Civil Engineering 163 February 2010
Eurocodes in Britain: the questions that still need answering
Alasdair Beal BSc CEng MICE FIStructE
Next month BSI is withdrawing a total of 57 structural design codes as they are deemed to have been superseded by the 58 parts of the ten new Eurocodes. However, as this paper sets out, despite the lengthy gestation of the Eurocodes there are still several issues that need to be resolved before the codes can be successfully implemented in Britain. Liability issues, difficulties of language and presentation, and problems associated with the complex partial factor system are investigated. The resources required for the necessary retraining of design engineers and building control officers are also considered.
In May 2008 BSI British Standards published the final two parts of the new suite of ten structural Eurocodes and has provided all of the required national annexes. The 58 Eurocode parts supersede a total of 57 UK design standards, such as BS 8110 (BSI, 1997b) for concrete and BS 5950 (BSI, 2000) for steel, and BSI is due to withdraw these in March 2010.
From 1 April, the withdrawn codes may still be used but they will not be updated and, according to a European public procurement directive (EC, 2004) -
Work on Eurocodes started in 1975, when the European Commission decided on ‘an action programme in the field of construction’ under article 95 of the Treaty of Rome aimed at ‘the elimination of technical obstacles to trade and harmonising technical specifications’ (BSI, 2002a, p.6). It took 17 years for a first draft of Eurocode 2 for concrete to appear, a further 12 years for this to be developed into the final version (BSI, 2004a), and the UK national annex for the general part (BSI, 2005b) was published in December 2005 -
However, in contrast to the lengthy drafting programme, the proposed changeover timetable is remarkably short -
Until fairly recently, structural designers in the UK have largely ignored Eurocodes. However with the 1 April 2010 deadline fast approaching, many now realise that they may not be able to do this for much longer. This paper reviews the issues involved and discusses the choices they now face.
At present, there is no requirement to use Eurocodes for private projects. For public projects, the technical specification must be based on Eurocodes but designs based on other codes may still be accepted if these can be shown to be technically equivalent. However this may well change in future: a stated purpose of the Eurocodes is ‘the elimination of technical obstacles to trade’. There has been little discussion of what this might mean and its possible legal implications for engineers and their work.
At present, UK building regulations and relevant codes of practice set minimum standards for structural design but engineers are free to apply higher standards if they wish, both in their own designs and in the performance specifications they prepare for contractor-
With Eurocodes available, is it still permissible for an engineer to specify different requirements, such as specifying that piled foundations for a project must comply with soon-
The engineer would explain to the court that he or she has a duty to the client to ensure that the piled foundations are properly designed. BS8004 was specified because the engineer knew from experience that it ensures satisfactory pile designs, whereas he or she has reservations about BS EN 1997-
However, the contractor would argue that a valid design to BS EN 1997-
Note that the question is not just about whether it is legal for an engineer to base a specification on withdrawn UK codes -
Intriguingly, any move to make Eurocodes mandatory for public projects could also be open to legal challenge. If public works contracts are restricted to a select coterie of engineers who have mastered Eurocodes, this would be a far greater technical barrier to trade than UK codes of practice have ever been -
Most of the 58 parts of the Eurocodes have a separate UK national annex. A code such as BS EN 1992-
However the ‘kit code’ problem does not stop there. When a new Eurocode part and national annex arrive from BSI they are not assembled in a ‘ready to use’ form. Not only do they arrive in loose-
To use a Eurocode, an engineer must first read what it says, then read what the national annex says, then work out what the result of combining them would be, then memorise this (because it is not written down anywhere) -
In theory, the partial factor system, load factors and load combination rules are all specified in BS EN 1990, characteristic loads are in the various parts of Eurocode 1 and design rules and material factors for particular structural materials are in Eurocodes 2-
BS EN 1990 states that the combination value of a load is ψ0 times the characteristic load. However, BS EN 1991-
BS EN 1990 clause 6.5.3 states that the ‘frequent’ value of imposed load (typically 50% of characteristic) should be used for ‘reversible’ serviceability limit states and the ‘quasi-
The most significant example of contradiction in Eurocodes is BS EN 1991-
‘Advisory note regarding BS EN 1991-
It is not clear how UK engineers are supposed to design to BS EN 1991-
Therefore, although in theory BS EN 1990 (and its national annex) defines all load factors and load combination rules, in practice this is not the case. The engineer must also check Eurocode 1 (actions) and the design Eurocodes in case any of these overrule it and say something different.
