CPD: installation of fixings in concrete and masonry
Mark Salmon of the Construction Fixings Association explains BS 8539:2012, the code of practice for the selection and installation of post–installed anchors in concrete and masonry.
Most construction projects involve hundreds of connections between components and structures, and are dependent on thousands of fixings that must be correctly selected and installed. Construction or design managers may well not have direct responsibility for either of these activities, but should an accident occur as the result of a failure of such a connection, then your indirect responsibility may come under scrutiny from the HSE.
But multi-tasking construction professionals do not need to become expert in construction fixings to prevent a failure – they simply need to ensure that all parties involved in the use of fixings are working to BS 8539. This article outlines how this code of practice details the roles and responsibilities of stakeholders in the use of fixings, and describes the guidance and tools assembled by the Construction Fixings Association (CFA) to help with their understanding and implementation.
Fixing failures are, thankfully, a relatively rare occurrence, and when considered in relation to the number of fixings actually installed, the percentage of failures is small. But there are probably more of them than you might imagine, and to the unfortunate victims of the accidents that do occur, their rarity is of little consolation. In addition, the impact on a construction project and those managing it can be dramatic.
In 2006, anchors embedded in epoxy resin in the concrete roofslab of Boston’s Big Dig tunnel failed and a ceiling panel crushed a car, killing the passenger
Structural-Safety is an independent body set up under the patronage of the HSE, the Institution of Structural Engineers and the Institution of Civil Engineers. It has set up a system called CROSS, which stands for Confidential Reporting on Structural Safety. This enables engineers with concerns about structural issues, including failures and collapses, to file a confidential report that can be made known to the wider industry through their monthly newsletter (see below). They have reported on several incidents involving failures that would otherwise not have had any exposure.
Fixing failures can range in scale from the collapse of suspended ceilings to the failure of major structural elements, both during construction and, in one case, six years after completion. As a result of these, the British Standards Institution (BSI) agreed to a request from the CFA to publish a standard that set down everyone’s roles and responsibilities in order to prevent further incidents. The finalised document was published in October 2012. Construction managers and other project professionals involved in drawing up project documentation should ensure that all project documents that concern fixings include the requirement that all stakeholders comply with the recommendations of BS 8539:2012.
Scope of the code
The scope of the code is broad. It covers all safety-critical applications using fixings that may be installed in holes drilled through concrete and masonry. (Although cast-in fixings per se are not included in the scope, there is no doubt that following the principles set down in the code will also make such installations safer.) The code covers all interested parties, from the manufacturer and distributor to the specifier, contractor, installer and tester.
Many causes of failures were identified in preparation for the drafting of this code and all have been addressed. One such is the failure of communication between stakeholders. The code sets out what information should be provided by each party to the others at stages in the process. Of course, the key responsibilities fall on those selecting the fixing and those installing it.
As far as selection is concerned, the code differentiates, for the sake of clarity, between the person with overall responsibility for the structure in which a fixing may be located, defined as the “designer”, and the person responsible for selecting the fixing – the “specifier”. They may, of course, be the same person.
A fixer's toolkit
To help construction managers understand and implement their responsibilities under the code, the Construction Fixings Association (CFA) has published a series of documents intended to summarise them in the form of the CFA 8539 Toolkit. It comprises four “how to” guides that cover selection, supply, installation and testing.
Five forms help specifiers to gather data, suppliers to recommend a fixing, specifiers to specify it, contractors to certify the completed installation and engineers to submit a test request. Three guidance notes directly connected with the subject are included: ETAs and Design Methods for Anchors used in Construction; Anchor Terminology and Notation; Procedure for Site Testing Construction Fixings – 2012. Finally, two flow charts or selector charts – one for concrete and one for masonry – guide specifiers to the most appropriate European Technical Approval guidance against which an ETA should be qualified. The CFA also offers CPD seminars.
