Eurocodes General Structure

Eurocodes form a set of documents that will enable building and civil engineering structures to be designed to common standards across the European Union using different structural materials. The documents are structured on a hierarchical basis, led by EN 1990, Eurocode - Basis of structural design, defining the basis of structural design, followed by EN 1991, Eurocode 1: Actions on structures, which comprises ten parts, defining the actions that have to be withstood. These documents are supported by a number of Eurocodes detailing the particular methods of design to be followed for the structural materials being used, i.e. structural timber, steel, concrete, etc.

The Eurocode for the design of timber structures is EN 1995, Eurocode 5: Design of timber structures. It comprises three parts:

• EN 1995-1-1 Design of timber structures - Part 1-1: General - Common rules and rules for buildings

• EN 1995-1-2 Design of timber structures - Part 1-2: General - Structural fire design

• EN 1995-2 Design of timber structures - Part 2: Bridges.

EN 1995 covers the requirements for strength, serviceability, durability and fire resistance, with matters such as thermal or sound insulation etc., having to be obtained from other standards.

The content of this book relates to the design of timber and wood-related products for buildings in accordance with the requirements of EN 1995-1-1. As stated in Chapter 1, the design of timber structures for the accidental situation of fire exposure should be carried out in accordance with the requirements of EN 1995-1-2, and this design condition has not been addressed in the book.

In the United Kingdom, the British Standard currently used for the structural design of timber is BS 5268-2 [1], and is based on a permissible stress design philosophy. With this approach the behaviour of the structure and its elements are assessed at the working/service load condition. In EN 1995-1-1 a limit states design philosophy in which the requirements concerning structural reliability are related to limit states, i.e. states beyond which the structure or its elements will no longer satisfy performance criteria, is used. The latter approach provides a more realistic representation of the overall behaviour of the structure and is the philosophy that has been adopted for the Eurocode design suite.

In every Eurocode each item is defined as being either a Principle or an Application rule. A Principle is a statement or requirement that must be fully complied with unless an alternative is given in the document and an Application rule is a rule that will satisfy the Principle. Alternative design rules can be used by the designer provided it can be demonstrated that these will fully comply with the Principles and will produce an alternative design equivalent in regard to serviceability, structural integrity and durability. An important point to note, however, is that in such a situation the design cannot be claimed to be fully compliant with the EC and this may prove to be a problem if an EC marking is required for the design or substantiation of a product. Where an item in a Eurocode is prefixed by a number in brackets followed by the letter P it is a Principle and where it is only prefixed by a number in brackets it is an Application rule.

Where it is considered that a national choice is appropriate for certain design rules or values of functions in a Eurocode, these items can be varied and are defined as Nationally Determined Parameters (NDPs). This information is given in a National Annex, which may also incorporate what is termed 'non-contradictory complimentary information' (NCCI), giving additional guidance on the interpretation or implementation of the design rules in the Eurocode. If not included in the National Annex, the NCCI should be published in a separate document.

For application in the United Kingdom, the Eurocodes are published by the British Standards Institution (BSI) incorporating the prefix BS before the Eurocode reference and when implemented nationally, the full text of each Eurocode will be preceded by the associated United Kingdom National Annex (UKNA). When designing to the Eurocode rules the NDP given in the UKNA must be used rather than the equivalent requirement in the Eurocode and because of the significance of the NDP in timber design, the authors consider it important that attention is drawn to these requirements when discussing the design rules in BS EN 1995-1-1:2004 [2]. This has been included for in the book.

As each Eurocode incorporating its associated UKNA has still to be published in its final version by the BSI, the UKNA associated with BS EN 1990:2002 [3] and BS EN 1995-1-1:2004 that has been referenced within the text and the examples given in the book are as follows:

• UK National Annex for Eurocode 0 - Basis of structural design [4]

• UK National Annex to Eurocode 5: Design of timber structures - Part 1-1: General - Common rules and rules for buildings [5].

The BSI publications of the following Eurocodes are regularly referenced in the book and the abbreviation used in the text for the relevant document is as follows:

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