How to select anchors in accordance with BS 8539:2012 Code of practice for the selection and installation of post-installed anchors in concrete and masonry. 1 Introduction BS 8539 is arguably the most significant development in the use of construction fixings since the advent of European Technical Approvals (ETA)1. It was published as a result of a series of failures of anchors due to the failure of various stakeholders to properly select or install them correctly and in order to prevent further such incidents. If fully adopted it will result in a reduction in failures of construction fixings with consequent savings in injuries, deaths and liabilities. This “How to” guide provides an overview of the responsibilities of the designer and specifier in selecting anchors for safety critical applications together with the actions which are key to satisfying those responsibilities. All stakeholders should acquire a copy of the code2. 1.1 Key themes Key themes of the code include the importance of using anchors with ETA if at all possible, the need for one person to carry the overall responsibility for anchors from selection to commissioning - including temporary works phases, the passing of information between stakeholders at appropriate stages and a recognition that anchor specifications should not be changed unless the full selection procedure is undertaken. 1.2 Terminology Terminology used in the code is based largely on that used in ETA, in ETAGs (the Guidelines used to award ETA) and CEN Technical Specifications3 for anchors. This has resulted in the adoption of terms and notation not previously used by the CFA or by some manufacturers. For a detailed explanation of the differences between the new and old terminology see4. The term “fixing” – integral to the name of the CFA – does not appear in the code having been replaced, for consistency and clarity, with “anchor”. Anchors are still fixings. The term “load”, is used only in the general sense having been replaced by “Action”, - the load to be transferred from the fixture to the base material, and “Resistance” - the capability of the anchor in resisting the action. BS 8539 defines selection5 as the overall process of choosing the anchor including both the type and size, and within that process design6 is the particular process of determining the size of anchor(s) required, referred to in conjunction with anchors carrying ETA as the Design Method. 1.3 Responsibilities BS 8539 defines the designer as the person with overall responsibility for the design of the structure, which includes the anchorage, and the specifier as the person responsible for the selection (including the design) of an anchor. Note: the designer and specifier may be the same person. CFA A “How to” guide to BS 8539 for specifiers and designers. (Issue 1. March 2013) 1 of 4 2 Preliminary considerations (BS 8539 Clause 5.2) The designer should take into account the preliminary design considerations listed below and supply all the necessary information to the specifier. a) Ability of the structure to support the applied actions. The design process will determine whether or not the local area of a structure is capable of accepting the applied actions but this process does not verify that the overall structure has sufficient integrity to sustain these actions. This must be verified independently by the designer. It should be noted that tests carried out on site to the procedures outlined in BS 8539 will not contribute to this verification. b) Concrete condition. The designer should determine if the concrete is cracked or noncracked. The default would be to assume it to be cracked. c) Robustness, redundancy and progressive collapse. Consideration must be given as to whether or not there is sufficient built-in redundancy to allow for load distribution in the case of individual anchor failure, i.e. is it statically determinate or indeterminate. This may affect the choice of ETAG to which the ETA of an anchor has been awarded. Clause 5.2 gives several helpful references on this subject. 3 Information to be provided by the manufacturer/supplier to the specifier (BS 8539 Clause 6.2) Full technical data should be provided by the manufacturer/supplier to the specifier, including the following: • • • • • • • • • designation of anchor including size and type material type used to manufacture the anchor, e.g. stainless steel/carbon steel design method to use e.g. ETAG 001 Annex C or CEN TS 1992-4 Parts 1 – 5 performance data (preferably given in an ETA), including: • characteristic resistance • design resistance (or partial safety factor, for the material, to enable it to be calculated) • recommended resistance (or appropriate safety factor to enable it to be calculated) effective embedment depth, minimum and maximum fixture thickness edge and spacing criteria minimum thickness of base material setting details including • temperature limits and curing times if appropriate • installation equipment • installation instructions The anchor manufacturers generally provide the required information in one (or all) of the following formats, usually available from the company website: o technical manuals o anchor design software o any associated information on anchor technology and design, ETAs and safety data sheets (SDS) CFA A “How to” guide to BS 8539 for specifiers and designers. (Issue 1. March 2013) 2 of 4 4 Information to be provided by the designer to the specifier (BS 8539 Clause 6.3) If the designer is not also the specifier, they should supply all the necessary information to enable the specifier to select and specify the anchor. This information should include, for example: The application details, including; • design actions, their directions and nature (static