Design Value Of The Effect Of Actions

To verify the ultimate and serviceability limit states, each design effect has to be checked and for each effect the largest value caused by the relevant combination of actions must be used. In the case of the ultimate limit states, the largest values will be derived from the application of the fundamental combination of actions referred to in 2.2.24 and given in Table 2.8. Where accidental situations have to be designed for, the combination of actions given in Table 2.9 must also be used. For the serviceability limit states, the combination of actions discussed in 2.2.25 will apply.

For the strength-related states, the design effect will also be a function of the strength modification factor, kmod, referred to in 2.2.20. Where a load combination comprises actions having different load duration classes, as explained in 2.2.24, the modification factor corresponding to the action with the shortest duration used in the combined load case is applied to the strength property being considered, and where there is a linear relationship between action and effect, the design condition will be that giving the largest value after division by the associated kmod factor.

With the equilibrium related states the design effects will apply solely to matters associated with static instability, and kmod will not be relevant.

For the SLS, the kmod modification factor is again not applicable as the design effects being considered are displacement and vibration under normal usage. The loading conditions to be used for these states are defined in 2.2.25.

An indication of the work involved in determining the critical load cases that will result in the greatest design effects at the ULS and the SLS is given in Example 4.8.1. The example covers the basic case of a simply supported beam subjected to permanent and variable actions, anchored at its ends to supporting structure, with all possible load cases being considered. Although the determination of the critical load cases is not difficult, it is time consuming, particularly when dealing with redundant structures. To be able to validate that the critical design effect of actions is being used, the design effects arising from all possible load combinations should be covered. For the examples given in the book, however, to ensure that attention is primarily focussed on the EC5 design rules for the timber or wood-based product being used, only the design load case producing the largest design effect has generally been given or evaluated in the calculations.

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