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

Conclusions
The following conclusions can be made from the limited analyses carried
out so far with respect to beam strengths.
– The present project on the determination of along-member beam
strengths seems to provide good prospects for the calculation of moment
configuration factors.
– It is desirable to gather information on the way and the extent to which
the strength reductions due to defects may spread along a beam.
– It is advisable to have information on the possible strength coefficients
of variation (COVs) with respect to the regression analysis predictions.
– It is necessary to study the effect of the possible subjectivity of manually
selected defect positions on the generated strength results. This
subjectivity occurs for beams with many smaller close depressions in
the graph of force vs. the beam length.
– The determination of valleys in the grading machine readings, which
may correspond to defects, need to be automated. For this purpose objective
criteria need to be developed.
– Firmer conclusions can be made only when the strength predictions of
all the 133 beams are simulated and a statistical analysis of results is
carried out.
28-12-2 E Aasheim, K H Solli
Size factor of Norwegian glued laminated beams
Background
A research project where the aim was to determine the size factor kh, of
Norwegian glued laminated beams has been carried out by The Norwegian
Institute of Wood Technology in co-operation with two Norwegian producers
of glued laminated timber. The purpose of this project was to compare
the laboratory results with the corresponding values of kh given in
Eurocode 5:
h
� � 0,2
600
k � h
with the following limits: 1�kh� 1,15
The limits correspond to h = 600 mm ( kh � 1)
and h = 300 mm (
k � 1,15 ).
h
Conclusion
Based on Eurocode 5 it should be expected that
kh � fm, k,300 fm,
k,600
� 1,15
The result from this project shows a lower value of k h . By using the values
from the Weibull-3 parameter distribution the following value of k h
was given for a depth of 300 mm:
f mk , ,300 34,1
� � � 1,07
f 32,0
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.128
k
h
mk , ,300
The corresponding size factor calculated from the mean values was
kh � 1,12 .
Based on observations of the lamellae and their distribution of defects it
is assumed that much of the size effect can be explained by the test method
and the grading accuracy.
If this assumption is correct, then the size factor k h is not only a function
of the depth, but also of the grading accuracy and the strength class.
29-6-1 N Burger, P Glos
Effect of size on tensile strength of timber
Introduction
When designing timber members according to Eurocode 5 the size of the
member has to be taken into account by means of size factors. These factors
were derived from investigations mainly carried out in North America
and Great Britain using wood species and grading rules common in these
countries. Since it is most likely that size effects may also depend on wood
quality, grading rules and, perhaps, wood species, the purpose of this study
was to investigate whether these size factors also apply to wood species
and grading rules currently in use in Germany.
Towards this end the influence of specimen size on the tensile strength
was investigated using 750 specimens from timber of native spruce and
200 specimens from timber of native Douglas fir, with varying crosssectional
dimensions and lengths.
Discussion of Results
Results from the tension tests conducted in the course of this study reveal
systematic effects of specimen dimensions on tensile strength. On the one
hand tensile strength decreases with increasing test length, which corre-