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)

A big advantage of this type of connection is tolerance. The commercial fixing shown above has fixed studs projecting through, thus tolerance is extremely limited. Some manufacturers do in fact quote tolerances as low as 10mm vertical and ±0mm horizontally. By using a combination of channels and slotted holes, far greater tolerance is achieved.

Channel allows great tolerance to compensate for casting errors. The sketch on the left shows a typical arrangement of channels and slots.

The images below show how the steel member can still be correctly placed and deal with horizontal, vertical, or rotational casting errors.

Capacities of such systems are limited only by the number of channels and bolts. Typical values in the face of a 200mm deep slab are in the order of 25 kNm moment and 60 kN shear, considerably higher than proprietary inserts.

For larger, structural, connections, the same principle applies but the channels are larger with more bolts. The two examples shown both take relatively high loadings. The upper bracket was designed to take a shear of 300 kN. This is taken by multiple bolts in two lengths of vertical toothed channel.

The lower example was designed to take a shear of 100 kN plus a moment of 100 kNm. The shear is taken by four vertical lengths of toothed channel, and the tension due to the moment is taken by heavy-duty horizontal channel.

Where multiple channels are used, it is normal practice to connect these together as a single assembly. This makes it easier to ensure that the channels are positioned correct in relation to each other. Reinforcing bars may be welded onto the channels instead of standard anchors to ensure load distribution.

The capacity of this type of connection is limited only by the number of channels and bolts that can be fitted into the space available.