By Dr-Ing Thilo Pregartner, Dr Fritz Wall and Thomas Holland- Letz, Construction Fixings Europe
Design of fastenings in concrete- a long way to EN 1992-4
It will be a historical moment when EN 1992-4 becomes available in the autumn of 2018 because it is the first time in the history of fastener design that the design provisions will be published in an official European standard and not in guidelines or state of the art bulletins. EN 1992-4 is part 4 of the Eurocode 2 (Design of concrete structures), providing a common approach for the design of reinforced concrete structures. It covers the design of fastenings for use in concrete and represents the current state of the art design method¹.
The publication of EN 1992-4 is a huge step forward for the fastening industry, as the presence of the design of fastenings in a code brings the importance of this topic to the attention of structural engineers. Furthermore, the safety concept for anchors is directly embedded into the general global safety concepts for structural design within the Eurocodes and specifically in the structural design for reinforced concrete structures. All denominations of design of fasteners were adapted to the nomenclature of the Eurocodes. Additionally, EN 1992-4 describes the design of various fastening systems in one document for:
EN 1992-4 covers the design of fasteners under static, quasi-static, fatigue and seismic actions, as well as under fire exposure.
The first anchor design guidelines were published by the European Organisation of Technical Approvals (EOTA) as Annex C of ETAG 001² under the approval system of the Construction Products Directive (CPD). In 1997, when Annex C was first published, only the design of mechanical anchors for use in concrete was covered. Later on, additional Technical Reports considering further design provisions for additional fastening systems or additional load situations were published by EOTA: TR029 for design of bonded anchors under static loading; TR045 for design of metal anchors under seismic actions; TR047 for design of anchor channels under static loading; and TR050 for design of anchor channels under fatigue loading³, ⁴, ⁵, ⁶.
In general, the idea of having a design standard for design of fastenings in concrete, published under the standardisation organisation CEN, had already evolved in the early part of the 21st century. However, due to many hurdles in the standardisation process, it took the corresponding CEN working group (CEN/TC 250/SC 2) until 2009 to publish the results of these long-lasting discussions in the Technical Specification CEN/TS 1992-4 ‘Design of fastenings for use in concrete’ ⁷, ⁸, ⁹. Although these design provisions were only published as a Technical Specification, they were widely accepted and the CEN/TS was cited in many European Technical Approvals as a design option.
With the change from the Construction Products Directive (CPD) to the Construction Products Regulation (CPR) in 2013 though, the new system for harmonised European standards or European Assessment Documents required that design provisions could not be part of the European Assessment Documents anymore. Therefore, the requirement for an official and published design standard arose again. Additionally, there was the request to reduce the extent of CEN/TS 1992-4 to a maximum of around 100 pages. Finally, EN 1992-4 is expected to be published as a result of all these developments in the autumn of 2018.
Current situation for design of fasteners
During these historical developments described, every year ETAs (European Technical Approvals and European Technical Assessments after June 2013) were published referring to a large variety of design guidelines (EOTA TR029, ETAG 001 Annex C, EOTA TR045, CEN/TS 1992-4). Especially in the last few years there has been obvious confusion about the publication of EN 1992-4 and the implications due to the citation of valid design standards in ETAs. This fact became even more obvious as the newly developed European Assessment Documents (EADs) for post-installed fasteners in concrete were referring to a pre-version of EN 1992-4, because the official publishing of the standard was repeatedly postponed. The assessment system was one step ahead of the standardisation work.
For ETAs issued during that time, the confusion is clearly visible within the ETA documents for post-installed fasteners. To enable structural engineers to use the performance data given in these ETAs, with alternative design methods, EOTA tried to close the gap by publishing the Technical Report TR055¹⁰, which compares the performance data for design according to EN 1992-4 with the performance data necessary for the design according to other EOTA design guidelines.
