Short Notes: Bolted Connections | Short Notes for Civil Engineering - Civil Engineering (CE) PDF Download

 Page 1


 
 
  
 
BOLTED CONNECTIONS 
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). 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 
Page 2


 
 
  
 
BOLTED CONNECTIONS 
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). 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 quit similar to riveted 
connections in behaviour 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. 
Uses 
1. 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. 
 
Types  
There are several types of bolts used to connect the structural elements. Some of the bolts 
commonly used are: 
a) Unfinished bolts 
b) Turned bolts 
Page 3


 
 
  
 
BOLTED CONNECTIONS 
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). 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 quit similar to riveted 
connections in behaviour 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. 
Uses 
1. 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. 
 
Types  
There are several types of bolts used to connect the structural elements. Some of the bolts 
commonly used are: 
a) Unfinished bolts 
b) Turned bolts 
 
 
  
 
c) Ribbed bolts 
d) High strength bolts 
e) Interference bolts 
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. The permissible 
stresses are as given in Table 8.1 of I.S:800-1984. 
TURNED BOLTS 
These are similar to unfinished bolts, with the differences that the shank of these bolts is formed 
from a hexagonal rod. The surfaces of the bolts are prepared carefully and are machined to fit in 
the hole. Tolerances allowed are very small. These bolts have high shear and bearing resistance 
as compared to unfinished bolts. However, these bolts are obsolete nowadays. The specifications 
for turned bolts are given in I.S:2591-1969. 
RIBBED BOLTS 
These are also called fluted bolts. The head of the bolt is like a rivet head. The threaded and nut 
are provided on the other end of the shank. From the shank core longitudinal ribs project making 
the diameter of the shank more than the diameter of the hole. These ribs cut grooves into the 
connected members while tightening and ensure a tight fit. These bolts have more resistance to 
Page 4


 
 
  
 
BOLTED CONNECTIONS 
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). 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 quit similar to riveted 
connections in behaviour 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. 
Uses 
1. 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. 
 
Types  
There are several types of bolts used to connect the structural elements. Some of the bolts 
commonly used are: 
a) Unfinished bolts 
b) Turned bolts 
 
 
  
 
c) Ribbed bolts 
d) High strength bolts 
e) Interference bolts 
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. The permissible 
stresses are as given in Table 8.1 of I.S:800-1984. 
TURNED BOLTS 
These are similar to unfinished bolts, with the differences that the shank of these bolts is formed 
from a hexagonal rod. The surfaces of the bolts are prepared carefully and are machined to fit in 
the hole. Tolerances allowed are very small. These bolts have high shear and bearing resistance 
as compared to unfinished bolts. However, these bolts are obsolete nowadays. The specifications 
for turned bolts are given in I.S:2591-1969. 
RIBBED BOLTS 
These are also called fluted bolts. The head of the bolt is like a rivet head. The threaded and nut 
are provided on the other end of the shank. From the shank core longitudinal ribs project making 
the diameter of the shank more than the diameter of the hole. These ribs cut grooves into the 
connected members while tightening and ensure a tight fit. These bolts have more resistance to 
 
 
 
 
vibrations as compared to ordinary bolts. The permissible stresses for ribbed are same as that for 
rivets. 
 
HIGH STRENGTH BOLT 
These bolts are called friction grip bolts. These are made from bars of medium carbon steel. 
Their high strength is achieved through quenching and tempering processes or by alloying steel. 
Steel washers of hard steel or carburized steel are provided as shown in Fig1.0 (d), to evenly 
distribute the clamping pressure on the bolted member and to prevent the threaded portion of the 
bolt from bearing on the connecting pieces. If the bolts are tightened by the turn of nut method, 
the nut is made snug and is tightened a half turn more by hand wrenches, then the washers are 
not required. The vibrations and impact resistance of the joint is also improved. The nut and head 
of the bolts are kept sufficiently large to provide an adequate bearing area. The specifications for 
high strength bolts are laid in I.S:3757-1972 and I.S: 4000-1967. These bolts have a tensile 
strength several times that of the ordinary bolts. High strength bolts have replaced rivets and are 
being used in structures, such as high rise buildings, bridges, machines etc. Due to their distinct 
advantages and vital use, high strength bolts are discussed below in some detail. 
Advantages of high strength bolts 
High strength friction grip (HSFG) bolts have replaced the rivets because of their distinct 
advantages as discussed below. However, the material cost is about 50% greater than that of 
ordinary bolts and special workmanship is required in installing and tightening these bolts. 
1. These provide a rigid joint. There is no slip between the elements connected 
2. Large tensile stresses are developed in bolts, which in turn provide large clamping force 
to the elements connected. High frictional resistances is developed providing a high static 
strength the joint. 
3. Because of the clamping action, load is transmitted by friction only and the bolts are not 
subjected to shear and bearing. 
Page 5


 
 
  
 
BOLTED CONNECTIONS 
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). 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 quit similar to riveted 
connections in behaviour 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. 
Uses 
1. 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. 
 
