Connections used in steel structures - Fastening, Rigidity and Bolts
The various steel members like beam, columns are interconnected using Rivets, Bolts and Welding. This is known as connection. Connections are of various types.
Connections, based on method of fastening
Rivets
Bolts
Welding
Connections, based on rigidity
Rigid: The moment capacity of the connection is the same as the moment capacity of members.So the angle between the connected members remains intact and no rotation takes place.
Simple: The moment capacity of the connection is zero. It acts as a hinge.
Semi Rigid: This connection carries a little moment, and some rotation is also there. In actual cases, all types of joints behave this way.
Practically, if the moment carried is more than the rotation it’s a rigid joint. If not, then a simple connection. For simplicity, we divide all connections into rigid or simple.
In IS:800-2007, the connections are dealt with bolts and not much talked about rivets. Rivets hammering pollute noise and fixing rivets require skilled labour. Repairing cost is also high and difficult in rivets connection. These are some of the reasons we prefer bolt in connections over rivets.
The shank dia is considered in bolt design whereas the dia of the rivets hole is considered for rivets design.
Type of Bolts
Unfinished bolts or black bolts: It will have a square or hexagonal bolt head and the shank area is not finished properly. So the contact between the plates and bolts is not proper. It is used for temporary connections/lightweight structures. Mild steel is used to manufacture these bolts. The bolts are designated as M16, M20, etc. The number represents the shank diameter. The bolt classes are marked as 4.6, 4.8, etc. For 4.6 class bolts, the ultimate strength is 400 N/m2 and its yield strength is 400 x 0.6 = 240 N/m2.
Finished bolts or turned bolts: The surface is finished properly. The contact would be proper. The diameter clearance is 0.15 to 0.5 mm.
HSFG bolts: It is made up of medium carbon steel. The class for this bolt is either 8.8 or 10.9. The force is transferred using friction, whereas in the other two cases it was bearing.
Friction bolt: The bolts are stressed upto a high stress called as proof stress ( = 0.7 x ultimate stress). The bearing and shearing action of the bolt don’t come into play.
Bearing bolt: These bolts transfer stresses using both bearing and shearing action. For our design purpose we consider the stress as uniform throughout the connection.
Types of joints
There are two types of joint, we make using bolts. One is a lap joint and the other is butt joint. The problem in lap joints is that it produces eccentricity in loading. Butt joints are of many times.
Single cover butt joint
Double cover butt joint
Single bolted single cover butt joint
Double bolted single cover butt joint
Single bolted double cover butt joint
Double cover double bolted butt joint
The butt joints, specifically double cover butt joints are more preferred over the lap joints, because of two reasons.
Shear capacity is more in butt joint than that of the lap joint.
There is no eccentricity in butt joint.
The connections can also be classified based on what sort of force the connection is resisting.
Types of Connections
Eccentric shear connection: The connection resists both shear and moment in the plane of connection.
Tension connection
Combined shear and tension connection