All of the 6.6

member strength properties that benefit from the load sharing system

For timber and wood-related products the design values may be taken to be the nominal values from product standards or drawings.

Where relevant, the design equations in EC5 include for the effect of the occurrence of the maximum permitted deviations specified in EC5, Section 10.

2.2.23 Design resistance (EC0, 6.3.5)

When dealing with timber and wood-related product structures, in line with the requirements of EC0, the design value of a resistance is expressed in EC5, 2.4.3 as

Km where kmod is a modification factor that takes into account the effect of load duration and moisture content (see 2.2.20), km is the partial factor for a material property at the ULS (see 2.2.20), and Rk is the characteristic value of the load-carrying capacity at the ULS.

In general, however, the resistance properties are defined in EC5 as F functions and the more representative expression for the design resistance for a timber or wood-related product is

Where they are relevant, the following ULS must be verified:

(a) Equilibrium (EQU). To confirm that the structure or any part of it is not unstable.

(b) Strength (STR). To confirm that the structure and its elements will not fail under stress, by element instability or at connections. Where displacements will affect the behaviour of the structure, their effect must be taken into account.

(c) Geotechnical (GEO). To confirm that the foundations of the facility provide the strength and stiffness required by the structure.

(d) Fatigue (FAT). To confirm that the elements of the structure will not fail under fatigue.

For timber or wood product structures, ULS (a), (b) and (c) will generally be relevant and any condition where fatigue could apply is taken into account in EC5 by the Strength (STR) requirements.

Load combinations are applied at each relevant ULS and by the application of the partial factor method (see 2.2.17) it must be verified that the design value of the effect of the design actions is less than or equal to the design value of the equivalent resistance. For example, considering the strength verification of the structure and its elements at the STR ULS, the requirement will be

and for a material property,

where Efd is the design value of the effect of actions (e.g. internal moment, internal stress, etc.) (see 2.2.20), FRd is the design value of the corresponding resistance (see 2.2.23), Xd is the design value of the timber or wood product material property (see 2.2.20), and nk is the product of those modification factors that will affect the design value. (The principal modification factors in EC5 are summarised in Table 2.7 and discussed in the appropriate chapters in the book.)

For each relevant limit state the design value of the effect of actions must be derived. To achieve this, those actions that are considered to be able to occur simultaneously are combined, and, where more than one variable action exists, each combination will include one of the variable actions in turn as the leading variable action.

To derive the combination of actions for persistent or transient design situations (referred to in EC0 as thefundamental combinations) and ignoring pre-stressing actions as they are not generally relevant to timber design, the combination to be satisfied is given in equation (6.10) in EC0 as follows:

^2 Yg, jGk, j + Yq,i Qk,i + YQi ft, Qk,i (EC0, equation (6.10)) (2.13)

The less favourable of the following combination expressions may be considered as an alternative to equation (2.13) for STR (and GEO) limit states,

^2 Yg, jG k, j + Yq, if 0,1 Q k, 1 + YQ,i f°'i Qki (EC0, equation (6.10a)) (2.14)

j2fjYg,jGk,j + Yq,i Qk,i + Y^ YQif0,iQk,i (EC0, equation (6.10b)) (2.15)

where y G is the partial factor for permanent loading, yq is the partial factor for variable loading, f 0 is the factor that converts a variable action into its combination value, f is a reduction factor for unfavourable permanent actions, Gk is the permanent action, Qk,1 is the leading variable action, and Qk is an accompanying variable action.

With accidental design situations, one combination of actions applies to all limit states and is given in equation (6.11b) in EC0,

J2Gk,j + ^d + (fi,i orf2,i)Qk,i +J2 fXiQki (EC0, equation (6.11b)) (2.16)

where Ad is the design value for a specific accidental event (e.g. the action due to an impact or the indirect thermal action due to a fire) or relates to the situation after an accidental event, in which case Ad = 0; f 1 and f 2 convert the variable action into the frequent and quasi-permanent value, respectively, and are referred to in 2.2.14.

Numerical values of the y and f factors to be used to derive the design values of actions for the EQU and STR (not involving geotechnical actions) states when subjected to persistent and transient design situations are given in Table 2.8, and the values applicable to all ULS when subjected to accidental design situations are given

Ultimate limit state (under persistent and transient design situations - fundamental combinations) |
Relevant equation in ECO |
Permanent actions |
Leading variable action |
Accompanying variable actions | |||

Unfavourable' |
Favourable' |
Main |
Others | ||||

Was this article helpful?

## Post a comment