Types  
There are several types of bolts used to connect the structural elements. Some of the bolts 
commonly used are: 
a) Unfinished bolts 
b) Turned bolts 
 
 
  
 
c) Ribbed bolts 
d) High strength bolts 
e) Interference bolts 
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. The permissible 
stresses are as given in Table 8.1 of I.S:800-1984. 
TURNED BOLTS 
These are similar to unfinished bolts, with the differences that the shank of these bolts is formed 
from a hexagonal rod. The surfaces of the bolts are prepared carefully and are machined to fit in 
the hole. Tolerances allowed are very small. These bolts have high shear and bearing resistance 
as compared to unfinished bolts. However, these bolts are obsolete nowadays. The specifications 
for turned bolts are given in I.S:2591-1969. 
RIBBED BOLTS 
These are also called fluted bolts. The head of the bolt is like a rivet head. The threaded and nut 
are provided on the other end of the shank. From the shank core longitudinal ribs project making 
the diameter of the shank more than the diameter of the hole. These ribs cut grooves into the 
connected members while tightening and ensure a tight fit. These bolts have more resistance to 
 
 
 
 
vibrations as compared to ordinary bolts. The permissible stresses for ribbed are same as that for 
rivets. 
 
HIGH STRENGTH BOLT 
These bolts are called friction grip bolts. These are made from bars of medium carbon steel. 
Their high strength is achieved through quenching and tempering processes or by alloying steel. 
Steel washers of hard steel or carburized steel are provided as shown in Fig1.0 (d), to evenly 
distribute the clamping pressure on the bolted member and to prevent the threaded portion of the 
bolt from bearing on the connecting pieces. If the bolts are tightened by the turn of nut method, 
the nut is made snug and is tightened a half turn more by hand wrenches, then the washers are 
not required. The vibrations and impact resistance of the joint is also improved. The nut and head 
of the bolts are kept sufficiently large to provide an adequate bearing area. The specifications for 
high strength bolts are laid in I.S:3757-1972 and I.S: 4000-1967. These bolts have a tensile 
strength several times that of the ordinary bolts. High strength bolts have replaced rivets and are 
being used in structures, such as high rise buildings, bridges, machines etc. Due to their distinct 
advantages and vital use, high strength bolts are discussed below in some detail. 
Advantages of high strength bolts 
High strength friction grip (HSFG) bolts have replaced the rivets because of their distinct 
advantages as discussed below. However, the material cost is about 50% greater than that of 
ordinary bolts and special workmanship is required in installing and tightening these bolts. 
1. These provide a rigid joint. There is no slip between the elements connected 
2. Large tensile stresses are developed in bolts, which in turn provide large clamping force 
to the elements connected. High frictional resistances is developed providing a high static 
strength the joint. 
3. Because of the clamping action, load is transmitted by friction only and the bolts are not 
subjected to shear and bearing. 
 
 
  
 
4. The frictional resistance is effective outside the hole and therefore lesser load is 
transmitted through the net section. Thus, the possibility of failure at the net section is 
minimized. 
5. There are no stress concentrations in the holes; therefore, the fatigue strength is more. 
6. The tension in bolts is uniform. Also the bolts are tensioned up to proof load hence; the 
nuts are prevented from loosening 
7. Few persons are require to make the connections, thus cost is reduced. 
8. Noise nuisance is not there as these bolts are tightened with wrenches. 
9. The hazard of fire is not there and there is no danger of tossing of the bolt. 
10. Alterations can be done easily. 
11. For some strength, lesser number of bolts are required as compared to rivets which brings 
overall economy. 
Principles of high strength bolts 
The shank of the high strength bolts does not fully fill the hole. So shear and bearing are not the 
criteria for load transmission as is in the case of rivets, which fill the hole completely. The nut is 
tightened to develop a clamping force on the plates which is indicated as the tensile force T in 
the Bolt. This tension should be about 90% of proof load. When a shear load is applied to the 
joint no slip will occur until the shear load exceeds the frictional resistance between the elements 
jointed. When shear load exceeds the frictional resistance a slip occurs. On further increase of 
this load, the gradual slipping brings the bolt in contact with the plate edges. 
The horizontal frictional forces F, is  induced in the joints which is equal to the tensile force T, as 
in Fig.1.0(d), in the bolts multiplied by the coefficient of friction.   
    F = µT  
This frictional force F should exceed the applied force P on the member.