Beam to column connection
Beam to column connections are very common and a variety of details can be used.
Connections between beams and columns are perhaps the most common structural connection type. A wide range of different types are used, and these include fin plates, end plates, web or flange cleats, and haunched connections.
The fin plate connection is simple and allows easy site installation.
Fin plate connections are based on a single plate welded to the column. Beams are normally attached using two or more bolts through the web. Where necessary adjustment can be provided using slotted holes (for instance horizontally slotted holes in the web of the section attached to the fin plate).
Fin plate connections are suitable for connecting open section beams to any steel column including tubular sections where a simple, principally shear type, connection is required.
End plate connections are simple and neat.
End plate connections have a single plate welded to the end of the beam. This is bolted to the column flange or web using two or more bolts arranged in pairs. Where necessary, adjustment can be provided by slotted holes and shim plates between the end plate and the column.
When the connections are made to hollow section columns it is not possible to install conventional nuts onto the ends of the bolts inside the section. Specially threaded holes or proprietary bolts which incorporate an expanding sleeve should therefore be used.
End plate connections may be partial, flush or extended. Partial depth end plates transmit the minimum bending effect into the column; flush end plates provide a neat detail and allow a greater number of bolts; extended plates enable significant transfer of bending between beam and column, but are not frequently used.
Haunched connections can develop fully rigid behaviour and are principally used in single storey portal frame construction.
Haunched connections are used where there is a need to achieve high moment transfer. The haunch locally increases the effective depth of the section. Beams are attached using multiple pairs of bolts through and endplate. Little adjustment is possible.
Because of the amount of fabrication involved, this is a fairly expensive type of joint. However, for single storey sheds the benefits are significant and overall economies can be realised. For multi-storey buildings, the advantages are much less and simple connections are widely used. Haunched connections are therefore common in portal frames, but are much less common in other buildings.
Beams can be positioned eccentrically to the column enabling continuity to be achieved.
By arranging the line of a beam to be a little eccentric in relation to the column centre line, both the beam and column can be treated as continuous. The beam can be connected to the face of the column using a modified fin plate detail. In practice, main beams are often arranged in pairs either side of the column, and this is referred to as the parallel beam system.
Web cleat connections are similar in principle to the fin plate but are now less widely used.
The web cleat connection is similar to the fin plate detail but can be entirely bolted. It is formed by bolting short angle cleats between the web of the beam and the column face. It was popular for rectangular (orthogonal) grids in which the beams and columns all meet at right angles, but is now less common.
Flange cleats provide a direct bearing for the beam; they were once popular but are now seldom used.
The use of a seating cleat provides a connection which is quick and easy to erect because the beam can be positioned directly onto the support angle. It can be fully bolted, with no welding necessary. A cleat can also be fixed to the top flange to provide additional restraint against twisting of the beam.
One disadvantage is that the seating cleat can impact on the ceiling finishes at the column position. Other connection details have become more popular and this connection is now seldom used.
A detail suitable for beam-column joints using hollow sections is to use a cut-out at the end of the tube to provide access for bolting.
End plate connections for tubes can be cumbersome because flush end plate details are difficult to achieve. The principal difficulty in using a bolted flush end plate connection is the inaccessibility from within the closed tube to secure the bolts. This can be overcome by introducing a cut-out in the end of the tube. This produces a very neat joint, particularly if the cut-out is arranged away from view. However, as the number of bolts which can be put inside the tube is limited, the connection is mathematically 'pinned', and the joint can be used for shear loads only.
Roof structures may be supported directly on columns using a seated connection.
In some low rise buildings seated connections are used for roof beams or trusses. The supporting column is fitted with a cap plate and the beam or truss bears directly on this, and is secured by bolts. This connection is treated as pinned.