To download the CFA 8539 Toolkit, go to www.fixingscfa.co.uk
The responsibilities of the designer and specifier are the most detailed. The designer, for instance, is expected to provide essential information to the specifier to enable them to make the correct choice of fixing, especially concerning the structure. For example, is the concrete considered to be cracked or non-cracked? To borrow the terminology of BS 8539:2012, is the application “statically determinate” or not? In other words is there any redundancy built into the system such that the failure of one fixing will not lead to the collapse of the whole structure? This sounds pretty drastic but fortunately the code also recommends that if an anchor with an “ETA” is available then that should be used.
ETA used to stand for European Technical Approval – a system by which fixings could be tested against the most onerous assessment regimes, be deemed to function reliably and have validated performance data that enabled the product to be designed into the structure. However, since the advent of the Construction Products Regulation in 2013, ETA now stands for European Technical Assessment.
In relation to ETA and redundancy, most ETAs are awarded specifically for use in cases where the failure of a single anchor could lead to the collapse of the whole, but are awarded on the basis that if correctly selected and installed, the anchor will not fail.
For applications such as suspended ceilings, where redundancy is desirable but difficult to calculate, the fact that an anchor has been awarded an ETA means that the designer/specifier need not consider the redundancy of the application. This is because, if it falls within certain conservative limits specifying the number of anchors per fixing point and the applied load, the ETA will have already taken the issue of redundancy into account.
Advice is given to the specifier regarding the full selection procedure, with that process divided into two stages. The first determines what type of anchor may be most appropriate, and the second determines how big it should be to support the applied load (also known as the unfactored load).
Factors in stage 1 include the nature of the structure – is the concrete cracked or non-cracked? Are the masonry units solid or hollow? What is the direction of loading: tensile, shear or combined? Is it static or dynamic? What is its duration: short or long term? What are the environmental conditions affecting the fixing? For example, is it subject to elevated temperatures on a short or long-term basis or exposed to corrosive conditions?
In stage 2, determining the size of anchor, the anchor must be put through the appropriate “design method” associated with it, as set out in the ETA. The deeper understanding the industry now has about the factors influencing anchor performance has led, inexorably, to a more complicated design process. That process is now itself about to be published as European Standard EN 1992-4 Design of Fastenings. This standard has been developed from the design annex of the main European Technical Approval Guidelines (ETAG) governing anchors. This is called ETAG 001 Metal Anchors for Use in Concrete, Annex C, to which most current ETAs refer.
Software for sizing
To determine the size of the anchor, all parameters of the application are taken into account, such as the magnitude of loading, its direction, the strength of the base material and the dimensional limitations of edge distances and anchor spacings. Fortunately, this degree of complication has led manufacturers to provide software designed to help professionals perform their calculations.
One benefit is that it forces the designer and specifier to gather all the design information they need in order to complete the questions the software will ask. And once fully and correctly completed, the resulting suggestion from the software will effectively be an endorsement from the manufacturer of the proposed fixing.
The Spit Triga Z safety anchor
Most fixing producers offer technical support to use the software and, in order to help designers and specifiers assemble the required information to give to the manufacturer or his distributor, the CFA has provided a form for this purpose, called CFA Form 8539/01: Design Information. It is part of a set of five that are themselves part of the CFA 8539 Toolkit (see panel above), and can be completed and given to the supplier or manufacturer.
Although BS 8539:2012 is not intended as an educational tool, the code does give a great deal of advice regarding subjects such as the positioning of fixings in brickwork, corrosion and its avoidance, and the types of fixings available. It even advises specifiers that they should consider the likelihood of the installer hitting rebar during hole-drilling and should therefore provide an instruction, as part of the anchor specification, as to what they should do in that eventuality.
Once the anchor has been selected it needs to be “specified”: this is, identified in project documentation in sufficient detail to ensure that the contractor acquires the right anchor and the installer fits it to the correct parameters. Again, the CFA has provided a form to help with this.
From the project management point of view, the code recommends that one person be designated as having overall responsibility for the fixing. This may not always be possible, but if not then steps should be taken to ensure that someone
is responsible at all stages including the erection of temporary works and during construction.
The lack of clear responsibility for a critical fixing during one phase of construction was deemed to be a factor in the death of a worker in an incident that contributed to the instigation of the code. Of even more significance was the changing of a fixing specification without the approval of the specifier.
