Because the position along the channel stays adjustable for the life of the structure, and bolt groups do not.

A channel is set out as a line, not as a set of points, so the exact fixing position does not have to be known before the pour.

That absorbs the tolerance stack between a concrete frame and the items later fixed to it, which is where most facade fixing problems come from.

It also allows fixings to be moved or added later, during fit-out or in future alterations.

Installation on the formwork is quicker than templating a bolt group, particularly over long runs.

The trade-off is cost per metre and a lower capacity in shear along the slot, which is the direction bolt groups handle best.

Much less than across it, unless the system is serrated and rated for longitudinal load.

Load across the channel and in tension is carried by bearing on the lips and by the welded anchors, which is a direct and predictable path.

Load along the slot has nothing to bear against in a plain channel. It is resisted by friction from the bolt's clamping force, which is limited and sensitive to how tightly the bolt was done up.

Serrated channels have teeth on the lips and matching teeth on the bolt head, giving a positive mechanical lock in the longitudinal direction.

Published capacities always separate the three directions, and combined loading has its own interaction rules.

Assuming a plain channel will carry longitudinal load is a common and consequential error.

Nailed or screwed through its back or its filler strip to the form face, with the slot kept sealed until after striking.

Most channels are supplied with a foam or plastic filler strip already in the slot. It keeps concrete and laitance out and is pulled out once the formwork comes away.

Fixing points are provided in the channel itself so it can be positioned accurately and cannot float or rotate during the pour.

The channel must sit flush with the form face, or it will finish proud of or below the concrete surface.

Clashes with reinforcement are resolved before concreting, since the welded anchors need cover and room.

Concrete that does get into the slot has to be chiselled out, which is slow and risks damaging the lips.

T-head bolts made for that channel profile - they are not interchangeable between systems.

The head is rectangular, inserted through the slot and rotated a quarter turn so it bears under both lips.

Head dimensions are matched to the specific channel profile, and a bolt from another manufacturer may enter but not bear correctly.

Serrated systems require the matching serrated bolt, or the longitudinal capacity is lost.

Most bolts carry a mark on the exposed end showing the head's orientation, so correct rotation can be confirmed visually.

Using the channel manufacturer's own bolts is what the published capacities and the approval assume.

By the orientation mark on the head end and by feel - a fully turned bolt stops positively against the lips.

A bolt rotated only part way bears on a corner rather than across both lips, and its capacity is a small fraction of the rated figure.

The visual mark on the exposed end is aligned with the head inside, so it shows the rotation without seeing the head.

Over-rotation is also possible in some systems, taking the head past its bearing position.

Tightening torque is specified, because clamping force is what resists longitudinal slip in a plain channel.

On safety-critical fixings, engagement is an inspection point rather than an assumption.

Facades, services support, rail and crane rail fixings, and anywhere the supported item is designed after the concrete.

Facade fixing is the dominant use: cladding brackets need three-dimensional adjustment, and the channel provides one axis of it permanently.

Pipe racks, cable trays and containment runs use them because the route changes during fit-out.

Rail and crane rail fixings use continuous channels to spread load and allow alignment along the track.

Tunnel linings use them extensively for the same reason - the services layout follows the structure by months or years.

They are less common in machine bases, where a fixed templated bolt group is both cheaper and stronger.

A compatible set, chosen for the exposure - and for permanent inaccessible fixings, usually stainless.

Hot-dip galvanising covers general external exposure and is the common specification for structural work in mild environments.

Stainless is used where the fixing is buried in a facade build-up and can never be inspected or replaced.

Coastal air, pool halls and de-icing salt all carry chlorides, which attack the standard grade wherever it is shielded from oxygen - and a channel slot is exactly such a place, so duplex and molybdenum-bearing steels are specified there.

The channel, bolt, nut, washer and bracket should be a compatible set; a mixed assembly corrodes at the junction.

Fire performance is a separate requirement in some applications and is qualified product by product.