As a result of the historical developments and the actual presence of various design documents, the manufacturers’ association CFE (Construction Fixings Europe) is of the opinion, that a clear statement of EOTA is necessary with the publication of EN 1992-4: The EOTA design guidelines TR029, ETAG 001 Annex C and TR045 for post-installed fasteners can be considered as not any more state of the art.
As alternative solution, the EOTA design guidelines could be amended such that the technical content and the denomination of variables is in accordance with EN 1992-4. This solution would ease the transition process and also give the opportunity to provide guidance on the application of EN 1992-4 in cases, where values, which are needed for EN 1992-4, are not stated in a European Technical Assessment, e. g by providing standard values.
Legal status of EN 1992-4 and the implications for anchor manufacturers and consumers
In general, construction design is national law. Nevertheless, the Eurocodes are recognised by the EU Member States and the European Free Trade Association (EFTA) as the reference design codes. Therefore, EN 1992-4 is, after its official publication, also the reference design code for fasteners for use in concrete.
The document is legally only a recommendation, which means that there is no obligation to carry out the design of anchorages in accordance with EN 1992-4. However, it is mandatory for the Member States to accept designs according to EN 1992-4.
According to the Commission Recommendation 2003/887/EC, the “Member States should adopt the Eurocodes as a suitable tool for designing construction works, checking the mechanical resistance of components, or checking the stability of structures.”
After publication of the National Standard of EN 1992-4 transposing the Eurocode and the National Annex, all conflicting national standards shall be withdrawn by the Member States. However, EN 1992-4 relies on product specific characteristic resistances stated in a European Technical Product Specification. A European Technical Assessment (ETA) represents such a document and provides technical data for the fastening element based on the latest state of the art product assessment. Otherwise the design of fasteners in practical applications cannot be carried out without further considerations. In practice this could cause mistakes in the design of fastenings, for which the design engineer would be responsible. Therefore, CFE recommends always utilising the state of the art design provisions in combination with corresponding information within European Technical Assessments referring to EN 1992-4 as valid design code.
To comply with the state of the art design method for post installed fastenings for use in concrete, anchor manufacturers should consider adapting their design software packages and the issued European Technical Assessments to comply with the latest European Assessment Documents and the state of the art design according to EN 1992-4. With these adaptations, the ‘circle’ between ETA and valid design standard will be closed and all confusion will be eliminated.
The design provisions according to EN 1992-4 represent the current state of the art. Therefore, designers can use the design provisions according to EN 1992-4 with a corresponding ETA for the verification of fastenings and they can be sure that this design method will be accepted within the EU and the EFTA.
Major changes for the design of adhesive anchors under sustained loading
One of the most important changes that was implemented in EN 1992-4 is the application of a product dependent reduction factor explicitly taking into account the behavior of bonded anchors under sustained loading (ψ°sus).
Based on the suggested reduction factor in the EN 1992-4, for cases where no specific information is provided for a product in e.g an ETA, the applicable bond strength is reduced up to 40% depending on the ratio of sustained action to the sum of the total actions. If in this case the percentage of sustained load is larger than 60% of the sum of the total load acting on the anchorage, the reduction factor ψ°sus needs to be applied to account for the decrease of the bond strength under sustained loading. Applications with 100% sustained load suffer the maximum reduction of 40% of the bond strength whereas for applications with less than 60% sustained load no reduction becomes effective.
The current value of the above mentioned reduction factor ψ°sus (ψ°sus = 0.6) given in EN 1992-4 considers the decrease of the bond strength of adhesive anchors due to sustained loading when the concrete temperature in the region of the fastener is at 43°C during a total time period of 10 years within the service life. The factor shall be applied for cracked and non-cracked concrete, as well as for all temperature ranges, although there is no guidance available in EN 1992-4 for maximum long-term temperatures higher than 43°C.
The problem with this change is the fact, that under distinct circumstances (applications with primary sustained tension loading) a reduction of the resistance of up to 40% may occur (depending on the loading situation). This may cause significant confusion for consumers and structural engineers, as a considerable difference in design results will occur depending on the applied design code EN 1992-4 or EOTA TR029 or CEN/TS 1992-4. These differences may also occur in design situations with alternative mortars of several manufacturers (depending on the utilised design provisions in the ETA Assessment Documents).
