Steel: Design Of Connections

Section 10 of IS 800:2007 deals with the design and detailing requirements for joints between members. The connections in a structure shall be designed so as to be consistent with the assumptions made in the analysis of the structure and comply with the requirements specified in section 10 of the code.

Connections shall be capable of transmitting the calculated design actions. In most structures connections are the weakest link. This leads often to failure in spite of the strong members used. This draws our attention to the design of connections with utmost care. The behaviour of connections is quite complex due to geometric imperfections and complexities, lack of fit, residual stresses etc; making it complex to analyse. This can be simplified by a number of assumptions and approximations based on past experience, experimental results and ductility of steel. It is the ductility of steel assists the distribution of forces generated within a joint. This is outlined in Cl. 10.1.4 of IS 800:2007.

The ultimate aim of connection design is to have a simple, compatible, feasible, easy to fabricate, safe and economical joint.

TYPES OF CONNECTIONS.

Connection elements consist of components such as cleats, gusset plates, brackets, connecting plates and connectors such as rivets, bolts, pins, and welds. Connections are classified based on the connecting element and the fixity of the joint

1. Classification based on the connector

Connections are classified based on the connecting element in to

(a) Riveted,

(b) Bolted,

(c) Pinned and

(d) Welded connection. Of these riveted, bolted and pinned connections behave in a similar manner.

2. Classification based on the fixity of the joint

Based on the fixity of the joint, connections are classified in to

(a) Rigid joint,

(b) Semi rigid joint and

(c) Flexible joints.

2. SELECTIONS OF TYPE OF CONNECTION

Riveted connections were once very popular and are still used in some cases but will gradually be replaced by bolted connections. This is due to the low strength of rivets, higher installation costs and the inherent inefficiency of the connection. Welded connections have the advantage that no holes need to be drilled in the member and consequently have higher efficiencies. However, welding in the field may be difficult, costly, and time consuming. Welded connections are also susceptible to failure by cracking under repeated cyclic loads due to fatigue which may be due to working loads such as trains passing over a bridge (high-cycle fatigue) or earthquakes (low-cycle fatigue). A special type of bolted connection using High Strength Friction Grip (HSFG) bolts has been found to perform better under such conditions than the conventional black bolts used to resist predominantly static loading. Bolted connections are also easy to inspect and replace. The choice of using a particular type of connection is entirely that of the designer and he should take his decision based on a good understanding of the connection behaviour, economy and speed of construction. Ease of fabrication and erection should be considered in the design of connections. Attention should be paid to clearances necessary for field erection, tolerances, tightening of fasteners, welding procedures, subsequent inspection, surface treatment and maintenance.