For busy engineers, Eurocodes are impractical to use in their present form and unless great care is taken errors are likely.
A new language
Each Eurocode includes a long list of unfamiliar abbreviations and a system of Greek symbols and complex suffixes which will be inconvenient for both hand and computer calculations. Many engineers will also find the language rather strange in English versions of Eurocodes, with obscure words such as ‘orography’ and words borrowed from French such as ‘normative’ and ‘consistence’.
There are clauses such as BS EN 1990 clause 2.1(1)P, where the words are familiar but their meaning may not be.
‘A structure shall be designed and executed in such a way that it will, during its intended life, with appropriate degrees of reliability and in an economical way sustain all actions and influences likely to occur during execution and use’.
At first sight, clause 1.3 (2) is also rather troubling:
‘The general assumptions of EN 1990 are: ... execution is carried out by personnel having the appropriate skill and experience’.
These peculiar clauses use recognisable English words, yet they make no sense if the words have their normal meanings. Structures are not usually ‘executed’, they do not ‘sustain’ influences and what are ‘actions ... likely to occur during execution and use’?
To decode the clauses, engineers need to understand the sometimes subtle linguistic traps of ‘Eurocode English’. Familiar English words are given new and unfamiliar meanings and then used in place of the words that would have normally been used. Thus instead of an appendix, a Eurocode has an ‘annex’, instead of resisting forces, the structure ‘sustains’ them and instead of buildings being constructed, they are ‘executed’.
The most surprising aspect is that Eurocodes also attempt to create a new technical language for engineering. All over the world, English-
Earlier drafts of the Eurocodes attempted to describe everything in terms of this new language, producing some very obscure passages. Fortunately in the final versions this has been moderated and there are welcome reappearances of words such as ‘load’ and ‘shear’. Unfortunately, instead of being fully rewritten in a consistent language, the Eurocodes now have a confused mixture of ‘action’ terminology and standard technical English. For example, BS EN 1990 clause 126.96.36.199 states,
‘Load Case: load arrangements, sets of deformations and imperfections considered simultaneously with fixed variable actions and permanent actions for a particular verification’.
‘In design situations when imposed loads act simultaneously with other variable actions (e.g. actions induced by wind, snow, cranes or machinery), the total imposed loads considered in the load case shall be considered as a single action.’
Does this mean that the complex load combination rules of BS EN 1990 should be ignored and instead all loads should simply be added up and considered as one loading?
Clause 3.3.2(1) adds to the confusion by stating,
‘on roofs, imposed loads and snow loads or wind actions should not be applied together simultaneously’.
Given that snow loads and wind loads are imposed loads, what does it mean?
According to BS EN 1991-
‘Imposed loads on buildings are those arising from occupancy’.
If that is the case, what happens to all the other types of imposed load? Is clause 3.3.1 (2)P based on the idea that they are now ‘other variable actions’? However BS EN 1991-
The clauses are all saying something important about load combinations, but their meaning is likely to remain unclear until they are translated into clear standard technical English.
γ, ψ0 and ξ
‘For structural engineers, the changes required to existing practice are relatively minor. As Chris Hendy, head of bridge design and technology at Atkins, has said, the impact of Structural Eurocodes can be summed up as ‘Same principles, different rules’ (Bond, 2007).