To prevent that recurring, the code sets out a strict change management procedure that should be followed if a change is requested. Essentially, the proposed alternative should be subjected to the full selection procedure to be sure that any differences in its performance characteristics are taken into account.
The responsibilities of contractors under BS 8539:2012 include ensuring that the correct fixing is obtained as specified, that their installers are competent, that is, they are trained in the installation of the particular anchor involved, and have the correct setting tools. Also that someone in a position of responsibility, such as a supervisor, certifies that the correct anchor has been properly installed in the right place. This neatly completes the circle of responsibility. Another form from the CFA 8539 Toolkit facilitates this certification.
On some jobs it is necessary to test fixings on site. BS 8539 lays down two cases for when this is needed. The first is for the determination of allowable loads if there is no manufacturer’s recommended load data available. This is usually only for needed for anchors that are to be used in masonry.
The second is to validate the quality of installation, in which case proof tests are carried out. This must be done on a sample of working fixings on all jobs, unless the anchors with ETA have been installed by trained installers working under supervision.
The code sets out the test regimes, how many tests are needed, with what load and how results should be treated. However, the test procedures themselves – how tests are actually carried out – are set out in the CFA Guidance Note: Procedure for Site Testing Construction Fixings – 2012, which is a normative annex to the code. In other words, it is considered to be “indispensible for the application” of the code.
Finally, the key point for construction managers is to insist that everyone involved in the use of fixings complies with the recommendations of the code.
Mark Salmon is general manager of the Construction Fixings Association
Three ways that fixings fail
Examples reported to structural-safety.org in 2014
1. Epoxy resin-bonded anchors in concrete on motorway works
A contractor was undertaking the removal of a concrete guide wall as part of enabling works on a major roadway. To facilitate the removal, concrete panels had been cut into sections and all post-fixed anchors had been tested prior to lifting. The anchors failed progressively when the crane lifted the section of wall 500mm off the ground. It became apparent that the incorrect pull test load had been applied: the anchors were tested at 35kN whereas the work package plan required testing to 55kN.
The ambient temperature of the resin dropped below the manufacturer's recommendations of between five and 25 deg C. The temperature on the morning of incident was recorded as being 2 deg C. The quality of the drilled holes was questionable as the manufacturer's installation best practice methodology was not followed.
CROSS comments: The Structural-Safety Alert Tension systems and post-drilled fixings – March 2014 has been published following a number of reports of similar failures. There are recommendations for the inspection of older systems and advice on installation of new anchors.
2. Partial collapse of suspended ceiling
The ceiling, which was much newer than the building, consisted of 1,200 x 600mm mineral-fibre tiles laid into a grid system supported by 2mm galvanized wires. These were connected to eye bolts that were screwed into expansion anchors that were themselves drilled into what appeared to be a plaster soffit. In the collapse area, some of the expansion anchors had become detached from the soffit. The building was originally constructed using an early reinforced concrete floor system, with hollow tiles, possibly clay-based, between in-situ ribs. The underside was then plastered. Some anchor holes had been drilled into the plaster and the tiles but neither was a suitable substrate material.
CROSS comments: There are two issues here. Firstly, there is the potential for an anchor coming out, commonly because they are too shallow. Secondly, and more widely, there is the possibility that any single failure will initiate a cascade-type collapse. BS 8539:2012 is the standard to use for fixings, and guidance is also given in Selection and Installation of Top Fixings for Suspended Ceilings, published by the Association of Interior Specialists and the Construction Fixings Association in 2012.
3. Corroded fixings
We had report where metal plasterboard fixings had deteriorated only a few months after installation. Apparently metal anchors were used to meet fire regulations, but using metal fixings in insulated panels generates cold bridges and can, at best, cause damp spotting on the surface of the wall, and, at worst, lead to corrosion if a low-grade metal is used.
CROSS comments: This is a good example of the role that the understanding of materials plays in safety. Corrosion and degradation always affects safety to some extent. In any wall construction the dew point should always be assessed so as to avoid interstitial condensation.
Visit the new CPD portal to take the questionnaire relating to this article and read more articles