The first day of April this year was a pretty quiet spring day for most people – the odd April Fools chuckle, a spot of rain but the big freeze was on the retreat and the prospect for that weekend’s Easter holiday looked good.
But while most people contemplated the pros and cons of gorging on Easter eggs hunts and the press feverishly speculated on the impending General Election, a quiet revolution had begun for the UK’s civil engineering and construction industry. That morning also heralded the withdrawal of conflicting British Standards and the major step taken by the UK towards the adoption of the Eurocodes.
After over 25 years in development, the Eurocodes are now with us. Published as 10 standards in a total of 58 separate parts, they cover the structural and geotechnical design of buildings and civil engineering works, including bridges.
The decision to withdraw conflicting British Standards was not unilaterally taken by the UK. All member countries of European standardisation body CEN were expected to withdraw their conflicting national standards before April 2010.
The withdrawal of a standard does not mean that it suddenly becomes unavailable or unsafe. In the UK, it simply means that it is no longer supported by a BSi committee. But for bridge designers working for public sector clients the withdrawal of conflicting standards has had a profound effect.
Public sector clients are bound by the requirements of the Public Procurement Directive. Major bridge clients, such as the Highways Agency and Network Rail, have assessed the requirements of this directive and concluded that Eurocodes must now be specified for all newly-procured design work.
Bridge engineers are now entering a period of considerable change and there has been much speculation about the consequences of the transition. Some of this has been well informed, some rather less so. But as construction starts on the first wholly Eurocode-designed bridge on the UK motorway network, it is possible to start to draw some early conclusions.
“We recognised the need to put in place special procedures to manage technical risks and drive financial performance”
Redhayes bridge has been designed by Parsons Brinckerhoff on behalf of Devon County Council. The twin-arch bowstring structure provides a segregated pedestrian and cycle crossing spanning 82m across a cutting immediately north of junction 29 of the M5 motorway.
“The bridge is a key part of the county council’s plan to support employment development to the east of Exeter including a science and business park and the new Cranbrook housing development,” explains Nick Bott, the council’s chief engineer for bridges and structures.
“It provides a landmark structure in a prominent location and re-establishes a route into the city that was lost when the motorway was originally constructed.”
Funding for the bridge came from a successful Community Infrastructure Funding (CIF) bid by the county council and is complementary to the ongoing cycle infrastructure improvements in the city that reflect Exeter’s status as a cycle demonstration town.
Parsons Brinckerhoff was initially commissioned to undertake a feasibility study and conceptual design for the scheme. When the project advanced to detailed design it was recognised as a candidate for a pioneering Eurocode design.
“Devon County Council took a progressive approach and saw the benefits of undertaking the design to Eurocodes,” comments Parsons Brinckerhoff projects director Steve Rees. “For some time, we had been suggesting to our clients that they procure an early Eurocode design to build their experience and knowledge base ahead of the withdrawal of conflicting British Standards.”
Although Devon County Council is the client for the scheme, formal technical approval is being undertaken by the Highways Agency because the bridge crosses a motorway.
This technical approval process requires the submission of an Approval in Principle (AIP) document, setting out the proposed form of structure and the planned approach to its design. Agreement of the Redhayes bridge AIP was a key moment in the project and a landmark in itself as it was the first wholly Eurocode-based submission signed off by the Highways Agency.
“The Redhayes bridge project has served as a fantastic opportunity to prove the application of the complete suite of Eurocodes in a live project environment”
Over the past decade a huge amount of work has been done by bridge engineers in the UK to prepare for the implementation of Eurocodes, led by major clients and a core of expert consultants and academics. National annexes have been prepared providing values for those parameters left open for national determination.
Supporting complementary guidance has been developed and clients’ standards and specifications have been revised.
Parsons Brinckerhoff has been heavily involved in these developments, contributing to key BSi and CEN committees and working with major bridge owners and authorities.
“All of our Eurocode work, together with a considerable internal investment, has undoubtedly put PB in a privileged position,” explains Tony Harris, Parsons Brinckerhoff’s head of discipline for bridges.
“The Redhayes bridge project has served as a fantastic opportunity to prove the application of the complete suite of Eurocodes, execution standards, national annexes, supporting documents and client requirements in a live project environment.”
“Despite the inevitable challenge of working to new standards, the design has gone really well,” says Vyvian Pike, who led Parsons Brinckerhoff’s design team.
“Importantly, at an early stage, we recognised the need to put in place some special procedures to manage technical risks and drive programme and financial performance.”
These procedures included establishing an internal peer review team of Eurocode experts to support the project design team, undertaking regular technical progress reviews and agreeing detailed design method statements for all key stages of the work.
“Bringing together a team that coupled strong practical design experience with leading Eurocode expertise has been a key factor in the success of the work to date,” says Rees. “It enabled us to deliver the design and specification to budget and on programme.”
“The project has demanded some quite sophisticated analysis,” explains Tony Harris, “and the way the Eurocodes give the designer flexibility and build upon first principles was certainly a benefit.”
Because of its complex geometry, both linear and nonlinear analyses of the structure were undertaken.
Geometric non-linearity was modelled to investigate the sensitivity of the structure to second order large-displacement effects and buckling behaviour was carefully investigated.
The modal dynamics were investigated to determine vertical, transverse and twist modes and frequencies. The primary dynamic modes were then used for more detailed pedestrian moving-load analysis in accordance with the requirements of the UK National Annex to BS EN 1991-2.
Because of the significance of the bridge, an independent check of the design was required and Atkins was appointed to undertake this work.
“The check went very smoothly,” says Atkins regional head of bridge engineering David Smith, who led the checking team. “This was certainly helped by a good mutual understanding between Parsons Brinckerhoff and Atkins of what is required for this kind of project.”
He also saw benefits in the use of Eurocodes.
“Achieving early agreement was made easier because the Eurocodes address aspects of design that were not well covered in previous standards”
“Achieving early agreement on the design approach was made easier because the Eurocodes address aspects of design of this form of structure that were not well covered in previous standards,” he says.
In May, the contract for construction of the bridge was awarded to Carillion. Work is now underway with the erection of the structure planned towards the end of the year and completion due by the end of March 2011.
So what early conclusions can be drawn from this project? It has certainly shown that it is possible to deliver designs to budget and programme, applying the complete suite of Eurocodes, execution standards and new Highways Agency requirements and procedures.
Moreover, the designers and checkers have seen clear benefits from their use.
So should bridge engineers fear the transition? With the right management and effort, there seems to be no need. Just one year after the withdrawal of conflicting British Standards a new landmark structure designed entirely to Eurocodes will be in place.
And most people using the bridge every day won’t know the difference – nor should they. They will simply enjoy the benefits of this re-established travel route for their community.
- Steve Denton is PB’s engineering director, a visiting Professor at the University of Bath and convenor of CEN/TC 250 Horizontal Group - Bridges, the international committee with responsibilities for the bridge Eurocodes.
KEY EUROCODE IMPACTS ON DESIGN APPROACH
● Application of complete new suite of Eurocodes, national annexes and supporting complementary information
● Use of new client requirements and procedures
● New methods to account for pedestrian induced dynamics
● Second order large-displacement effects examined
● Elastic critical buckling analysis applied to complex form of structure
● New loading requirements applied, and new approach used to combine actions
● Specification aligned with European execution standards