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