UK codes have a variety of γ factors for different load types and the values of these vary depending on the load combination being considered. In most situations Eurocodes adopt a ‘two sizes fit all’ approach to basic ultimate-
In traditional codes, a structure is first designed for combination 1 (dead load + imposed load) and then it is checked for combination 2 (dead load + imposed load + wind load) with increased permissible stresses (permissible stress design) or reduced load factors (limit-
Eurocode load combinations work in a different way: combination 1 no longer exists. Instead there is a series of load combinations, each of which includes all relevant loads and imposed deformations but applies different combinations of partial factors to them. In the BS EN 1990 simplified approach (equation 6.10), load combinations take the form
Design load = γ(DL + PID) + γ(leading VL or VID) + sum of (γψ0 (other VLs)) + sum of ψ0 (other VIDs))
where DL is load; PID is permanent imposed deformations; VL is variable load; VID is variable imposed deformation; γ is the partial load factor (1.35 on permanent loads, 1.5 on variable loads); and ψ0 is the ‘combination factor’ applied to imposed loads and deformations (typically 0.7 for floor and roof loads (except storage), 0.5 for wind and snow loads and 0.6 for thermal movement).
Eurocode load combinations involve considering each imposed load in turn as a ‘leading variable action’, while other imposed loads and deformations are applied as reduced ‘accompanying variable actions’. All the different possible permutations of factors must then be considered to find which has the worst effect. Thermal movement and differential settlement must be included in every load combination -
BS EN 1990 (BSI, 2002a) also allows an alternative approach where the design load is the worse of equations 6.10a and 6.10b These are
Design load = γ(DL+PID) + sum of γψ0(VLs) + sum of γψ0(VIDs)
Design load = ξγ(DL+PID) + γ(leading VL or PID) + sum of γψ0(other VLs) + sum of γψ0 VID
where ξ =0.925 in the UK.
For those who enjoy calculations and working with numbers, there is certainly fun to be had. However, before equation 6.10 can be applied the engineer must identify which variable actions can be considered as being separate actions in the calculation and which cannot, so as to apply the factors correctly. This also affects safety and economy, because if the total imposed load can be divided into separate actions, this reduces the design load and the structure's safety factor. The more the loading on the structure can be divided up into separate actions, the lower the safety factor becomes.
Figure 1 Floor plan
Consider a beam which supports an office and shop on one side and a roof and car park on the other (Figure 1). Are these four separate variable actions, or are they all parts of one variable action (the imposed floor load)? The total characteristic imposed load on beam A = 4x3(2.5+l.5+4+2.5)= 126kN. If all of the imposed loads are considered as parts of one variable action, then the factored load for design = 1.5 x 126 = 189 kN. However if the four different imposed loads are considered to be separate variable actions, then the design factored load = 1.5 x 4 x 3((0.7 x 2.5) + (0.7 x 1.5) + 4 + (0.7 x 2.5)) = 153.9 kN. If the beam was designed to BS EN 1993-
BS EN 1990 gives no rules to define which imposed loadings (or parts of imposed loadings) can be considered as separate actions in equations 6.10, 6.10a and 6.10b. This is fundamental to the operation of the equations and these in turn are fundamental to all Eurocode design, yet BS EN 1990 leaves the question unanswered. Perhaps the answer is in clause 3.3.1(2)P in BS EN 1991-
The Eurocode safety factor system appears to have several anomalies.
* Design safety factors should include an allowance for the effects of corrosion, minor damage, other deterioration, or other unanticipated influences which may affect the structure. However, in Eurocodes, γF only covers analysis uncertainties and unfavourable load deviations (BS EN 1990 clause 6.3.1 and 6.3.2) and γM only covers geometrical deviations and possible deviations from characteristic strength (BS EN 1990 clause 6.3.4). Logically, corrosion and so on should be covered by γM but, in BS EN 1993-
* A low partial safety factor (γF = 1.35) is applied to all permanent loads and a high factor (γF = 1.5) is applied to all variable loads. This seems inappropriate as some permanent loads (e.g. earth pressure or weight of screeds and finishes) are highly variable, whereas there are variable loads (e.g. weight of water in a tank) which can be calculated to within 1%.
* Eurocode design rules appear to aim for constant failure probability, regardless of circumstances and consequences of failure. This produces some unusual results: the safety factor on a 1:50 years, 3s wind gust is higher than the safety factor on permanent earth pressure and, because of the load combination rules, a structure subjected to only dead load and wind load (e.g. a signpost) is given a higher safety factor than a structure which also carries imposed floor loads (e.g. a house or public building).