and/or dynamic) • whether the application is statically determinate or statically indeterminate • confirmation that the structure is capable of sustaining the characteristic action • details of the base material type, thickness and likely strength at the time of installation • in the case of concrete - its status i.e. cracked or non-cracked • edge distance and anchor spacing used for the design • details of the base plate, material and thickness • environmental conditions, including elevated temperatures or fire rating requirements, corrosion conditions and required durability or life expectancy • special requirements (e.g. need for through fixing, immediate loading, flush fixing for ease of removal, etc.) 5 Selection process BS 8539 considers the selection process in two phases – determination of the anchor type (BS 8539 Clause 5.3) and determination of the anchor size (BS 8539 Clause 5.4) this part of the process being referred to as the “Design” of the anchor, see section 6 below. Determining the anchor type means considering – among other things - anchor reliability, base material, positioning, environmental considerations (temperature and corrosion) and various aspects of the loading. Anchor reliability BS 8539 recommends that anchors should be selected which will function reliably in all potential site conditions and that one way of achieving this is to select an anchor with an ETA. Base material considerations include its type (concrete or masonry and whether solid or hollow) its nature (if concrete is it cracked or uncracked?), structural thickness, etc. Loading aspects include the nature, magnitude, direction and duration of the applied load. Nature: - is the overall loading statically determinate or indeterminate? (as discussed in 2 c) above) and is it a static or non-static action? (BS 8539 Annex E). The magnitude of the characteristic action (a.k.a. applied [unfactored] load) is fundamental to the anchor selection and handled within the design process (See BS 8539 Annex A). Direction. Different anchors have different capabilities in tension and shear and a special approach is needed for combined tensile and shear loads. Few anchors cope with bending loads very well. Duration. Is the action applied over the short or long term? Temperature considerations may influence the type of anchor in terms of limitations placed on service temperatures (plastic anchors and resin bonded anchors) or installation temperatures (resin bonded anchors). Corrosion aspects are discussed within BS 8539 in clause 5.3.5.2 and Annex F. CFA A “How to” guide to BS 8539 for specifiers and designers. (Issue 1. March 2013) 3 of 4 6 Design method (See BS 8539 Annex A) The design method for an anchor with an ETA will be referred to in the ETA itself and will commonly be to ETAG 001, Annex C, for mechanical anchors or TR 029 for bonded anchors, the details of which may be downloaded from the EOTA website at http://www.eota.eu. Some ETA may refer to design methods of CEN TS 1992-43. The design would, in all cases, require reference to the ETA for the particular anchor to obtain the relevant design data. The practical approach is to download design software from the anchor manufacturers’ website, which will provide an accurate and efficient design along with the widest choice of suitable anchor types to select from. Most companies also offer technical support to assist in this and engineers who can visit your office/site to discuss the application. The manufacturers also provide technical manuals that allow “simplified” designs to be carried out by hand, but these are usually much more conservative than the software and the data is updated far less frequently. 7 Changing specifications (BS 8539 Clause 10) Anchor specifications are frequently qualified with the phrase “or equivalent” or “or similar approved”. With or without such a phrase changes to specifications should only be made following a full change management process carried out by the specifier. It might be necessary to change the specification of an anchor for a variety of reasons including: • unavailability of the specified anchor; • contractor has a preferred supplier; • a change in the design loading information means the current anchor specification is inappropriate; • economic reasons. Whatever the reason for the change request, the alternative specification should be determined by either the original anchor specifier or by a responsible engineer on site who assumes the role and responsibilities of the specifier and has access to the original design data. The full process must be completed. It is not sufficient to change the specified anchor for one which appears to be similar, or to compare headline performance figures quoted in catalogues of the proposed alternative anchor with those of the originally specified anchor. To do so might not take into account the way in which the performance of different anchors changes with particular application factors . Nor is it acceptable to carry out proof load tests of the proposed alternative anchor on site, as this does not validate the required safety margin. 1 For more information on ETAs see CFA Guidance Note: ETAs and design methods for anchors used in For copies of British Standards relating to construction fixings, at a discount of 15%, contact the CFA via our website www.the-cfa.co.uk. 3 Design methods for anchors with ETA may be contained in CEN Technical Specifications in the series CEN TS 1992-4 Parts 1 – 5. (Otherwise they are as the relevant ETAG.) 4 CFA Guidance Note: Anchor terminology and notation 5 BS 8539 Clauses 3.1.39 & 5. CFA Guidance Note: Anchor selection. 6 BS 8539 Clause 5. 2 CFA A “How to” guide to BS 8539 for specifiers and designers. (Issue 1. March 2013) 4 of 4