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

– The factor (kL) for adjusting bending and tension stresses to other
member lengths when the width remains constant, is given by
kL = (A / B) 0.17
where A is the length relevant to the stress value to be adjusted and B is
the length relevant to the required stress value.
24-6-3 I Czmoch, S Thelandersson, H J Larsen
Effect of within member variability on bending strength of structural
timber
Introduction
In this paper a simple model of the length wise variation of strength of a
piece of timber is used. The strength is modelled by means of composite
random point series: Random series of strength are assigned to randomly
distributed weak zones.
It is assumed that
– timber is composed of short weak zones connected by sections of clear
wood
– the weak zones correspond to knots or groups of knots and are randomly
distributed
– failure occurs only in the middle of the weak zones
– the strengths of the weak zones are random.
The basis for this model is the fact that failure almost always occurs in the
vicinity of knots because of grain distortions around knots resulting in
stresses perpendicular to the grain, stress concentrations caused by knot
holes and encased knots, and because of differences between the properties
of the knot and the surrounding wood.
The model can be used to evaluate
– the influence of length on the strength of timber members
– the influence of load configurations deviating from the standard test
configuration used for assigning characteristic strength values
– the influence of different test procedures e.g. the difference between
North American practice (where the length to be tested is chosen randomly)
and European practice (where Eurocode 5 prescribes that the
tested length shall contain a grade determining defect)
– the influence of the test procedure on the reliability parameters (e.g.
safety index).
Standard test procedures result in extreme value distributions. If they are
used instead of the parent distribution (i.e. the distribution of the strength
of the weak zones), the reliability of timber structures will be higher than
for material like steel and reinforced concrete for the same safety index or
partial safety factors.
Comments
The suggested procedure will give a result which is very close to the minimum
strength over a length equal to the original length of the boards. The
result will depend on this length, and an adjustment with respect to this
length might be considered in the evaluation of the characteristic value.
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.124