Swapping commas for decimal places, executing a structure rather than building it, and different concrete strengths for cylinders over cubes are just some of the new realities facing engineers using Eurocodes, the recently introduced structural design standards.
The switch to Eurocodes is an enormous challenge for the industry. The Institution of Structural Engineers calls it, ‘the biggest change to codified structural design ever experienced in the UK’. Never before has an entire set of codes been replaced in such a manner – and the magnitude of the change is greatly compounded by the sheer number of new documents.
On 4 June 2011, a new Network Rail standard for the design of bridges came into force which mandates the use of Eurocodes. Against this backdrop, Network Rail arranged for Parsons Brinckerhoff to provide an intensive period of Eurocodes training for its engineers and managers to ensure project and asset staff have an awareness and understanding of the significant changes that arise from the new codes.
The Eurocodes have been over 25 years in development but are now with us. Published as 10 codes in 58 parts, they cover the structural and geotechnical design of buildings and civil engineering works. They include bridges, buildings, retaining walls, earthworks or minor foundations for location cabinets – in fact, virtually all aspects of civil engineering.
The conflicting British Standards were withdrawn at the end of March 2010. Gone are the codes familiar to many engineers, including BS 5400 for bridges, BS 8110 for concrete, BS 5950 for steel, BS 6399 for loads, and BS 8004 for foundations.
Withdrawal of these standards does not mean they have become unsafe; rather, that they are no longer supported or developed. However, for structural and geotechnical engineers working for public sector clients the withdrawal has had a profound effect.
The European Public Procurement Directive requires publicly funded works to be designed to current standards. Major clients, including Network Rail, have concluded that the Eurocodes must therefore be specified for newly-procured design work.
Network Rail has issued and updated key documents to implement this requirement. The new Design of Bridges standard NR/L3/CIV/020, published in March, confirms that the Structural Eurocodes must be used for design of new structures and gives additional client requirements particularly for the application of railway loading. A new Engineering Assurance standard, NR/L2/CIV/003, was published in June and brings significant changes to the technical approval process – these changes include aligning the process with the requirements of the Eurocodes.
In addition to the 58 parts of the Eurocodes, there are 58 corresponding National Annexes which must be used in conjunction. These contain values, procedures, and decisions for nationally determined parameters. Designs for the UK must use the UK-specific decisions on these parameters. Other countries may have made different decisions. Typical examples of parameters found in the National Annex are partial factors and climatic data such as wind and temperature charts.
There is new and different terminology – a classic pitfall is the major axis bending modulus, now termed IYY, which was previously the minor axis modulus – not one to get the wrong way round. The language is English, but it is a Euro-English designed to ensure accurate and unambiguous translation into the different national languages. This gives a few surprising terms – ‘action’ to encompass ‘load’, ‘execution’ instead of ‘construction’. And the codes use the European convention of using the comma for a decimal point.
It is not just the design standards that are changing. A new series of harmonised European product standards has been introduced over the last few years, affecting specification of steel, concrete, reinforcement, and other construction materials. Amongst other things, these give the relationship between concrete cylinder strength, used in design, and cube strength, used in UK site tests. Similarly, there is a suite of new construction standards covering tolerances, quality and supervision – these are having a profound effect on manufacturers and fabricators.
So for the archetypal project engineer with the ability to reel off a string of relevant standards and clause numbers, there is a lot to re-learn. One of the few areas which will not change is bridge assessment and related strengthening and repair works to existing structures, which will continue to be carried out using the existing standards.
Training and support
Parsons Brinckerhoff has been at the heart of Eurocodes development for many years – Parsons Brinckerhoff’s Engineering Director, Steve Denton, chairs the European Committee with responsibilities for the Eurocodes relevant to bridges and sits on many of the British Standards Eurocodes committees. The company has been providing strategic and technical support to clients for much of the last decade and has had extensive involvement in drafting passages of the UK national annexes. It was therefore natural for Network Rail to select Parsons Brinckerhoff to support its transition to the Eurocodes.
Bespoke courses were developed and delivered specifically for Network Rail, including three-day courses on bridges, buildings and geotechnical design. During 2010, Parsons Brinckerhoff’s Eurocodes experts travelled the length and breadth of the country, from London to Larbert, Watford to Walsall, delivering 28 training courses to over 350 Network Rail staff. More courses are planned for the rest of 2011.
In addition, Parsons Brinckerhoff has assisted in addressing strategic implementation challenges, in particular collaborating with Network Rail’s in-house experts to define technical requirements for the application of Eurocodes.
Clearly time and investment are required to climb the learning curve – but once this is done, the Eurocodes are expected to bring a range of benefits: they are based on the latest science; they are more consistent and rational; and there have been some positive advances in terms of methods used.
The Eurocodes approach begins with general principles and then develops specific application rules. In contrast the traditional British Standards tended to be more procedural. Therefore, although the Eurocodes demand a greater technical understanding from designers, they offer greater flexibility and the ability to deal with non-standard situations which could be a particular advantage in the rail environment with its many constraints.
These design objectives are set out in Section 6 of Network Rail’s Design of Bridges standard and include maintenance, robustness, mechanical resistance and economic considerations. Thus, whilst the Eurocodes’ flexibility could permit the use of advanced modelling techniques to optimise designs, particularly the buckling behaviour of steel structures, this advance needs to be balanced against other requirements. Following a bridge strike, for example, a rapid and safe decision is required on whether a bridge can be loaded, preferably without needing extensive high-end computer analysis.
Rail traffic loads are still based on international rail standard UIC 776-1, so the basic load models are unchanged. New material has been included drawing on European experience with high speed lines, including high speed train models and information and rules on dynamic analysis. This will be of particular relevance as High Speed 2 progresses as this project, like Crossrail, will use Eurocodes.
A question of principle
Designers will clearly need to immerse themselves in the detail of the new codes. This is already happening – Redhayes Bridge is the first major bridge in this country designed to Eurocodes, by Parsons Brinckerhoff for client Devon County Council, and was erected in February 2011. Likewise, Network Rail’s new standard underbridge designs have been checked by Mott MacDonald to be Eurocode-compliant – the Approval in Principal documents list treatment of matters not covered by the standards such as calculation of U-frame restraint forces.
Client staff in a technical approval role will be in a particularly challenging position, albeit a step removed from the nitty-gritty of detailed design. Staff will need to have an awareness and understanding of the changes that will result, and ensure that appropriate processes are put in place to manage this change.
Additional measures may be needed during the transition period to assure the competency of engineers in using the new codes. These may include upgraded independent checking or increased levels of design review by in-house Eurocodes experts.
Fortunately, the benefit of many years of experience using the old codes can still be brought to bear, and the fundamental principles of design and construction should not change. In practice, for Eurocode designs this means that there should be few conceptual differences to emerging proposals up to the Approval in Principle stage.
Associated with the new Engineering Assurance standard are a series of forms, due to be published later this year, which cover the Eurocode technical design requirements. Over 200 clauses in the Eurocodes permit project-specific choice – some are expected to be specified by Network Rail, some will require technical approval, others may require a design choice to be recorded on the forms and some may only be relevant in unusual circumstances. A particularly important example is the load classification factor α, a multiplication factor for rail traffic loads.
The future’s Euro
With effective implementation, the Eurocodes offer a number of significant benefits in the areas of best value, best practice and innovation. It will take the right training, management and effort to realise these. Parsons Brinckerhoff has been at the forefront of this industry-wide effort, with involvement in Eurocodes development, leading-edge design work, strategic and technical assistance and training programmes. The first Eurocode designs are now being constructed – soon, every new structure will be a Eurocode structure.