Types of Bolts
• Unfinished bolts or black bolts or C
Grade bolts (IS: 1363-1992)-bearing type
• Turned bolts - Expensive & used in Spl. jobs
• Precision (A-Grade)& Semi-precision (B-Grade) bolts (IS: 1364-1992) -They are used when no slippage is permitted
• Ribbed bolts (Rarely used in ordinary steel structures)
• High strength bolts (IS: 3757-1985 and IS:4000 - 1992)-Friction type connections
High-Strength Friction Grip (HSFG) Bolts
• Special techniques are used for tightening
the nuts to induce a specified initial
tension in the bolt (called the proof-load),
which causes sufficient friction between
the faying faces.
• Such bolts are called High-Strength Friction Grip bolts (HSFG).
• Due to this friction, the slip in the joint is eliminated; joints with HSFG bolts are called non-slip connections or friction type connections
Bolt Tightening Techniques
• When slip resistant connections are not
required, high strength bolts are
tightened to a 'snug-tight' using an
ordinary spud wrench.
• When slip resistant connections are desired with HDFC bolts, three methods are used:
> Turn-of-the-nut tightening (part-turn method) -Cheap, more reliable, and common method.
> Direct tension indicator tightening,
> Calibrated wrench tightening (torque control method).
Advantages of Bolted connections
• Bolted connections offer the following advantages over riveted or welded connections:
• Use of unskilled labour and simple tools
• Noiseless and quick fabrication
• No special equipment/process needed for installation
• Fast progress of work
• Accommodates minor discrepancies in dimensions
• The connection supports loads as soon as the bolts are tightened (in welds and rivets, cooling period is involved).
• Main drawback of black bolt is the slip of the joint when subjected to loading
Advantages of HSFG Bolts
• HSFG bolts do not allow any slip between the
elements connected, especially in close tolerance
holes, Thus they provide rigid connections.
• Because of the clamping action, load is transmitted by friction only and the bolts are not subjected to shear and bearing.
• Due to the smaller number of bolts, the gusset plate sizes are reduced.
• Deformation is minimized.
• Holes larger than usual can be provided to ease erection and take care of lack-of-fit. However note that the type of hole will govern the strength of the connection.
• Noiseless fabrication, since the bolts are tightened with wrenches.
• The possibility of failure at the net section under the working loads is eliminated.
• Since the loads causing fatigue will be within proof load, the nuts are prevented from loosening and the fatigue strength of the joint will be greater than in welded/connections.
• Since the load is transferred by friction, there is no stress concentration in the holes.
• Unlike riveted joints, few persons are required for making the connections.
• No heating is required and no danger of tossing of bolt. Thus safety of the workers is enhanced.
• Alterations, if any (e.g. replacement of defective bolt) is done easily than in welded connections.