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

19-9-4 U Korin
Non-linear creep superposition
Preface:
A method is presented for the representation of the behavior of viscoelastic
materials under complex load histories. The method was applied
during a study of the creep of glass reinforced polymers for the prediction
of the creep of reinforced specimens. It was also applied to the transformation
between various viscoelastic functions, like creep to relaxation,
creep to stress-strain. The method is based on the basic creep superposition
principle, and it was modified to treat non-linear viscoelastic materials.
It is the author's feeling that the method may be used also for analysis
of creep of timber and creep superposition of timber. The paper presents
the theoretical background of the proposed method and the development of
the proposed superposition model.
The proposed method is programmed in computer language and may be
easily applied for the analysis of creep data and for the analysis and prediction
of creep superposition behavior.
19-9-5 R Kliger
Determination of creep data for the component parts of stressed-skin
panels
Abstract
The main aim of this investigation has been to predict the creep characteristics
of stressed-skin panels consisting of wood-based materials in the
web and compression flange and a steel sheet in the tension flange. This
paper describes studies of the long-term bending and shear characteristics
of wood beams, the compression characteristics of particle board and the
shear properties of glued joints between
(a) wood and particle board and
(b) wood and steel.
Two different rheological models are used to fit the creep data, which corresponds
to the behaviour of a linear visco-elastic material, which creeps
under sustained loading. It is shown that the selection of the time for the
initial measurement (after application of the load) is of great importance.
19-9-6 P Glos
Creep and lifetime of timber loaded in tension and compression
Summary
As a supplement to existing investigations on small, clear specimens and
to full-size bending tests, long term tension and compression tests on fullsize
European spruce specimens (picea abies) were carried out for the first
time. The material tested consisted of glulam laminates, collected as representative
samples from German glulam plants. The tension and compression
specimens had cross-sections of 30x120 mm and 30x180 mm and a
gauge length of 1.65 m. In 10 test series a total of 212 specimens was subjected
to constant load tests (stress level 14 N/mm 2 and 21 N/mm 2 ; loading
time 6 months) and to ramp load tests (time to failure 10 sec, 1hr and 240
or 368 hrs). All tests were carried out under constant climate conditions at
20 C and 65 % r.h.
Both under tension and compression tests a pronounced duration of
load effect was noticed which was clearly stronger in the lower range of
strength distribution than reported in earlier bending tests carried out in
Canada. The duration of load effect seems to be equally pronounced in
both tension and compression despite differences in creep behaviour. Its
magnitude corresponds approximately to that of the Madison curve. The
test results support the assumption that unless the applied stress surpasses
a threshold of roughly 50 % of the respective short term strength of timber
apparently no damage or strength degradation over time is caused. A modification
factor which would take into account the duration of load effect
in structural timber design codes would depend not only on load history
but also on strength distribution and consequently on the timber grading
system.
Conclusions
All test results presented here are based on a very limited number of tests
on European spruce (picea abies). With this reservation the following conclusions
can be drawn:
1. Structural timber exhibits a significant duration of load effect in tension
and in compression under constant climate conditions. The test results indicate
that this effect is more pronounced in the lower range of strength
distribution than was the case for bending tests carried out in Canada. Nor
were Canadian results supported by new ramp load bending tests with
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.61