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

the pith and with the height in the stem, and also about the influence from
growth conditions on the properties examined.
Some concluding remarks
The choice of proper stiffness parameters to be used in structural grading
of timber is by no means a simple matter. Usually the coupling effects between
tension, bending in two directions and twist are neglected. This may
lead to a much too crude approach for grading of high quality timber and
in the structural analysis of beam structures.
Due to strongly varying material properties, especially from pith to
bark, it is often of great importance for the grading where in the log the
sawn board comes from. This is today normally not considered in practice.
The sawing pattern influences the positions of the boards to be sawn and
might thus have a considerable influence on the strength and stiffness
properties of the timber. Another phenomenon that must receive much
more attention in grading of timber is the drastic reduction of strength and
stiffness caused by compression wood.
Fibre misalignment, like spiral grain deviations, may have a considerable
influence on the stiffness properties. It might substantially reduce the
stiffness moduli for tension and bending and it must be considered properly
in stiffness grading for high quality timber.
31-5-2 R H Leicester, H Breitinger, H Fordham
A comparison of in-grade test procedures
Abstract
The design properties derived for structural timber depend on the in-grade
test procedures used to measure these properties. In this paper, the data obtained
from a limited set of bending and tension tests undertaken according
to European, North American and Australasian procedures is used. It
was found that the European procedure can underestimate the in-service
bending strength and stiffness and slightly overestimate the in-service tension
strength.
Conclusions
The data measured provides useful information on the differences that can
be obtained between different in-grade test procedures. It also provides
information for calibrating equivalencing models proposed in an earlier
paper. The size and grade of timber used in this study probably provides
the greatest differences likely to occur in practice.
Additionally, it is of interest to compare the data of the CEN and
AS/NZS procedures. The AS/NZS procedure may be considered as an attempt
to measure the properties of timber that would occur in-service. By
comparison, the human bias applied in selecting the test specimens according
to the CEN procedure leads to an underestimate of design bending
strength and stiffness but (because it is imperfect) a small overestimate of
tension strength.
32-5-1 K Frühwald, A Bernasconi
Actual possibilities of the machine grading of timber
Introduction
In the Austrian and German sawmilling and wood construction industry
structural timber is visually graded according to DIN 4074. But visual
grading only allows a prediction of the very wide variation of mechanical
properties within a piece of lumber or between several pieces. The dimensioning
values including relative high safety factors have to be established
to reach the lowest strength and stiffness values, which can not be realized
by visual grading. Therefore of that only a limited utilisation of the potential
timber properties is possible. Additionally visual grading only fulfils
partly existing requirements of quality control. Generally the competition
between wood and other material in the structural building sector puts
structural lumber under high technical and economical pressure. This requires
more efficient utilisation of the technical possibilities of timber
which is only possible with a reliable prediction of the mechanical properties
of timber. This is one reason why research and the industry have been
engaged in the further development and practical utilisation of machine
strength grading.
The aim of the study "Actual possibilities of machine grading of timber"
is the comparison of the various technologies for strength grading for
timber in Europe. The efficiency of grading is described as
a) yield in each strength class and
b) their mechanical properties which have to meet the requirements of the
international EN- or national DIN-standards.
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.90