None of these seems to make much sense from an engineering point of view. Engineers would thus be well advised to be cautious in any situation where they find Eurocodes appear to justify significantly more economical' designs than past practice.
As noted earlier, it is also necessary to apply load factors in serviceability calculations: ψ0, ψ1 or ψ2. With so many different factors to be applied, great care needs to be taken to avoid mistakes, particularly as some design Eurocodes and national annexes contain clauses which sometimes amend or contradict the BS EN 1990 rules.
Also, if reduction factors are applied to loads for serviceability calculations, then limits on deflection and so on may need to be reduced correspondingly to avoid trouble.
It is worth remembering that every structure has foundations -
‘When I last counted, there were 112 partial factors to choose from in EN 1997-
BS EN 1997-
Dots and commas
In the UK and most other English-
For some reason, the English editions of Eurocodes have adopted continental decimal notation, using ‘,’ as the decimal point. Presumably this was done for convenience during drafting but it is not clear why it has been retained in the published final editions. Was it an oversight, or is the intention that continental decimal notation should be used for Eurocode designs in the UK? The Eurocodes do not actually say what is intended. UK national annexes only add to the confusion: the national annexes for BS EN 1990 and Eurocodes 1, 2, 5 and 6 use continental notation but the national annexes for Eurocodes 3, 4 and 7 use UK notation.
Is the UK construction industry supposed to change its decimal notation as part of the Eurocode changeover or not? If so, this would need careful planning -
The government has not announced any plans for the UK to change to continental decimal notation, so presumably if engineers are to adopt it for Eurocode calculations they will be doing this on their own. It is not realistic to ask engineers to do this if everyone else in the country is continuing to use standard UK notation and schoolchildren are still being taught that ‘.’ is a decimal point and ‘,’ means thousands. It would also require office computers to recognise as a thousands marker in financial calculations but as a decimal point in engineering calculations.
BS EN 1993 introduces another change which with potential safety implications: the major and minor axes, which have always been ‘x-
Some may see these as relatively trivial matters that engineers ‘will just have to get used to’. However, anyone with knowledge of engineering failures knows that the dangers of introducing even apparently trivial changes to standard conventions and notation without careful thought and planning should not be underestimated.
BS EN 1990 clause 1.3 (2) states:
‘The general assumptions of EN 1990 are ... adequate supervision and quality control is provided during execution of the work, i.e. in design offices, factories, plants, and on site’.
In the UK today there is rarely a clerk of works or resident engineer or architect on site checking the contractor’s work: when architects and engineers are appointed on ‘design only’ contracts, they usually do not check the contractor's work at all. Work on site is usually carried out by subcontractors, often with minimal supervision and checking of their work by the main contractor provided it is completed on time for the agreed price. Does this constitute ‘adequate supervision and quality control’ as assumed by BS EN 1990? If it does not, then Eurocodes may require changes on construction sites as well as in design offices.
Support and training
A considerable amount of work has gone into textbooks, guides and manuals for Eurocodes. In addition, the Institution of Civil Engineers and the Institution of Structural Engineers have created the Eurocodes Expert website at www.eurocodes.co.uk to help engineers deal with the proposed changeover. This gives access to basic information about the availability of Eurocodes and related publications, training courses and software. It also includes a list of frequently asked questions.
Despite the amount of work that has gone into publications and websites to help engineers with Eurocodes, the scale of the retraining challenge posed by the proposed changeover is daunting. In 2004, a study for the Institution of Structural Engineers (IStructE, 2004) estimated the cost for an office of 16 engineers as £255,000, or an average of £16,000 per engineer. However this assumed that the 16 engineers could manage with only one set of Eurocodes between them and that three man-
If it is assumed that an office of 16 engineers would need several sets of Eurocodes and if we allow a rather more cautious estimate of 15 days on courses plus 15 days of individual study for each engineer to learn to use the new codes, plus increased costs since 2004, the cost could easily be double the previous estimate -
In practical terms, UK consulting engineers may struggle to find the staff and the money to cover the training required for the Eurocode changeover. Furthermore, Britain's construction training industry is unlikely to find sufficient resources to retrain all of the country's engineers before 1 April 2010.
