Terms and definitions EC

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 requirements 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:

• During its intended life it must sustain all actions likely to occur and remain fit for use.

• It must have adequate structural resistance, serviceability and durability.

• The fire-resistance requirements must be fully met.

• 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 requirements 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 index j, 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 recommended 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 10-4 and 10-5 at the ULS and 10-1 and 10-2 at 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

Table 2.1 Recommended minimum values for the reliability index p*
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