Structural Eurocodes are gradually being phased in, with the latest publication covering wind loading. So is this good news or bad? Scott Brownrigg and Barbour Index offer a guide
British Standards for structural design are progressively being replaced by Eurocodes, which are intended to harmonise structural design across Europe.
The latest Eurocode to be published covers wind loading and will affect designers of all disciplines. Here, we take a look the implications for specifiers.
The new structural standards
The standards covering all major building elements will be replaced by several groups of codes and some of these new documents have already been published. The structural Eurocodes are scheduled to be fully published by 2007 and all existing standards will be withdrawn after a three-year transition period. BSI has produced a leaflet on the new documents. There are 10 codes in the structural section covering structural and civil engineering aspects numbered BS EN 1990 Eurocode 0 to 1999 Eurocode 9. Some of these are also subdivided, making a total of 32 documents that have been announced. With a cover price of more than £100 each, this is an expensive set of documents - although only the bigger consultants will require all of these and accompanying software.
Of more concern is the lack of guidance or any commercial imperative to use the new codes, except for on public buildings where they must be applied. Adoption of the new standards is therefore likely to be slow. As was the case when the current standards came in, the industry will have to get accustomed to the use of both old and new for some time to come.
What is the difference between the old British Standards and the Eurocodes?
The good news is that the Eurocodes are part of a new range of standards seen as being co-ordinated and organised around all the key subjects from the outset.
They have a common layout and language, encouraging a unified European approach. They will also be used by research and development bodies and assist with stronger European technical consolidation with new products based around them.
Additionally, as the new standards are less prescriptive, they are intended to allow the designer more choice and encourage innovation. Certainly initially, designs will cover a range of solutions making comparisons difficult and commercial objectivity unclear.
The bad news is that yet another change in this area is likely to be adopted very slowly by industry, resulting in a long transition period where dual standards are in use, which may cause conflicting designs and confusion.
The latest Eurocode on wind loading
The publication of BS EN 1991-1-4:2005 Actions on Structures continues the theme of a single standard being replaced by several arguably inferior standards. As well as wind loading, Eurocodes 1990 1-4 cover issues including fire, snow and thermal design and will be fully published later this year. This will include the national annex that will hopefully enable these to be utilised as the replacements for existing standards such as BS 6399.
Specifiers may also need to take account of the general standard entitled BS EN 1991-1-2002 General Actions.
Problems with transition from old to new
It was not long ago that British Standard Code of Practice CP3 was widely quoted as the definitive document for assessing wind loading on structures. For the specifier, this was a relatively straightforward element of the specification concerning the building fabric. This was replaced by BS 6399 Part 2, which was widely ignored for a time as it was too complex, overly detailed and unnecessary, primarily because it required more work to interpret. Further confusion was caused because Part A of the Building Regulations continued to quote CP3.
In recent years, BS 6399 has been adopted both by industry and in the regulations. Now it is set to disappear with the publication of Eurocode One, BS EN 1991-1-4:2005.
The new standard will allow a range of possible solutions, which could be potentially confusing. The specifier must understand the options and make a clear recommendation to the client complete with the implications of accepting any particular design. This will introduce another challenge for the specifier as it will potentially affect areas including cladding, roofing and external fittings. It may be prudent for the specifier to stick with the old standard for as long as possible.
Common sense dictates that clear guidance and a unified approach is necessary for specifiers. Yet the Eurocodes follow the same trend as the Building Regulations in that there is less definitive information in the new standard, creating the need for third-party reference material. This is not a helpful trend for specifiers and designers as this approach is more time-consuming and requires greater understanding of a range of information. It will take a long time for a common approach to be hammered out and clear direction established using the new standards. Furthermore, updated computer software will also be needed, making the whole process of adopting the new standard even more onerous.
How to avoid the pitfallsn Decide if you are going to adopt the new standard
- Ensure you have all the necessary accompanying documents
- Whatever you choose, make sure the specification is unambiguous
- Quote and accept only either the old or the new standard
- Remember that the new standard must be used on all public projects
- At least for the moment, do not accept interpretations between the old and new.