Bolt ....
Bolt is a metal pin with a head at one end,a shank threaded at the other end to receive a nut.
Types of bolt
1- unfinished bolts or black bolts or C grade bolts.
2- Turned bolts
3- Ribbed bolts
4- High strength bolts
1-Unfinished bolts
Other name of unfinished bolts- ordinary bolt, rough bolt, black bolt
Unfinished bolts generally used for light structures subjected to static loads and for secondary members such as purlins, bracing ,roof trusses etc.
They are not recommended for connections subjected to fatigue, vibration and impact load. These bolts are forged from low carbon rolled steel circular rods.
2- Turned Bolts
These are similar to unfinished bolts, with the difference that the shanks of these bolts are formed from a hexagonal rod. The surface of these bolts are prepared and machine carefully to fit in the hole.
3- Ribbed Bolts
These type of bolts have a round head similar to a rivet and raised ribs parallel to the shank. The actual diameter of a ribbed bolt is slightly larger than the hole into which it is driven. While driving the bolt into the hole, it cuts into the edges around the hole, producing a relatively tight fit. These type of bolts are rarely used in ordinary steel structure.
4- High strength Bolts
These bolts are made from bars of medium carbon heat treated steel and from alloy steel. These bolts may be tightened until they have very high tensile stresses , two or more times that or ordinary bolts ,so that the connected parts are clamped tightly together between the bolts and nut heads.
For high strength bolted joints transfer of forces is accomplished through the friction between the interfaces formed between load carrying elements jointed.
The bolts are made from alloy steel with proof stress in the range of 600 to 900 MPa.the bolts are made of heat treated steel and are extensively used in variety of construction.
Advantage of high strength bolts
1- High strength friction grip bolts provide a rigid joint.
2- there are no stress concentration in the holes and therefore, the fatigue is more.
3- The tension in bolts is uniform. Also , the bolts are tensioned up to proof load preventing the nuts from loosening.
4- Few person are required to make the connection, thus cost is reduced.
5- Alternations can be done easily.
Bolts joints
There are two types of bolted joint as discussed below.
1- Lap joint
2- Butt joint
1- Lap joint.
When two members or plates are overlapped and connected together such a joint is called lap joint . These may be two types
a- A single bolted lap joint
b- A double bolted lap joint
2- Butt joint
When two members are connected end to end then this is called butt joint. Additional plates provided on either one or both sides, called cover plates ,are placed and are connected to the main plates .
Note - if cover plate is provided on one side it is called single cover butt joint.
If cover plates are provided on both sides of the main plates, it is called double cover butt joint.
Failure of bolted joints
1- shear failure of bolts
2- Bearing failure of bolts
3- Bearing failure of plates
4- Tension failure of bolts
5- Tension or tearing failure of plates
6- Block shear failure
1- shear failure of bolts
Shear failure are generated when the plates slip due to applied forces. The maximum factored shear force in the bolt may exceed the nominal shear capacity of the bolt. The bolt may be fail in single shear or double shear .
2- Bearing failure of bolts
The bolt is crushed around half circumference. The plate may be strong in bearing and the heaviest stressed plate may press the bolt shank.
3- Bearing failure of plates
When an ordinary bolt is subjected to shear forces, the slip takes place and bolt comes in contact with the plates. A possible failure mode resulting from excessive bearing is shear tear out at the end of connected member as shown in fig.
4- Tension failure of bolts
Bolts subjected to tension may fail at the stress area .in case if any of the connecting plates is sufficiently flexible additional prying forces induced in the bolt must also be considered.
5- Tension or tearing failure of plates
Tearing failure occurs when the bolts are stronger than the plates , tension on both the gross area and net effective area must be considered. The tension failure of the plate shown in the fig.
6- Block shear failure
Bolts may have been placed at a lesser end distance than required causing the plates to shear out which , however ,can be checked by observing the specifications for end distance .
Some points for fabrication of HT bolt
Minimizing the bolt materials
Minimizing bolt tightening and erection operation.
Smaller gusset plates are the number of bolts required are rather few,
Minimizing deformation.
Read more
Comments
Post a Comment