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

On the basis of long-term experience it is possible during tests without
reaching the ultimate load capacity to consider the component as possessing
sufficient bearing capacity under the assumption that the deformation
ratio (permanent deflection/total deflection) does not exceed the
value 0.3. As regards this criterion experience has proved that it can be
applied even in other cases. For example, in this way we have tested also
frames with screwed comers of hall structures.
Conclusion
It is obvious from the results of experimental testing of thin-flanged beams
and their subsequent modelling by means of the Finite Element Method
(See section 5) that modal analysis is a convenient method for the verification
of computation models.
Modal Analysis has recently proved useful in the determination of
characteristic values of the mechanical properties of structural timber and
its grading. Up to the present day we have carried out three series of experiments
with structural timber taken from three different regions of the
Czech Republic. The tests were carried out with samples with the dimensions
of 100 mm x 120 mm x 2 800 min and of 100 mm x 150 mm x 2 950
mm. Structural timber first underwent visual grading according to the
standard DIN 4074. Further tests were carried out by means of the modal
analysis method and the ultrasonic method. Eventually destructive tests
were carried out according to the standards EN 408 and EN 384. The purpose
of the destructive test was the verification of non-destructive test results.
The test results of one series of 43 samples of structural timber,
which corresponded to the strength class C 22 according to the standard
EN 338, are shown in Figure 6.1.
Fig.6.1 Results of experimental testing of structural timber
Another non-destructive method which we have recently applied in the
analysis of a frame comes detail in the frame construction of a Prague
sports hall, with a span of 50 m, was the photoelastic measuring method.
31-5-1 S Ormarsson, H Peterson, O Dahlblom, K Person
Influence of varying growth characteristics on stiffness grading of
structural timber
Introduction
The common practice in investigating stiffness and strength properties of
sawn timber is to load the specimens and measure the deflection. The longitudinal
modulus of elasticity (MOE) is obtained as some kind of average
value, determined on the basis of elementary beam theory. This value is
correlated to the strength of the board. The results obtained from measurements
are strongly influenced by grain deviations with respect to the
longitudinal direction and variation of material properties with the position
in the log (often explained by juvenile wood and influence of compression
wood). The value obtained from a measurement may therefore be regarded
as an "effective modulus of elasticity". In addition to influence from fibre
misalignment and property variation, the grading procedure is often disturbed
by twist deformations of the board caused by spiral grain.
To improve the stiffness and strength grading process for sawn timber,
it is important to clarify how the material properties and the internal structure
affect the stiffness properties. In the prediction of timber stiffness, the
fibre orientation, growth ring width distribution, juvenile wood and compression
wood are of considerable importance. Because of the complex
growth characteristics of wood and of various imperfections, the stiffness
prediction may require computer simulations based on experimental data.
In the present study, finite element simulations have been performed to
investigate how a number of basic parameters primarily affect the stiffness
properties and indirectly the strength properties. Some preliminary results
are also presented from an experimental investigation of basic material
properties of spruce. The properties studied are stiffness and shrinkage parameters
and grain deviations. The measurements have been carried out
for stems from different stands. The specimens have been sawn at different
distances from the pith and at different heights in the stem. The aim is to
gain information about the variation of properties with the distance from
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.89