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

gether with their correlation to the indicator. An optimization problem can
be defined for identifying the grading procedure, leading to the optimal set
of timber grades.
Discussion and Conclusions
It has been demonstrated how an optimal (in terms of monetary benefit)
set of timber grades can be identified through the solution of an optimization
problem. The objective function of the optimization problem is defined
based on the findings reported previously by the authors where the
statistical assessment of timber material properties has been considered
with special emphasis on the modeling of the grading of timber. The identified
timber grades can be described by means of the probabilistic characteristics
of the relevant material properties as e.g. the bending strength, the
bending modulus of elasticity and the density of the timber. The simplex
algorithm for the optimization of non-differentiable object functions in
conjunction with a simulation procedure has been applied for the identification
of the optimal grading procedure. The constraints to the optimization
problem, in terms of the requirements for timber grades according to
EN 338 have been incorporated directly into the objective function.
An example has been presented illustrating the suggested approach to
optimal timber grading. The assignment of monetary benefit to the different
grades of timber has, however, been based on judgment rather than
true values. In practice the benefit associated with timber of a particular
grade would depend on a number of factors such as the size of the individual
timber specimen, the total amount of available timber for a given grading,
the production capacity of a given sawmill, the available grading machines
and not least the market price for the different timber grades. The
implementation of the proposed approach in practice would have to incorporate
these and other factors more accurately into the formulation of the
benefit function. Further studies in close collaboration with the timber industry
should be undertaken and discussed to clarify these aspects and to
set up a rational basis for their assessment. However, according to the
preferences of a sawmill owner the proposed approach facilitates the identification
and the calibration of a grading procedure and thus an increase in
the overall production benefit.
36-7-11 A Ranta-Maunus, A Kevarinmäki
Reliability of timber structures, theory and dowel-type connection failures
Introduction
This paper has two separate parts both dealing with the reliability of timber
structures. The first part is a theoretical one related to strength distribution
of timber materials and to the theoretical effect of lower tail strength
on safety factors in code calibration. Some results on the strength distributions
of machine strength graded timber are discussed.
In the second part a recent failure case is reported. This is considered
urgent because the failure was caused partly by inadequacy of the prestandard
version of Eurocode 5, which can now be used in many European
countries. Because of the accident, supplementary national guidelines will
be given in Finland concerning the design of dowel-type joints, including
block shear failure. The draft guidelines are given in appendix.
Summary
This paper reports on the theoretical investigation of strength of sawn timber,
which suggests that higher grades are relatively much better than low
grades, with the consequence that structures made of C40 have clearly
higher structural safety than structures made of C18, when present European
strength grading and structural design standards are used.
38-5-1 M Arnold, R Steiger
Are wind-induced compression failures grading relevant
Introduction
Compression failures (CF) are defects in the wood structure in the form of
buckled cell walls of the wood fibres. They may be wind-induced in the
standing trees, if the stems are bent so much by frequent or strong winds
that the proportionality limit of the wood in axial compression is locally
exceeded on the inward (leeward) side of the bow. The distorted fibres of
CF remain as weak points in the wood and can lead to brittle fractures in
processed timber already at a relatively low stress in bending or tension
(Fig. 1). Therefore CF are regarded as unwanted structural defects.
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.94