DESIGN OF STRUCTURE 3 : BOLTED  CONNECTION

BOLT:- A Bolt is a metal pin in which a head formed at the one end and shank threaded at the second end  to receive a nut. A bolt is used for joining together pieces of the metal by inserting them through the hole in the metal and tightening the nut at threaded ends.

There is less skilled labor are required to the bolted connection.

TERMINOLOGIES USED IN THE BOLTED CONNECTION

·       Pitch of the bolt (p) :- It is the center to center spacing

·       Gauge Distance :- the distance between two consecutive bolts of adjacent rows and measured at right angle to the direction of the load.

·       Edge Distance :- Edge distance is the distance of the nearest bolt from the adjacent edge of plates.

·       End Distance :- It is the distance of the nearest bolt hole from the end of the plate.

·       Staggered Distance :- It is the center to center distance of staggered bolts measured obliquely on the member .

CLASSIFICATION OF THE BOLTED CONNECTION

On the basis of the resultant force transferred

On the basis of the resultant force transferred the bolted connection may broadly divided into the  three main categories that are classified below ;-

1.    CONCENTRIC CONNECTION :-  When the load passes through the center of gravity of the section. For example – Axially loaded tension or compression members.

2.    ECCENTRIC CONNECTION :- When the load is away from the center of gravity of the section. For example – Bracket connection , Seat connection.

3.    MOMENT RESISITING CONNECTION :- When the joints are subjected to the moments. For example – Beam of the column section in the framed structure.

On the basis of the type of the force

Connections are further classified on the basis of the type of force

1.    SHEAR CONNECTION :- When the load transfer is through the shear.

2.    TENSION CONNECTION :- When the load is transferred by the tension on the bolts. For example Hanger connection

3.    COMBINE SHEAR AND TENSION CONNECTION :- When an inclined member is to be connected to column through brackets . For example – connection of the bracings.

On the basis of force mechanism

Connections are classified are as fallow

1.    BEARING TYPE :- Bolts bear against the holes . For example – slip-type connection.

2.    FRICTION TYPE :-  the force is transferred by the friction between two plates due to the tension in bolts. for example slip-critical connection.

 

TYPES OF THE BOLT

There are mainly three types of the bolts l:-

1.    Unfinished or black bolts

2.    Finished or turned bolts

3.    HSFG bolts (high strength friction grip bolt)

UNFINISHED OR BLACK BOLTS

Unfinished or black bolt are made from the mild steel rods with square or hexagonal head. The shank of the this bolt is left unfinished.

The nominal diameter (Diameter of the shank size) size of the black bolt is 12 , 16 , 20 , 22 , 24 , 30 and 36 are available. These bolts are designated as the M20 , M16 , M24 etc.

FINISHED OR TURNED BOLTS

Turned bolts  made of the mild steel . they are formed from the hexagonal rods , that are finished by turning to a circular shape.

The dimension of the turned bolts are kept 1.2 mm to 1.3 mm larger than the nominal diameter , and the bolt hole is kept 1.5 mm larger than the nominal diameter.

HIGH STRENGTH FRICTION GRIP (HSFG) BOLTS

HSFG bolts are made of the high strength steel rod and the surface of the shank in the HSFG bolt is kept unfinished as like the Black bolts.

These bolts are tightened to a proof load using calibrated wrenches , Hence they grip tightly to the member. In the HSFG bolts nut are prevented by using clamping devices.

TYPES OF THE BOLTED JOINTS

Fallowing are the various types of the bolted joints

·       LAP JOINTS

·       BUTT JOINTS

LAP JOINTS

In the lap joints two members which are to be connected are overlapped and connected together , this types of the joints are called lap joints.

The load in the lap joints has eccentricity as the center of gravity of the load in one member and the center of gravity of load in the second member lies in another line. So a couple is formed which causes undesirable bending in the connection and the bolts may fail i n the tension.

To minimize the effect of bending in the lap joints at least two bolts should be provided in the same line.

Presence of the eccentricity in the connection the stresses are distributed unevenly in the contact area between the bolts and member which are to be connected.

BUTT JOINTS

In the butt joints two members which are to be connected are placed together end to end . Additional plates provided on the either one or both sides , called cover plates.

If the cover plates are provided in only one side , it is called single cover butt joints and when the cover is provided to the both sides of the main plates it is called a double cover butt joints.

It is more beneficial to provide a butt joints instead of the lap joints for snug tight bolts for two main reasons.

1.    The shear carrying capacity of the bolt in the double cover butt joints is twice than that of the bolts in the lap joints.

2.    In double cover butt joints , eccentricity of forces does not exist and bending is removed.