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Sir In defence of Eurocode 7

NEWS :

The article 'No room for new methods in 'academic' Eurocode 7'(Ground Engineering news, June 2001), states that EC7 inhibits the development and application of new, well-proven and established methods.

It is not clear whether this refers to developments in construction techniques or in design methods. We can see no way that EC7 inhibits new construction techniques, so we presume the latter is intended; but we would disagree totally.

The Eurocodes are divided into two types of clauses: principles and application rules. The Principles are not generally contentious, while 'It is permissible to use alternatives to the Application Rules given in Eurocode 7-1, provided it is shown that the alternative rules accord with the relevant Principles' Even in the application rules, precise methods of calculation are not prescribed, leaving the designer considerable freedom in methods of analysis and design. The only prescription is in the factors of safety to be demonstrated, which do not preclude any form of design method.

EC7 is accused of aiming to provide compatibility with structural codes. The drafters would certainly plead guilty to this, knowing that lack of such compatibility in British and other national codes is a major difficulty for designers.

However, the code has been drafted by geotechnical engineers who have been very keen to ensure that it does not compromise sound geotechnical engineering procedures. In our view, it has achieved this.

EC7 has certainly become more complicated since its publication as an 'ENV'in 1995. This is because it now allows several different approaches to the application of factors of safety. It is intended that each country will specify the approach(es) allowed on its territory.

This is unfortunate, and does infringe the principle that the code can be used without change throughout Europe. However, this was found to be the only way to bring disparate European practice together in a single document, and, in fact, affects only a small part of the code's text and significance to the user.

The associated fact that different countries may specify different factor values within the same method is regarded as a secondary problem; the fact that groups of countries will use the same general method should be seen as a significant step forward.

Most methods of factoring can be adjusted to give reasonable, and generally similar, results in the majority of situations. However, some methods also give unreasonable results in some practical situations, and the 'old, lump factor'methods fall into this category. For this reason, a partial factor approach has been preferred throughout the Eurocodes.

It is correct to say that compatibility with previous design methods has been one of the guiding requirements in choice of factor values. No apology is made for this.

It is certainly a wish of geotechnical designers to have a better control of deformation and of serviceability design. However, there are two main difficulties: (a) serviceability limits are very difficult to specify in general, though it is sometimes possible to give project-specific requirements, and (b) the calculations associated with serviceability are generally much less robust and reliable than those associated with collapse mechanisms.

For this reason, geotechnical codes, almost universally, concentrate on collapse mechanisms, and use factors of safety to give at least a first level of control over deformations.

The TC288 documents do not, in principle, cover design. In places, they give methods of design for very specific elements, but are not relevant to complete designs involving several geotechnical elements, such as walls, foundations, ties and props, including both geotechnical and structural design.

It has proved to be difficult to get agreement on the style of such a complete method, yet its achievement is surely crucial to designers and to the future development of geotechnical engineering.

Brian Simpson, director, Ove Arup & Partners International Richard Driscoll, director, Centre for Ground Engineering & Remediation, BRE

Brian Simpson is the ICE member of the British Standards Committee dealing with EC7 and leader of the BSI delegation to the CEN committee writing the code.

Richard Driscoll is the chairman of the BSI geotechnical committee and an independent adviser on the project team producing the Eurocode.

They are the joint authors of Eurocode 7: a commentary.

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