Any steel structure is an assemblage of different members such as beam, columns, and tension members, which are fastened or connected to one another, usually at the member ends. Many members in a steel structure may themselves be made of different components such as plates, angles, I-beams, or channels. These different components have to be connected properly by means of fasteners, so that they will act together as a single composite unit. Connections between different members of a steel framework not only facilitate the flow of forces and moments from one member to another but also allow the transfer of forces up to the foundation level.
The basic goal of connection design is to produce a joint that is safe, economical, and simple (so that it can be manufactured and assembled at site without any difficulty). It is also important to standardize the connections in a structure and to detail it in such a way that it allows sufficient clearance and adjustment to accommodate any lack of fit, resists corrosion, is easy to maintain, and provides reasonable appearance.
There are several types of bolts used to connect structural members. Some of them are listed as follows:
- Unfinished bolts or black bolts or C grade bolts (IS 1363 : 2002)
- Turned bolts
- Precision bolts or A grade bolts (IS 1364 : 2002)
- Semi-precision bolts or B grade bolts (IS 1364 : 2002)
- Ribbed bolts
- High strength bolts (IS 3757 : 1985 and IS 4000 : 1992)
Advantages of Bolted Connections
The black bolts 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
- HSFG bolts do not allow any slip between the elements connected, especially in close tolerance holes, thus providing rigid connections.
- Due to 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.
- Since HSFG bolts under working loads do not rely on resistance from bearing, holes larger than usual can be provided to ease erection and take care of lack of fit. Thus the holes may be standard, extra-large, or short/long slotted. However, 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 and better than welded and riveted joints. Moreover, 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, the safety of the workers is enhanced.
- Alterations, if any (e.g. replacement of the defective bolt) are done easily than in welded or riveted connections.