â€‹Cast-in channel was invented almost 100 years ago in 1913 by Anders Jordahl and in the time since then it has become established as one of the most efficient and reliable means of connecting things to concrete. As can be seen from the original patent paper, the principle is largely unchanged.
Historically, most channels used in UK were manufactured by two German manufacturers, and their range of channel profiles is virtually identical. They published loading data based on their own tests, allowing users to select a suitable product for a given situation. However, as there were no laid down procedures for the testing or the interpretation of the test results, we had the undesirable position where two apparently identical profiles could have somewhat different published capacities. Until recently there were no standards or agreed rules governing the design of channels.
Two recent events prompted action on this situation. Firstly, cheap clones of profiles came in from the Far East. These often suffered from quality problems and do not have the technical backup of the main manufacturers.
For example, if the material is too thin then the lips can bend under load, and if too thick, cracking can occur during rolling causing longitudinal failure.
Secondly, the standard BS EN 1992-1-1:2004 – Eurocode 2: Design of concrete structures .General rules and rules for buildings (EC2) was published, and has now taken over from BS8110 as the principle design standard covering concrete structures. This states in clause 2.7 that “The performance of fasteners should comply with the requirements of a CEN standard or should be demonstrated by a European Technical Approval (ETA)”. In simple terms this means that using a channel without an ETA will result in a non-compliant design. This could have significant implications for insurers of projects and the liability of designers and suppliers.
To address this, the major channel manufacturers began a process in 1999 to evolve a completely new, comprehensive test and design method.
The old loading data was based on uncracked concrete and included limitations on parameters such as edge distance, corners, concrete depth, centres of fixing etc. If the minimum values given were not achieved, then no solution was found. The new methods go far beyond this.
A new Standard, CEN/TS 1992-4-3:2009 - Design of fastenings for use in concrete Part 4-3: Anchor channels was developed and the UK version published by BSI in 2009. This sets out the factors that have to be applied to characteristic resistances under differing circumstances. The actual characteristic resistances and the reduction factors are established by very extensive testing.
This testing establishes a whole range of resistances for the various possible failure modes. There are typically up to seven failure modes in tension and five in shear as shown below. It should perhaps be stressed that these are theoretical/potential modes rather than ‘likely’ modes.
Possible failure modes for steel in tension
Possible failure modes for concrete in tension
Possible failure modes for steel in shear
Possible failure modes for concrete in shear
The reduction factors are then applied to these values to take account of parameters such as edge distance etc as listed above. This can result in 26 individual verifications, plus interaction proofs. These figures and any limitations on their use are gathered together in the ETA document. The ETA is non-transferable; i.e. it applies only to the specific channel manufacturer’s products. It is not permitted to use the results on a similar product by another manufacturer, even if the profile is identical. Similarly it is not legal to claim that one channel is equivalent to another just because the profile is the same.
The ETA also lays down the range within which the values apply. An ETA is not just a test certificate. It typically contains 19 annexes covering matters such as physical dimensions, materials, intended use, minimum edge distances, effect of reinforcement, etc. In some cases, taking the reinforcement can result in an increase in capacity of more than 40%. Whilst edge distances may now be as low as 30mm in some cases, the reduction factor that this triggers will be considerable. Rather than a standard concrete grade of C20/25, strengths may now range between C12/15 minimum and C90/105 maximum in accordance with EN 206-1:2000. These higher strengths are particularly beneficial to precast concrete. Both cracked and uncracked concrete are permitted.
Possession of an ETA also permits the use of CE marking, which acts as further assurance that the product has gone through the rigorous testing and attestation process. The first such ETA was awarded to Jordahl in February 2010, but it took a further year to interpret the results and create usable data.
The CEN Standard, plus the ETA work hand-in-hand to give allowable values, and also to give the all-important compliance with EC2.
Whilst all these results and proofs can be tabulated, it is no longer an easy matter to use printed tables to check a design. All of the illustrated failure modes should be checked even when unlikely, for all bolts and anchors and the sheer complexity of the design can be daunting. The major manufacturers have issued various software packages to allow designs to be easily done.
This software allows rapid input of the physical parameters such as concrete details, channel length, number of loads, etc. Tee bolts can be automatically moved within the length of the channel to arrive at a worst case loading. The load in each channel anchor is calculated separately and checked against the permitted values from the ETA with the various reduction factors applied. Quick checks can now be made on different circumstances such as another channel nearby, or a corner on a slab. As the software will print out the results, these can be included in the overall design package.