One of the trickier connections to make on site is a steel member onto a concrete wall or column. This might be required for a cantilever steel balcony, or for a larger steel beam. Traditionally the options were a little limited.

A bolted connection can be formed either by casting-in a threaded device, or drilling and post-fixing. These offer very little in the way of tolerance. If drilling hits reinforcement, forcing repositioning of the insert, then end plates on beams may need re-drilling. Another method used is to cast in a steel plate to which a welded connection is made. This allows a higher degree of tolerance but has health and safety implications with regard to the welding. It is also difficult to make any adjustments or remove a member after welding. When welding, great care has to be taken not to damage the concrete behind the cast-in plate.

There are purpose-made commercial inserts available, but again these offer only limited tolerance. These are primarily intended to connect steel balconies onto a concrete structure. They are generally only suitable for lightly loaded members, taking up to about 15 kNm moment and 12 kN shear. Capacities are fixed, and the choice is made by selecting a suitable size from a catalogue. They normally incorporate insulation surrounding the connection, to minimise heat flow.

An alternative method is to use a combination of cast-in channels designed to suit the connection. These generally fall into two main types. The first is intended to connect to the edge of a concrete slab carrying a steel balcony or a similar member, and the second is used to connect substantial members to a wall or column. Typically, in both cases the vertical shear is taken by vertical 'toothed' channel, and any moment is taken by a tension fixing at the top acting in combination with a compression area lower down.

For balconies, there is normally a steel stub between the face of the concrete and the balcony itself. This can incorporate an insulation shroud to minimise heat loss. It also allows a first-fix at the time of building the external envelop, taking the fixing of the balconies off the critical path.

Typical steel balconies fixed using cast-in channel
(Photos courtesy of Hubbard Architectural)