What to watch out for in Connections in Structural Steelwork.

Connections form an important part of any structure and are designed more conservatively than members. This is because, connections are more complex than members to analyse, and the discrepancy between analysis and actual behaviour is large.

Further, in case of overloading, we prefer the failure confined to an individual member rather than in connections, which could affect many members.

Connections account for more than half the cost of structural steelwork and so their design and detailing are of primary importance for the economy of the structure.

The type of connection designed has an influence on member design and so must be decided even prior to the design of the structural system and design of members.

For example, in the design of bolted tension members, the net area is calculated assuming a suitable number and diameter of bolts based on experience. Therefore, it is necessary to verify the net area after designing the connection.

Similarly in the analysis of frames, the member forces are determined by assuming the connections to be pinned, rigid, or semirigid, as the actual behaviour cannot be precisely defined.

Just as members are classified as bending members or axially loaded members depending on the dominant force/moment resisted, connections are also classified into idealised types while designing. But the actual behaviour of the connection may be different, and this point should always be kept in mind so that the connection designed does not differ significantly from the intended type.

Take for example, the connection of an axially loaded truss member at a joint. If the truss is assumed to be pin jointed, then the member should ideally be connected by means of a single pin or bolt.

However, in practice, if the pin or bolt diameter works out to be larger than that possible, more than one bolt will be used. The truss can then be considered pin-jointed only if the bending due to self-weight or other superimposed loads is negligible. Note that the connection behaviour will also influence the calculation of the effective length for the buckling analysis of struts.