Creep represents the increase of with time, under a constant applied action. It is a particular aspect of behnviour of a material. Wood is generally considered be a viscoelastic material. As i n Figure the creep
of the deformation begins after. the
Experimentally, is by progressively applying load within well defined and under defined conditions, up a fixed value.
A of of i s
Creep
-
RIn representation, creep exhibits two principal
- a rapid part at beginning, as by the steep
variable slope of curve. The slope of the part of is the rate.
- range a creep
On unloading, a progressive recovery, total or partial, to the initial state occurs.
Total recovery corresponds to pure Partial recovery that plastic even microscopic, has
In a environment and for stresses less than about 35% of instantaneous of wood, the deformations under two different stresses have a constant ratio, independent time and to stress ratio. The
ratio) is independent
of
the stress. behaviour is then considered linear.In the design all conditions, and particularly the load (or stress) are such this assumption of linearity is always appropriate.
Principal parameters of creep
parameters influencing creep in wood are:
-
load duration,-
content,temperature, and
-
stress level.interactions occur between all of them but only the effects of load duration and moisture content are taken into account in the design rules.
Some brief comments on the temperature and the stress level effects are presented for information.
of load
steady-state environmental conditions, creep deformation increases with load duration. Its amplitude strongly depends on load which must be maintained lower when duration of load is extended in order to ensure an acceptable deformation limit of the structure with time.
For instance, under load, creep of solid timber may increase from two to four times for load duration ranging from six months to twenty years.
3.1.6 the calculation rules, five load duration classes are defined. value of creep, for a class, is as constant.
of
Solid and board materials are all, to different degrees, sensitive to humidity, a very effect on creep beyond certain limits. Creep increases with content. Generally, under the same conditions, it is considered that creep of wood based panels is higher and, of these materials, plywood has the creep. Creep amplitudes, from once to twice the instantaneous deformation, are
values for indoor use under permanent load. can to four times the instantaneous deformation when the moisture content is close to 20%.
Research results on structural sized solid timber and (Rouger et al., 1990) show elsewhere that creep is practically equivalent for two materials when the average moisture content does not exceed the value of The experimentally observed differences between the two materials are mainly caused by the differences of the instantaneous deformation. These differences are directly related to the modulus of elasticity to the moisture content.
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1-1: The design rules clearly distinguish three service classes, corresponding to three different contents of the wood.
An additional situation must be underlined for solid timber, whose moisture content, near to fibre saturation point at time of' erection on site (25 to will greatly decrease to the state in service. Where there is variable content under stress, there is an acceleration of creep and the final deformation is even greater.
Experimental on solid timber and under protected external conditions, show creep values ranging once to twice the instantaneous deformation, more than one year under stresses of 2 5
1992).
Different curves, illustrating four moisture content effects on creep are in Figure 2.
2 for
A, 12%
12% D,
to is days.
of 1962).
The nature of' wood makes it also sensitive to temperature.
For purposes, higher the temperature, the of
creep. Further, variable result in acceleration of the creep (Dinwoodie et 1991).
in use, when the does not exceed about 50 its influence on creep is negligible and masked by effects of moisture content variations, even these variations are low.
Nevertheless, it must be kept in mind the notions of temperature and content in situ, have to be related to parameters:
-
thickness of wooden elements added to that, for instance of encasements,Programme A
finishes, or similar means which decrease exchanges the environment.
- the speed the frequency of the ambient atmospheric variations.
Unsteady gradients occur the section and one consequence is a reduced creep amplitude with a humidification or drying, a steady state
be reached.
Provided that the stress and the of the are
the creep curves for wood versus generally terminate, just as
with in an stage preceding which
can explained by progressive damage of wood. The higher the the higher the of and is time before fracture (see Figure 3).
3 a,
is
the design the stress levels are calculated so that creep remains within stable phase, where the of is low and stable during lifetime
of the results, in Figure 4, present
stability, for stress levels than 35% of resistance.
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Influence of load duration moisture content
according to EC5
The combined effect of load duration and moisture content is quantified by the in general expression of final deformation:
Writing,
it can be seen that the value of creep is equal to
In timber structures effects must be calculated for component such as or members and then to obtain the deformation (see STEP lecture
Mechanical joints
Similar exists in limber joints, because of local
deformation of wood, in under the fastener. The magnitude of creep may also be at least as as that in wood. Maximum creep is obtained
one of the jointed pieces of wood is loaded perpendicular to grain.
Concluding summary
-
duration, moisture particularly moisture content variations, and stress level influence the extent of creep deformation. Even if are complex interactions among these variables and even if thesechange from one to another, the of the
two first parameter-s can be considered as most significant in use of and wood based panels.
-
Creep deformation is by factor- by the initialdeformation. The factor depends on the load duration and service classes of the structure.
References
(1992). of solid timber and
(in French). Proc. IUFRO, :
Kingston. R.S.T. of
of wood of Applied Scicncc,
J.M., J.A., (1991). and
understanding of crccp in Proc. COST 508. on
in wood, 18. Sweden.
D. nnd P. (1988). cn Son
Annalcs 469 33-83.
Rougcr F., Govic C., R. J. (1990). of
Proc. Inter. Vol 2 : Tokyo.
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