2.1 Fundamental requirements

P(1) A structure shall be designed and constructed in such a way that

— with acceptable probability, it will remain fit for the use for which it is required, having due regard to its intended life and its cost, and

— with appropriate degrees of reliability, it will sustain all actions and influences likely to occur during execution and use and have adequate durability in relation to maintenance costs.

P(2) A structure shall also be designed in such a way that it will not be damaged by events like explosions, impact or consequences of human errors, to an extent disproportionate to the original cause.

(3) The potential damage should be limited or avoided by appropriate choice of one or more of the following:

— avoiding, eliminating or reducing the hazards which the structure is to sustain

— selecting a structural form which has low sensitivity to the hazards considered

— selecting a structural form and design that can survive adequately the accidental removal of an individual element

— tying the structure together.

P(4) The above requirements shall be met by the choice of suitable materials, by appropriate design and detailing and by specifying control procedures for production, design, construction and use as relevant to the particular project.

2.2 Definitions and classifications

2.2.1 Limit states and design situations Limit states

P(1) Limit states. Limit states are states beyond which the structure no longer satisfies the design performance requirements. Limit states are classified into:

— ultimate limit states

— serviceability limit states.

P(2) Ultimate limit states are those associated with collapse, or with other forms of structural failure which may endanger the safety of people.

P(3) States prior to structural collapse which, for simplicity, are considered in place of the collapse itself are also treated as ultimate limit states.

(4) Ultimate limit states which may require consideration include:

— loss of equilibrium of the structure or any part of it, considered as a rigid body.

— failure by excessive deformation, rupture, or loss of stability of the structure or any part of it, including supports and foundations.

P(5) Serviceability limit states correspond to states beyond which specified service requirements are no longer met.

(6) Serviceability limit states which may require consideration include:

— deformations or deflections which affect the appearance or effective use of the structure (including the malfunction of machines or services) or cause damage to finishes or non-structural elements

— vibration which causes discomfort to people, damage to the building or its contents, or which limits its functional effectiveness

— cracking of the concrete which is likely to affect appearance, durability or water tightness adversely

— damaging of concrete in the presence of excessive compression which is likely to lead to loss of durability. Design situations

P(1) Design situations are classified as:

— persistent situations corresponding to normal conditions of use of the structure

— transient situations, for example during construction or repair

— accidental situations 2.2.2 Actions Definitions and principal classifications5-1

— a force (load) applied to the structure (direct action), or

— an imposed deformation (indirect action); for example, temperature effects or settlement. P(2) Actions are classified:

i) by their variation in time

— permanent actions (G), e.g. self-weight of structures, fittings, ancilliaries and fixed equipment

— variable actions (Q), e.g. imposed loads, wind loads or snow loads

— accidental actions (A), e.g. explosions or impact from vehicles ii) by their spatial variation

— fixed actions, e.g. self-weight [but see for structures very sensitive to variations in self-weight].

— free actions, which result in different arrangements of actions, e.g. movable imposed loads, wind loads, snow loads.

(3) Prestressing (P) is a permanent action but, for practical reasons, it is treated separately (see 2.5.4).

(4) Indirect actions are either permanent Gind (e.g. settlement of support) or variable Qind (e.g. temperature) and are treated accordingly.

P(5) Supplementary classifications relating to the response of the structure are given in the relevant clauses. Characteristic values of actions

P(1) Characteristic values Fk are specified

— in Eurocode 1 or other relevant loading codes, or

— by the client, or the designer in consultation with the client, provided that minimum provisions, specified in the relevant codes or by the competent authority, are observed.

P(2) For permanent actions where the coefficient of variation is large or where the actions are likely to vary during the life of the structure (e.g. for some superimposed permanent loads), two characteristic values are distinguished, an upper (Gk,sup) and a lower (Gk,inf). Elsewhere a single characteristic value (Gk) is sufficient.

(3) The self-weight of the structure may, in most cases, be calculated on the basis of the nominal dimensions and mean unit masses.

P(4) For variable actions the characteristic value (Qk) corresponds to either:

— the upper value with an intended probability of not being exceeded, or the lower value with an intended probability of not being reached, during some reference period, having regard to the intended life of the structure or the assumed duration of the design situation, or

— the specified value.

P(5) For accidental actions the characteristic value Ak (when relevant) generally corresponds to a specified value.

5) Fuller definitions of the classifications of actions will be found in the Eurocode 1, Bases of Design and Actions on Structures. Representative values of variable actions6-1

P(1) The main representative value is the characteristic value Qk.

P(2) Other representative values are expressed in terms of the characteristic value Qk by means of a factor ?i. These values are defined as

P(3) Supplementary representative values are used for fatigue verification and dynamic analysis. P(4) The factors ? are specified

— in Eurocode 1 or other relevant loading codes, or

— by the client or the designer in conjunction with the client, provided that minimum provisions, specified in the relevant codes or by the competent public authority, are observed. Design values of actions

P(1) The design value Fd of an action is expressed in general terms as

Fd = Yf Fk P(2) Specific examples are:

Ad = Ya Ak (if Ad is not directly specified) Pd = Yp Pk where

Yf, Yg Yq, Ya and Yp are the partial safety factors for the action considered taking account of, for example, the possibility of unfavourable deviations of the actions, the possibility of inaccurate modelling of the actions, uncertainties in the assessment of effects of actions, and uncertainties in the assessment of the limit state considered.

P(3) The upper and lower design values of permanent actions are expressed as follows [see (P2)]:

— where only a single characteristic value Gk is used, then: VJd,sup Yg ,sup Gk

— where upper and lower characteristic values of permanent actions are used, then:

d,sup = % ,sup Gk ,sup Gd,inf = 1G,inf Gk,inf where Gk,sUp and Gk,inf and YG,sup and YG,inf are the upper and lower characteristic values of permanent actions are the upper and lower values of the partial safety factor for the permanent actions

6) Fuller definitions of the classifications of actions will be found in the Eurocode 1, Bases of Design and Actions on Structures. Design values of the effects of actions

P(1) The effects of actions (E) are responses (for example internal forces and moments, stresses, strains) of the structure to the actions. Design values of the effects of actions (Ed) are determined from the design values of the actions, geometrical data and material properties when relevant:

where ad is defined in 2.2.4

(2) In some cases, in particular for non linear analysis, the effect of the randomness of the intensity of the actions and the uncertainty associated with the analytical procedures, e.g. the models used in the calculations, should be considered separately. This may be achieved by the application of a coefficient of model uncertainty, either applied to the actions or to the internal forces and moments.

(3) One possible procedure, called "linearization procedure", may be schematically represented by the following equation:

and involves making a non-linear analysis until the level Yg Gk, Yq Qk ... and then increasing E by the application of the factor YSd.

2.2.3 Material properties Characteristic values

P(1) A material property is represented by a characteristic value Xk which in general corresponds to a fractile in the assumed statistical distribution of the particular property of the material, specified by relevant standards and tested under specified conditions.

P(2) In certain cases a nominal value is used as the characteristic value.

(3) A material strength may have two characteristic values, an upper and a lower. In most cases only the lower value will need to be considered. In some cases, different values may be adopted depending on the type of problem considered. Where an upper estimate of strength is required (e.g. for the tensile strength of concrete for the calculation of the effects of indirect actions) a nominal high value of the strength may have to be established.

(4) The approach in P(1) above does not apply to fatigue.

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