Cast-in channel is one of the few pieces of building technology whose origin is clearly known. The first patent was granted to Anders Jordahl on 11 December 1913, and the Company he founded is still one of the two major suppliers of channel in UK.
There are two distinct types of channel available, cold-rolled and hot rolled. Cold rolled is formed from a strip of steel passing through a series of rollers. It can be recognised by having rounded corners and being the same thickness across the whole cross section. Hot rolled is formed from an incandescent block, and is recognised by squarer corners and a non-uniform thickness. It is particularly free from built-in stresses, and is therefore better suited to dynamic loadings and welded fabrications.
The material used can be carbon steel (grade S235) or stainless steel (grade 1.4401) for the channel. Carbon steel channel is usually hot-dip galvanized.
Channel is frequently used as an alternative to cast-in sockets and post-fixings as it offers considerable tolerance along the length of the channel.
Channel is normally designated by a series of letters denoting the manufacturer and whether it is cold or hot rolled, followed by the size of the channel. Sizes vary from 28/15 (28mm wide x 15mm embedment) to 74/48.
Channel is used in conjunction with Tee headed bolts. These are inserted into the channel and rotated 90o to prevent them coming out. Bolts are available in carbon steel (grades 4.6 and 8.8) and stainless steel (grade A4-50 or 70). Diameters range from 6mm to 30mm with capacities up to 44 kN (factored). Grade 8.8 bolts should only be fully torqued with hot rolled channel as the high forces developed could deform the lips of cold rolled channel. Suppliers will advise on individual cases.
A slot at the free end of the tee bolt indicates the orientation of the head during installation.
If loads are to be resisted along the length of the channel, standard channel offers only limited capacity. Special channel is available with ‘teeth’ along the lip of the channel. These teeth match similarly toothed tee bolts and provide a positive fixing.
Although a very efficient fixing used on its own, channel can be greatly improved by being incorporated into a fabrication. At its simplest, this may consist of additional reinforcement welded to the channel or its anchors. By carrying out this operation, loads may be distributed into the base concrete, thus overcoming limitations such as edge distance. Fabrications are also frequently used for particular circumstances such as casting into precast ‘plank’ flooring, precast stair well and lift shaft elements, or for attaching balconies. These subjects are covered separately elsewhere.