2.1 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 dif- ferent 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 sup- ported 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:
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EN 1995-1-1 Design of timber structures – Part 1-1: General – Common rulesand rules for buildings
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EN 1995-1-2 Design of timber structures – Part 1-2: General – Structural firedesign
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EN 1995-2 Design of timber structures – Part 2: Bridges.EN 1995 covers the requirements for strength, serviceability, durability and fire resis- tance, 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 implemen- tation 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:
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UK National Annex for Eurocode 0 – Basis of structural design [4]r
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:
Abbreviation
British Standard title used in text
BS EN 1990:2002 ‘Eurocode – Basis of structural design’ [3] EC0
BS EN 1991-1-1:2002 ‘Eurocode 1 – Part 1-1: General Actions – Densities, self-weight and imposed loads for Buildings’ [6]
EC1
BS EN 1995-1-1:2004 ‘Design of timber structures – Part 1-1: General – Common rules and rules for Buildings’ [2]
52 Structural Timber Design to Eurocode 5
Because of the importance of the design framework set by EC0, those matters that have a significant relevance to designs carried out in accordance with the requirements of EC5 are briefly reviewed in Section 2.2.
2.2 EUROCODE 0: BASIS OF STRUCTURAL DESIGN (EC0)
EC0 provides the framework within which design must be carried out and, as stated in Section 2.1, uses the limit states design method.
In this section, the methodology to be used and its application to the design of structures made from timber and wood-related products are discussed, drawing on the content of EC5 to show how the requirements have been addressed and interpreted.
2.2.1 Terms and definitions (EC0, 1.5)
Some of the terms and definitions used in EC0 are slightly different to those normally used in UK timber design practice and the following, including some terms for those not familiar with limit states design, are to be noted:
Action. This is the term used for a load or force applied to the structure (i.e. a direct action). This term is also used for imposed displacements, e.g. settlement (i.e. an indirect action).
Effect of action. This is the term used for the internal stress resultants or displacements in the structure arising from the effect of the action.
Permanent action. This is the term used for an action that will always act in the same direction (i.e. is monotonic) over a given reference period with negligible variation in magnitude, e.g. self-weight.
Variable action. This is the term used for an action that is not monotonic and can vary with time, e.g. live loading.
Limit states. States beyond which the structure will not comply with the design requirements that have been set.
Ultimate limit states (ULS). Limit states associated with collapse or equivalent forms of failure.
Serviceability limit states (SLS). Limit states beyond which defined service criteria will not be met.
Irreversible SLS. SLS where some effects of actions having exceeded the SLS criteria will remain after the SLS actions have been removed.
Reversible SLS. SLS where no effect of actions exceeding the SLS criteria will remain after the SLS have been removed.
Serviceability criterion. A design requirement for a SLS.
Resistance. This is the capacity of a structural element to withstand actions without failing, e.g. shear resistance, bearing resistance.
Strength. This is the withstand capacity of the material at a failure condition, e.g. shear strength, bearing strength.
Reliability. This is the ability of a structure or structural element to fulfil its design re- quirements over the design working life and is normally expressed in probabilistic terms.
2.2.2 Basic requirements (EC0, 2.1)
The fundamental Principles that must be satisfied by any structure are given in EC0, 2.1 and summarised as follows:
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During its intended life it must sustain all actions likely to occur and remain fitfor use.
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It must have adequate structural resistance, serviceability and durability.r
The fire-resistance requirements must be fully met.r
It must not be susceptible to damage disproportionate to the original cause.The adequacy of the design for structural resistance, serviceability and durability will be satisfied by compliance with EC5; fire resistance will be met by designing in accordance with the requirements of BS EN 1995-1-2:2004 [7] and robustness will be achieved by:
(1) minimising hazards to which the structure can be exposed,
(2) choosing a structural form that will be least affected by the types of hazard to be designed for,
(3) selecting a structure that can survive localised damage including the removal of an individual member or a limited part of the structure,
(4) avoiding as far as possible structural systems that can collapse without warning, (5) tying the structural members together.
General guidance on the approaches that can be used to satisfy robustness requirements is given in the Designers’ Guide to EN 1990 [8].
2.2.3 Reliability management (EC0, 2.2)
The design of the structure must satisfy reliability criteria and the conceptual require-
ments to be met are given in EC0, 2.2. Consequences classes, categorised as high
(CC1), medium (CC2) and low (CC3), dependent on the consequences of the loss of human life as well as economic, social or environmental consequences in the event of failure, have been set. The consequence category for most facilities in which timber or timber-related materials are used for structure or structural elements will be CC2. Each consequence class is linked to a reliability class (RC), with classes CC1, CC2 and CC3 being linked to reliability classes RC1, RC2 and RC3 respectively. A reliability class
has an associated reliability indexβ, which can be considered as the safety index to be
achieved for that class.
For those facilities that come within the CC2 consequence category, the recom- mended minimum values of the reliability indices are given in Table 2.1 and, as stated in EC0, structures designed in accordance with the requirements of EC0, BS EN 1991, Eurocode 1: Actions on structures, and EC5 will generally result in a structure having a reliability index greater than 3.8 for a 50-year reference period. For a 50-year reference period, the recommended reliability indices will result in a probability of failure of the
structure between 10−4and 10−5at the ULS and 10−1and 10−2at the SLS.
It should also be noted that achievement of the above reliability levels will depend on the checking standard used for drawings, calculations and specifications and for
54 Structural Timber Design to Eurocode 5
Table 2.1 Recommended minimum values for the reliability indexβ∗
Minimum values forβ
Reliability class (RC) associated with CC2: RC2 ULS SLS (irreversible)
1-year reference period 4.7 2.9
50-year reference period 3.8 1.5
∗Based onTables B2 and C2 in EC0.
compliance with the RC2 class, the minimum standard will require checking by differ- ent persons to those originally responsible for the design and in accordance with the
quality management system of the organisation (EC0,Annex B, Table B4).
2.2.4 Design working life (EC0, 2.3)
At the start of the design process the ‘design working life’ of the facility to be designed must be specified and is defined in EC0 as the
‘assumed period for which a structure or part of it is to be used for its in- tended purpose with anticipated maintenance but without major repair being necessary’.
In EC0,Table 2.1, five categories of design working life are specified and the one
most typically associated with facilities supported by timber structures designed in accordance with the requirements of EC5 will be Category 4. The indicative design working life of this category is 50 years. Where necessary or beneficial, it is acceptable to have a Category 4 design working life for the structure and incorporate structural elements with a shorter design working life provided these can readily be replaced without any adverse effect on the facility.
It is to be noted that the design working life may or may not coincide with the reference period used to determine the design values for environmental factors, i.e. wind speed and temperature extremes etc. However, it provides a guide for selection.
Provided the client implements a sound maintenance inspection policy and properly undertakes the maintenance requirements of the building and structure, the facility will remain fit for use for the design working life. When dealing with timber structures, it is of particular importance that the maintenance policy will also ensure that the environment within which the structure functions complies with the service class (see Section 2.2.20) for which it has been designed.
2.2.5 Durability (EC0, 2.4)
Durability is the ability of the structure and its elements to remain fit for use when properly maintained during the design working life and must include for the effects of deteriorating factors that can arise during this period.
The particular factors highlighted in EC0,2.4(2) that are relevant to timber structures