Steve Armstrong of Mabey Bridge looks at the increasing use of Eurocodes in railway bridge design to explain the differences in approach from BS5400 and highlights the implications of the changes.
In recent years, engineers have had to adapt to the increasing use of Eurocodes in all forms of design. Rail bridges are not exempt from this, and in issue 82 of the rail engineer (August 2011), Mungo Stacey looked at the implications for designers.
Since then, the first steel rail bridges designed using Eurocodes have reached the fabrication and installation stage. The steel fabrication industry has been preparing for this evolution over several years in order to be ready for the changes and new approaches contained within the new standards.
Two of the first rail structures designed and installed to Eurocodes are currently in the process of fabrication. These are the Hitchin Flyover in Hertfordshire, and Loughor Viaduct in South Wales. As is reported elsewhere in this issue, the first steel spans have been completed at Hitchin, with the assembly work at Loughor due to start later this year.
Steel bridges are designed in accordance with EN1993 and EN1994 if a composite structure. In addition to this, fabricators work in accordance with EN1090-2: The Execution of Steel Structures which covers the fabrication of hot rolled, cold formed, plate, section and stainless steels up to S690 and S700 grades. Whilst EN1090-2 applies primarily to Eurocode designs, it can be used for structures designed according to other design rules.
In addition to EN1090-2, a specification document is also required by the fabricator. As yet, Network Rail has not produced a standard specification for Eurocode designs, so current designs have specifications which are produced on a scheme-by-scheme basis. Designers produce these by combining guidance from the SCI (Steel Construction Institute) Model Project Specification, PD6705 and any existing Network Rail requirements. It is expected that, as more structures are installed using the new codes, then a standard Network Rail specification will be developed.
One of the key differences between BS5400 and EN1090-2 is the concept of the Execution Class. This relates to a set of requirements specified for the execution of the works as a whole, or of an individual component or of a detail of a component. For structural steel there are four classes ranging from 1 to 4, with 4 reserved for the most critical of steel fabrications.
There are two elements in the definition of an execution class. The first is the selection of a consequence class; fundamentally what would happen should the element fail. The second is the service category or the usage of a structure. The default class for bridges has been set at EXC 3 with additional elements at EXC 4 if required.
As might be expected, there are differences between EXC 3 and EXC 4 requirements and these will result in higher fabrication costs for EXC 4 work. These differences include tighter thickness tolerances, edge quality limits on thermal cutting, higher acceptance criteria on fillet and butt welding, and higher levels of inspection post welding and on bolt tightening. Clearly, designers should ensure that they only specify EXC 4 when justified, otherwise cost will be added to projects.
Tolerances and traceability
Within EN1090-2 there are three types of geometrical tolerance defined, and these are; essential tolerances, functional tolerances and special tolerances.
Essential tolerances set basic limits in order to satisfy the design assumptions for structures in terms of mechanical resistance and stability. This would include controlling eccentricity on stiffeners. Functional tolerances are those which are required to meet a function other than mechanical resistance and stability, for example, appearance or fit up requirements. Functional tolerances are further divided into Class 1 and Class 2, with Class 2 being more onerous than Class 1. It is therefore recommended that Class 1 tolerances are specified. Special tolerances are those not covered by the tabulated types given in EN1090-2 and which need to be specified in a particular case.
Within EN1090-2 there is a requirement for traceability of all elements in a fabricated structure. Not only do these requirements cover plate or rolled sections, but fasteners, consumables and paint. These items can be controlled by either ensuring that constituents are selected which are produced from standards listed in EN1090-2, or by checking that a particular product has conformity against a relevant standard. Additionally, controls must be in place within the factory to record which components are used and where.
As might be expected, there are a large number of other areas controlled by EN1090-2, but the requirements and controls are broadly similar to requirements in previous standards. Such areas include records and record keeping, inspection and testing, mechanical fasteners, preparation and assembly, welding, surface treatment, corrosion protection and erection.
Meeting the requirements of EN1090-2 is essential to achieving CE marking for structural steelwork and this will be mandatory from 2013. However, CE marking does not only depend on compliance with EN1090-2, but also requires the fabricator to be certified for Factory Production Control in accordance with EN 1090-1. Key to this certification process is ensuring that welding is of an appropriate quality and organisations will be required to ensure a high level of technical competence by providing welding engineers and coordinators.
In the course of fabricating the first two structures to the requirements of Eurocodes, Mabey Bridge has encountered a few issues where the current requirements in the Eurocodes are unclear. Post assembly plate butt welds which are ground flush are not defined for fatigue in EN1993-1-9. This classification means that this type of weld is now more critical than a transverse fillet weld onto the same plate. The consequence of this is that butt welds cannot be positioned near the mid-point in a girder without a fatigue assessment being completed. Under BS5400 this type of weld would have been permitted. One other change that has been noted is in the limits on plate thickness for subgrades which result in NL grade, which calls up a low temperature impact test as part of the inspection regime, being required on thinner plates than previously. This can lead to increased material costs and longer lead times for the delivery of plate.
Clearly, the introduction of Eurocodes and CE marking has required a significant investment in time and development by fabricators. However, many are now ready to meet the challenges presented by the new requirements. With the first structures fully constructed to Eurocodes, there will be a period of time where Network Rail, engineers and fabricators develop and refine the technical specifications to arrive at a common set of guidelines which are well understood. These guidelines will enable Network Rail to produce a specification document which can be followed by everyone, but until this is complete there will be differences in interpretation.
It is important to understand that the Eurocodes have a different language and approach to BS5400. However, the fabrication industry is ready for the challenges and understands the requirements. Fabricators can provide a significant amount of technical support and guidance throughout the development of a project. Designers are always encouraged to engage with the industry to discuss these issues, preferably earlier rather than later in the project. Through this engagement, technical issues can be clarified early in the design process, thereby ensuring the most efficient solutions are developed.