CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL ...

when a dry period is followed by a humid period, and both periods are
long.
When the effect of moisture changes is compared to the effect of mechanical
loading, we can conclude that the moisture load caused by the analysed
cycles corresponds to an extra load of 0.15 to 0.35 MPa when acting
simultaneously with a mechanical load causing stress of 0.2 MPa,
when the beam is not surface coated. A good coating will decrease the
moisture load by 70 %. A single fast change from 65 % RH to 90 % RH
seems to be more effective than the test cycles analysed.
While moisture gradients proved to be more important than realised in
advance, some other factors appeared to be less important: creep after several
moisture cycles seems not to change the level of stresses from that of
the first cycle. Accordingly, for the analysis of the duration of load behaviour
under tensile stress perpendicular to grain, it is of great importance to
consider the largest moisture cycle or change. All other duration of load
effects are of much less importance and can well be estimated on the basis
of the traditional stress ratio vs. log time to failure graph based on ideal
constant humidity experiments. Under ideal constant conditions duration
of load effect in tension perpendicular to grain is slightly less severe than
suggested by the Madison curve.
Volume effect during long term loading is found to be of the same order
than adopted in Eurocode 5. The strength of a curved beam in comparison
to a tensile specimen is higher than expected justifying the use of a
higher kdis value, 1.85 for curved beams. On the other hand kmod factors
should be lower, about 0.5 for medium and long term loads, or an additional
moisture load should be calculated, depending on the surface coating
or impregnation of curved beams.
32-9-1 P Becker, K Rautenstrauch
Bending-stress-redistribution in tension and compression and resulting
DOL-effect
Introduction
Creep is generally assumed to be larger for compression than for tension to
some degree. This results in a change of stress-distribution. The stress for
the tension edge will increase and decrease for the compression edge. An
analytical solution of this behaviour can be easily derived with the assumption
of creep limits for both impacts.
The resulting stress-distribution will lead to a decreasing computational
bending strength value, because the stress for the tension edge, which will
be finally responsible for failure, has already increased during lifetime of a
structural element. The exact difference can be determined by simulation.
For initial and resulting stress-distribution load increments are applied until
tension strength is reached for the tension edge. Plasticating ability of
wood subjected to compression is considered.
Conclusion
Different creep in tension and compression was observed by many researchers.
The exact difference can hardly be quantified because it strongly
depends on climatic and loading conditions and also on the quality of
the material.
It generally leads to changing edge stresses, which under the assumption
of linear viscoelasticity can be easily determined using creep factors.
If certain factors are given for compression and tension creep a bending
creep factor, which is slightly lower than the mean of tension and compression
creep, can be determined also.
Because of the strong ability of the material to plasticise under compression,
the influence of an increasing tension stress on the duration-ofload
phenomenon turns out to be quite low, almost negligible for normal
and high quality material. Taking timber of low quality, the influence becomes
larger; a big difference in tension an compression creep can rather
be not expected for this kind of material though.
32-9-2 R Grantham, V Enjily
The long term performance of ply-web beams
Abstract
Ply-web beams have been used for a number of years in floor and flat roof
constructions, such as swimming pools, that require medium-span structural
beams. Many of the advantages associated with ply-web beams stem
from their structural efficiency, utilising structural timber flanges and plywood
webs. This produces lightweight components that are easily installed
on site and reduce the dead weight of construction when compared to solid
timber beams. The design of ply-web beams has been thoroughly investigated
with the exception of their long-term performance. Very few studies
have covered this aspect of design, which may be critical in service condi-
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.67