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Anchor Selector

Anchor selection

Anchor selection is a complicated business with a lot of factors to be taken into account.

The factors affecting the selection of anchors can be considered in two groups, with some overlap.

A) Factors affecting the type of anchor

B) Factors affecting the size of anchor

For more detail on all these aspects refer to BS 8539:2012 Clause 5 once published.

In the mean time the following CFA Guidance Notes cover the relevant aspects:

  • Anchor selection
  • ETAs and design methods for anchors used in construction
  • Anchor Terminology and Notation.

*Within the following Sequence diagrams some terms may need explaining see below. 

Three charts are available here to help with this process: click to view

Anchor Selection Chart

Anchor Selection Chart

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Choosing the most appropriate ETAG for use in Concrete.

Sequence diagram* for choosing the most appropriate ETAG for use in Concrete

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Choosing the most appropriate ETAG for use in Masonry.

Sequence diagram* for choosing the most appropriate ETAG for use in Masonry

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"Statically Determinate" and "Statically Indeterminate".

This subject looks more complicated than it is, especially when you look at a definition of these terms:

Applications in which the failure of any anchor supporting the fixture will lead to its collapse are referred to as being "statically determinate"while those which have been designed with sufficient redundancy that the failure of one anchor will not lead to progressive collapse are referred to as "statically indeterminate".  The problem appears to be in deciding whether or not the failure of a single anchor would lead to the collapse of the whole fixture. In theory this can be tricky and may require complicated calculations, but if we look instead at the criteria needed to be satisfied in order to use anchors that are qualified for these applications things become easier.

 

If an application can satisfy the requirements for "Multiple use" as set out in ETAG 001 Part 6Anchors for multiple use in non-structural applicationsor ETAG 20Plastic anchors for multiple use in concrete or masonry for non-structural applications,then it can be regarded as statically indeterminate and anchors with ETA to these ETAGs may be specified without special consideration of the stiffness of the supported structure. If not then an anchor with and ETA to ETAG 001 Parts 1 - 5 or to ETAG 029 may be used.

            Appropriate ETAGs are shown in the sequence diagrams and anchor selector chart referred to above.

"Multiple use"

For anchors with ETA to ETAG 001 Part 6Anchors for multiple use in non-structural applications and ETAG 020Plastic anchors for multiple use in concrete or masonry for non-structural applications, the same basic definition applies:

The design of the fixture is such, that in the case of excessive slip or failure of one anchor the load can be transmitted to neighbouring anchors without significantly violating the requirements on the fixture in the serviceability and ultimate limit state.

For example the design of the fixture may specify the number n1 of fixing points to fasten the fixture and the number n2 of anchors per fixing point. Furthermore by specifying the design value of actions NSd on a fixing point to a value n3 (kN) up to which the strength and stiffness of the fixture are fulfilled and the load transfer in the case of excessive slip or failure of one anchor need not to be taken into account in the design of the fixture.

In the absence of a definition by a Member State** the following default values may be taken:

  For ETAG 001 Part 6

         n1 ≥4; n2 ≥1 and n3 ≤ 3.0kN  or

          n1 ≥3; n2 ≥1 and n3 ≤ 2.0kN

   For ETAG 020

           n1 ≥4; n2 ≥1 and n3 ≤ 4.5kN or

           n1 ≥3; n2 ≥1 and n3 ≤ 3.0kN.

  ** These default values apply in the UK.