are determined by assuming the connections to

be pinned, rigid, or semi-rigid, as the actual behaviour cannot be precisely defined.

Just as members are classified as bending members or axially loaded members depending on the

dominant force/moment resisted, connections are also classified into idealised types while designing.

But the actual behaviour of the connection may be different and this point should always be kept in

mind so that the connection designed does not differ significantly from the intended type. Take for

example, the connection of an axially loaded truss member at a joint. If the truss is assumed to be pin

jointed, then the member should ideally be connected by means of a single pin or bolt. However, in

practice, if the pin or bolt diameter works out to be larger than that possible, more than one bolt will

be used. The truss can then be considered pin-jointed only if the bending due to self-weight or other

superimposed loads is negligible. Note that the connection behaviour will also influence the

calculation of the effective length for the buckling analysis of struts.

The connections provided in steel structures can be classified as 1) riveted 2) bolted and 3) welded

connections. Riveted connections were once very popular and are still used in some cases but will

gradually be replaced by bolted connections. This is due to the low strength of rivets, higher

installation costs and the inherent inefficiency of the connection. Welded connections have the

advantage that no holes need to be drilled in the member and consequently have higher efficiencies

However, welding in the field may be difficult, costly, and time consuming. Welded connections are

also susceptible to failure by cracking under repeated cyclic loads due to fatigue which may be due to

working loads such as trains passing over a bridge (high-cycle fatigue) or earthquakes (low-cycle

fatigue). A special type of bolted connection using High Strength Friction Grip (HSFG) bolts has been

found to perform better under such conditions than the conventional black bolts used to resistpredominantly static loading. Bolted connections are also easy to inspect and replace. The choice of

using a particular type of connection is entirely that of the designer and he should take his decision

based on a good understanding of the connection behaviour, economy and speed of construction.

Bolts

A bolt may be defined as a metal pin with a head at one end and a shank threaded at the other end to

receive a nut as in Fig 1.0(a).

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Steel washers are usually provided under the bolt as well as under the nut to serve two purposes:

1. To distribute the clamping pressure on the bolted member, and

2. To prevent the threaded portion of the bolt from bearing on the connecting pieces.

In order to assure proper functioning of the connection, the parts to be connected must be tightly

clamped between the bolt between the bolt head and nut. If the connection is subjected vibrations, the

nuts must be locked in position. Bolted connections are quite similar to riveted connections inbehaviour but have some distinct advantages as follows:

1. The erection of the structure can be speeded up, and

2. Less skilled persons are required.

The general objections to the use of bolts are:1. Cost of material is high: about double that of rivets.

2. The tensile strength of the bolt is reduced because of area reduction at the root of the thread and also due to stress

concentration.

3. Normally these are of a loose fit excepting turned bolts and hence their strength is reduced.

4. When subjected to vibrations or shocks bolts may get loose.

Uses1. Bolts can be used for making end connections in tensions and compression member.

2. Bolts can also be used to hold down column bases in position.

3. They can be used as separators for purlins and beams in foundations, etc.

TypesThere are several types of bolts used to connect the structural elements. Some of the bolts commonly used are:

a) Unfinished bolts or black bolts or C grade bolts (IS 1363:2002)

b) Turned bolts

Precision bolts or A grade bolts (IS 1364:2002) Semi-precision bolts or B grade bolts (IS 1364:2002)

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c) Ribbed bolts

d) High strength bolts (IS 3757:1985 and IS 4000:1992)

UNFINISHED BOLTS

Unfinished bolts are also called ordinary, common, rough or black bolts. There are used for light

structures (purlins, bracings, etc.) under static loads. They are not recommended for connections

subjected to impact load, vibrations and fatigue. Bolts are forged from low carbon rolled steel circular

rods, permitting large tolerances. Ordinary structural bolts are made from mild steel with square or

hexagonal head, as shown in Fig 1.0(b). Square heads cost less but hexagonal heads give a better

appearance, are easier to hold by wrenches and require less turning space. The bolt hole is punched

about 1.6mm more than the bolt diameter. The nuts on bolts are tightened with spud wrenches,

producing little tension. Therefore, no clamping force is induced on the sections jointed. Sometimes a

hole is drilled in the bolt and a cotter pin with a castellated nut is used to prevent the nut from turning

on the bolt, as shown in Fig 1.0(c). The connections with unfinished bolts are designed in a similar way

as all the riveted connections except that the permissible stresses are reduced to account for

tolerances provide on shank and threaded portion of the bolts. The requirements regarding pitch and

edge distance are same as that for rivets.

Table 1. Tensile properties of fasteners (bolts) used in steel constructionSpecification Grade/Classification Properties

Yield stress,

MPa (Min)

Ultimate

tensile stress,

MPa (Min)

Elongation

percentage

(Min)

IS 1367 (Part

3) (ISO 898)

Specifications

of fasteners-

threaded

3.6 180 330 25

4.6 240 400 22

4.8 320 420 14

5.6 300 500 20

5.8 400 520 10

6.8 480 600 8

8.8 (d

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Table 2.Bolt Edge distances

Nominal

Size (d)

Clearance

Hole(D),

mm

Pitch

(A),

mm

Min Edge

distance(B),

mm

Min Edge

distance

(C), mm

M12 14 30 20 18

M16 18 40 26 23

M20 22 50 31 28

M24 26 60 37 33M27 30 68 42 38

M30 33 75 47 42

M36 39 90 55 49