* Based on data given in Table 3.2, EC5.

* Based on data given in Table 3.2, EC5.

Where an action is not permanent, to obtain the creep effect it must be converted to an equivalent permanent action and this is done by using the quasi-permanent value of the action, referred to in

EC5 uses the characteristic combination of actions to derive the instantaneous deformation and the quasi-permanent load combination to derive the creep deformation. In 3.4(2)P of EC0, which is a Principle, a distinction is made between reversible and irreversible SLS and where it is acceptable to the client to adopt a reversible SLS condition, to obtain an economical design the frequent combination given in equation (2.25) should be used rather than the characteristic combination to derive the displacement.

Assuming irreversible SLS conditions will apply, the final deformation under permanent and variable loading will be as follows:

(i) For permanent actions, G, on a member or connection:

«fin,G = «inst, G + «creep,G = «inst,G(1 + kdef) (EC5, equation (2.3)) (2.29)

(ii) For the leading variable action, Q1, on the member or connection:

« fin, Q, 1 = U inst, Q, 1 + U creep,Q,1 = «mst,Q,l(1 + kdef)

(iii) For the accompanying variable action(s), Qi, on member i or connection i:

«fin, Q,i = «inst, Q,i + «creep, Q,i = «inst, Q,i ('^'0,i + ^2,i'kdef)

(iv) The final condition for n variable actions will be n

The f values are obtained from Table NA.A1.1 in the UKNA to EC0 (reproduced in part in Table 2.2) and where there is only one variable action, equation (2.31) is not relevant.

(b) Structures comprising members, components and connections having different creep behaviour

In these situations, the creep behaviour will affect the stiffness and stress distribution and the linear relationship between the instantaneous and the creep deflection referred to in 2.3.2(a) will not apply.

The instantaneous deformation is calculated as in 2.3.2(a) but the creep deformation is derived using only the instantaneous loading condition (i.e. the quasi-permanent load combination is not used) and reduced stiffness properties for the elements of the structure. The reduced stiffness properties are given in

For structures or members complying with the above conditions, the final deformation, ufin, will be obtained from ufin u (inst+creep)

where u (inst+creep) is the deformation derived from a linear elastic analysis of the structure subjected to the instantaneous loading condition and based on the reduced stiffness properties.

For both situations, i.e. 2.3.2(a) and 2.3.2(b), when timber is being used and installed at or near its fibre saturation point, but is to function in an environment where it is likely to dry out under load, as required by EC5, 3.2(4), the value of kdef given in EC5, Table 3.2 (reproduced in part in Table 2.3), used in the analysis must be increased by 1.0.

Further, as required by EC5,, when dealing with a connection, if it is constituted of timber elements with the same creep behaviour, the value of kdef used in the analysis is to be taken as double the value given in Table 2.10, and if the connection comprises two wood-based elements with different creep behaviour, kdefj1 and kdefj2, the value to be used will be kdef = 2^kdef,1kdef,2.

The application of the factor 2 to derive kdef values for connections may be inappropriate for certain conditions and sizes of fixing; however, as no guidance is given in the UKNA to EC5 on this matter, the code requirements should be complied with.

2.3.3 Load duration and moisture influences on strength (EC5,

The adjustment of strength and resistance properties due to the effect of load duration and moisture content is taken into account in the design process by the modification factor, kmod. The factor is obtained from Table 3.1 in EC5, which has been reproduced in part in Table 2.4, and is discussed in 2.2.20.

When the strength of a connection is being considered and it comprises two timber elements, each having a different time-dependent behaviour, kmodj1 and kmodj2 (e.g. as shown in Figure 2.7), the requirement of EC5, is that the modification factor kmod to be used to calculate the design load-carrying capacity of the connection be obtained from k mod — V kmod,1kmod,2

side member - knod>1

side member - knod>1

side member - kmoAil central member - kmod,2

Fig. 2.7. A connection comprising two timber elements with different time-dependent behaviour.

side member - kmoAil central member - kmod,2

Fig. 2.7. A connection comprising two timber elements with different time-dependent behaviour.

2.3.4 Load duration and moisture influences on deformations (EC5, SLS analyses

At the SLS, in order to demonstrate compliance with SLS criteria at the instantaneous and final displacement conditions, displacement analyses must be undertaken at each condition and, where a structure comprises members, components or connections having different time-dependent properties, the effect of creep on stiffness properties must be taken into account:

(a) At the instantaneous condition, the analysis is undertaken using the design value of the combination of actions for the SLS, i.e. either equation (2.24) or (2.25) depending on whether the characteristic or the frequent combination of actions will apply. As the creep behaviour of the member is not relevant at this condition, in accordance with the requirements of EC5, 2.2.3(2), the mean value of the appropriate modulus of elasticity, shear modulus and slip modulus should be used to derive the stiffness properties.

(b) For analysis at the final deformation condition, where the structure has a linear elastic behaviour and consists of members, components and connections having the same creep behaviour, the design value of the combination of actions will be the summation of the actions used in the instantaneous condition analysis referred to in (a) plus the quasi-permanent combination, i.e. equation (2.26). In this situation, creep behaviour will not influence stress behaviour, and consequently the stiffness properties remain the same as those used for the instantaneous condition analysis.

(c) Where the structure comprises members, components and connections that have different creep behaviour, this will influence displacement behaviour. For this condition, the loading used for the final deformation analysis will be the same as for (a) and the creep effect on displacement behaviour will be achieved by using a reduced stiffness property for the structural elements. In accordance with the requirements of EC5,, reduced stiffness properties must be derived using the mean final values of the modulus of elasticity, shear modulus and slip modulus as given in the following equations,

0 0

Post a comment