Introducing reliability into the design

Reliability is introduced into design against loss of serviceability by selecting suitable limiting values of displacement, as illustrated in Figure 8.3.

Partial factors for serviceability limit states are normally taken as 1.0.

Hence the equation for verification of serviceability (see Section 8.1) becomes: Ed = E {F ; X,; a } < Cd d [ rep > , > nom J — d and no partial factors are introduced in the flow diagram of Figure 8.3.

The combination factors ^ shown on Figure 8.3 are applied to accompanying variable actions only and are those specified for the characteristic, frequent, or quasi-permanent combinations (see Chapter 2), i.e. ^ =

In ultimate limit state (ULS) verifications, combinations of actions for permanent and transient design situations employ combination factors ^ = Since is numerically greater than for most actions, representative actions are usually larger for ultimate than for serviceability limit states.

It is important to recognize that actions and material properties may vary during the structure's design life and hence serviceability limit states may need to be checked at various times. Of critical importance in verifying serviceability is appropriate selection of the limiting effects of actions. These must represent a realistic assessment of what is necessary for the long term performance of the structure, rather than overly conservative limits which simplify structural analysis.

Representative actions F„p

Geometrical parameters strength

Design actions

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