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1-5^0,1 2k,1

* Based on Tables NA.A1.2(A) and NA.A1.2(B) of the UKNA to ECO.

t The values apply when static equilibrium of the structure or element does not also involve the resistance of structural members. If the static equilibrium involves structural members, to assess the integrity of the structural element(s) the design values for STR (c), (i) must be considered in addition to EQU (a).

i Where static equilibrium involves the resistance of structural members, as an alternative to the option referred to in the note above (t), a combined verification based on the maximum design value derived from combinations EQU (b), (i) and EQU (b), (ii) should be adopted.

§ At the STR limit state, the designer is given a choice of using the design values given in STR (c), (i) or the less favourable of STR (d), (ii) and STR (d), (iii). The examples given in the book are based on STR (c), (i).

" For timber structures, where the variability of Gk is small, GkjSUp and G^j^ shall be replaced by a single value, G^j, based on the mean value of density.

Table 2.9 Design values of actions for accidental combinations*

Ultimate limit states (under an accidental combination of actions)

Relevant equation in EC0

Permanent actions

Leading variable action

Accompanying variable actions

Unfavourablef Favourablef

Main Others

Any ultimate limit state

(6.11a/b)

1.0Gk, j, sup 1.0Gk,j,inf

Ad

1.0f 1,1 Qk,1 1.0f 2,iQk,i

* Based on Table NA.A1.3 of the UKNA to EC0.

t For timber structures, where the variability of Gk is small, Gk,j,sup and Gk, j,inf shall be replaced by a single value, Gk, j, based on the mean value of density.

* Based on Table NA.A1.3 of the UKNA to EC0.

t For timber structures, where the variability of Gk is small, Gk,j,sup and Gk, j,inf shall be replaced by a single value, Gk, j, based on the mean value of density.

in Table 2.9. Based on the content of Table NA.A1.1 in the UKNA to EC0, values for the f factors are summarised in Table 2.2.

For the STR limit state, when subjected to combinations of actions under persistent and transient design situations, as stated previously and shown in Table 2.8, alternative combinations of actions can be used. From calibration work undertaken by Gulvanes-sian and Holicky [19], the use of equation (6.10) in EC5 will result in the highest reliability index, and closely approximates that achieved by design in accordance with current BS requirements, generally well exceeding the minimum reliability index given in Table 2.1. The alternative use of the less favourable of equations (6.10a) and (6.10b) will, on the other hand, achieve a more uniform reliability that is better aligned with the minimum level set in the code, and is likely to be more economical, but will be below the reliability index achieved by design in accordance with BSI codes. Unless otherwise stated, equation (6.10) has been used in the examples given in the book to determine the design values of actions at the ULS.

To determine the load case producing the greatest design effect (i.e. the maximum bending moment, shear force, etc.), the load combination equation(s) must be applied in turn with each variable action acting as the leading variable. Also, where the variable loads are not related, all possible combinations must be considered. For example, consider the STR limit state for a simply supported beam loaded by its own weight, Gkj1, a permanent load, Gk,2, a medium-term duration variable load, gk>1, and an unrelated short-term variable load, Qk,2. Adopting equation (6.10), the alternative loading conditions that have to be considered to determine an effect, Ef, e.g. a bending moment, are

1.35(Gk1 + Gk2) + 1.5Qk1 + 1.5f0,2 Qk2 ^ Ef4 (2.20)

1.35(Gk1 + Gk2) + 1.5Qk2 + 1.5f0,1 Qk1 ^ Ef5 (2.21)

Where a load combination consists of actions belonging to different load duration classes, the effect of load duration on the property of the timber or wood product has to be taken into account by the use of the kmod modification factor discussed in 2.2.20.

The effects of combinations of permanent and variable actions have a less degrading effect on strength properties than permanent action alone, and where a combination of permanent and several variable actions is applied, the design condition will be dictated by the variable action having the shortest duration. On this basis, and as required by EC5, 3.1.3(2), the modification factor corresponding to the action having the shortest duration used in the combined load case is applied to the strength property being considered. Where there is a linear relationship between actions and effects, the design condition will be that having the largest value after division by the associated kmod factor. For the example given in equations (2.17)-(2.21), taking kmod,perm, kmod,med and kmod,short as the modification factor for the permanent, medium-term and short-term actions, respectively, and with a linear relationship between action and corresponding stress, the design value Efd of effect Ef will be the largest value given in equation (2.22):

( Efl/ kmod perm » Efl/ kmod ,med Ef?)/ kmod , short Efn/ kmod ,short \Ef5/kmod,short /

It should be noted that when a favourable value of the variable action is to be applied (i.e. yq = 0), this means that the variable action is not being applied in that particular load case and the kmod to be used will be the one associated with the shortest duration of the variable actions that are being applied.

2.2.25 Serviceability limit states: General (EC0, 6.5)

For timber structures the following SLS shall be verified:

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