CFE (Construction Fixings Europe) therefore recommends proactively informing the customers and structural engineers about the upcoming changes. In the opinion of CFE the current state of the art shall always be applied for the design of post installed fastenings, which means after its publication, EN 1992-4 is quasi mandatory for future projects.
Construction Fixings Europe – a competent voice for safe anchoring
Construction Fixings Europe (CFE) represents leading European manufacturers of metal, plastic and bonded anchors, and direct fixings (powder actuated fixings). Because drill bits for concrete and masonry are important for the installation of anchors, also this product sector is represented by CFE.
CFE represents 25 companies of all sizes – from small family owned businesses and SMEs up to larger companies. 50% of CFE members have less than 50 employees. Additionally, three national associations for anchor fixings from France, Germany and the UK are CFE members. Most of the companies hold one or more ETAs for their anchors.
Benefits and synergies arise from the combination of the broad knowledge and the resources of the bigger companies with the specific input and the product know how of the SMEs.
CFE was found in 2016 by conversion of the former professional section ‘Anchors’ of the European tool association Comité Européen de l’Outillage (CEO). This was done to give this section its own identity, but CFE remains an integrated part of CEO.
CFE is actively involved in the development and maintenance of European Assessment Documents (EAD). CFE experts represent the member companies in the EOTA WG. The formation of the CFE opinions is done by online meetings and via a web portal in the members’ area of the CFE website.
CFE members stand for:
- Research, development and manufacturing in Europe.
- Innovative and safe construction fixing products.
- Reliable CE Marking based on state of the art ETAs.
- Correct use of CE Marking and references to ETAs in advertising, defined in a Code of Conduct.
- Level playing field for European manufacturers of all sizes.
- Short time-to-market for innovative products with new ETAs under fulfilment of all legal requirements for CE Marking.
- Robust products and user friendly installation instructions to prevent installation failures.
CFE is a member of the Construction Products Europe and of the European association of the mechanical industry – ORGALIME. Other associations and institutions like the association of the European construction industry, FIEC, are part of the CFE network.
Conversion of ETAGs into EADs
The conversion of European Technical Approval Guidelines (ETAGs) coming from the former Construction Products Directive (CPD) into European Assessment Documents (EADs) under the Construction Products Regulation, in the field of anchors, is nearly finished.
The table lists these EADs for anchors, which are covered by EOTA product area 33 ‘Fixings’.
Other EADs have been developed due to an ETA application for products that were not covered by ETAGs. During the conversion of the ETAGs into the first EAD versions (version 00), no technical changes or amendments were possible for formal reasons. Therefore, these EAD versions 00 do not reflect in all cases the current state of the art and have to be amended now by EOTA.
Another formal requirement for the conversion of ETAGs into EADs was a change of the philosophy from the CPD to the CPR. Under the CPD, a product got an approval (ETA) based on an approval guideline (ETAG). Under the CPR, products are no longer “approved”, but “assessed” to get a European Technical Assessment, which can be seen as a “snapshot” of the product at the time of the assessment.
Upcoming CPR revision
In the upcoming revision of the Construction Products Regulation, CFE is strongly advocating the maintenance of the proofed and reliable ETA-route to CE Marking. CFE has signed, together with major umbrella associations active in the construction sector, an open letter to Members of the European Parliament, the European Commission, and Members of Standing Committee for Construction (SCC), promoting the importance of the ETA route to CE Marking of construction products.
This aspect is also addressed in the current CFE Position Paper together with some proposals on procedural improvements. The paper can be found on the CFE website: construction-fixings.eu
Having joined the magazine in 2012, Claire developed her knowledge of the industry through the numerous company visits, exhibitions and conferences she attended both in the UK and abroad.
Claire prides herself on keeping readers well informed and up to date with the latest industry news.