It is a similar problem for local authority building control departments: the government has given them no extra money to retrain their staff yet, unless this is done, it is difficult to see how they can check Eurocode calculations submitted.
Some UK engineers may be tempted to continue to use withdrawn UK codes of practice and hope for the best. This would be a decision based on simple economics: without substantial government assistance (which is not on offer) they may not be able to afford to do anything else. They would also know that withdrawn codes will still be accepted under the Building Regulations for many years to come and that, even if a project is specified as ‘design to Eurocodes’, they could probably get away with a design based on withdrawn codes without anyone noticing.
For engineers who wish to try to use Eurocodes, there are two choices: either use the codes directly or use Eurocode manuals published by the Institution of Structural Engineers and others. Though they may have some limitations, a lot of work has gone into these manuals, they are written in English, they combine Eurocode and national annex requirements in one document and they simplify and clarify some of the requirements.
However, ‘designing to a Eurocode manual’ is not the same as ‘designing to Eurocodes’. If a contractor’s design does not comply with the Eurocode manual, it may still comply with the Eurocodes, but the engineer can only check this by referring to the Eurocodes themselves. Therefore, the manuals only offer a partial solution. ‘Eurocode manual design’ is easier than ‘Eurocode design’ and it should produce results which comply with the code -
Although the proposed changeover to Eurocodes on 1 April 2010 is now imminent, there are still important questions about the project which need to be answered.
* Can the idea of using Eurocodes to remove technical barriers to trade be reconciled with an engineer's professional responsibility to their clients?
* Is it realistic to expect busy practising engineers to use Eurocodes in their present form, without any published consolidated texts that combine Eurocodes with their national annexes?
* Why are English Eurocodes not written in standard technical English?
* How can the BS EN 1990 system of partial factors and load combinations be used if the code does not define which imposed loads are to be considered as separate ‘variable actions’ and which are not?
* Is it necessary to introduce load factors for serviceability calculations as well as for strength calculations?
* Are UK engineers to change to continental decimal notation for Eurocode calculations and, if so, how are the practical problems and safety risks associated with this to be overcome? There are similar safety implications with the proposed change to axis labels for steel members.
* Where are consulting engineers to find the resources to fund retraining of their staff for Eurocodes and where can the training industry find the resources to train the number of engineers involved?
* Unless the government injects substantial resources into local authority building control departments for retraining, how are Eurocode designs to be checked under building regulations?
* Unless the government injects substantial resources into local authority building control departments for retraining, how are Eurocode designs to be checked under building regulations?
These questions are unlikely to worry anti-
Bond A (2007) ‘When an irresistible force meets an immovable object’, Geodrilling International, April, p. 22. BSI (British Standards Institution) (1986) BS 8004:1986: Code of practice for foundations. BSI, London. BSI (1997a) BS 6399-
BSI (1997b) BS 8110-
BSI (2000) BS 5950-
BSI (2002a) BS EN 1990:2002: Eurocode: basis of design. BSI, London.
BSI (2002b) BS EN 1991-
BSI (2003a) BS EN 1991-
BSI (2003b) BS EN 1991-
BSI (2004a) BS EN 1992-
BSI (2004b) BS EN 1992-
BSI (2004c) BS EN 1995-
BSI (2004d) BS EN 1997-
BSI (2005a) BS EN 1993-
BSI (2005b) NA to BS EN 1992-
BSI (2006) NA to BS EN 1993-
BSI (2008) NA to BS EN 1991-
EC (European Community) (2004) Directive 2004/18/EC of the European Parliament and of the Council of 31 March 2004 on the coordination of procedures for the award of public works contracts, public supply contracts and public service contracts. Official Journal of the European Communities, L134/114.
HMG (Her Majesty’s Government) (2006) Public Contract Regulations 2006. The Stationery Office, London, Statutory Instrument 2006 No. 5.
IStructE (Institution of Structural Engineers) (2004) National strategy for implementation of the structural Eurocodes: design guidance. The Institution of Structural Engineers, London, p. 13.