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

emphasis on the modelling of the effect of different schemes for quality
control and grading of timber. The suggested approach not only forms a
very strong tool for the statistical quantification of the material characteristics
of timber but furthermore provides a consistent basis for quantifying
the efficiency of different quality control and grading procedures. The
probabilistic models for the graded timber material properties have been
formulated such that they readily may be applied in structural reliability
analysis.
It is of utmost importance that the statistical characteristics of timber
material properties are assessed and treated in consistency with the implemented
quality control and grading procedures. Only then a consistent basis
may be established for the quantification of the reliability of timber
structures - the basis for codification of design and assessment. The suggested
probabilistic modelling seems to provide the required framework
for establishing such a basis by means of quantifying the efficiency of the
different quality control and grading procedures. It is envisaged that different
quality control grading procedures may be described by means of
their regression characteristics and acceptance probability curves corresponding
to different grading criteria. A format for the standardisation of
the probabilistic modelling of timber materials subject to different quality
control and grading procedures is suggested by Faber et al. It is important
that the appropriateness of such a format is discussed and that a consensus
is achieved in this respect in the near future.
Based on Köhler and Faber in section 4.2 it has been demonstrated how
an optimal (in terms of monetary benefit) set of timber grades can be identified
through the solution of an optimisation problem. The objective function
of the optimisation problem is defined based on the methodology presented
in section 4.1. The implementation of the proposed approach in
practice would have to incorporate factors such as the marked price of
timber, potential demand of the building sector, etc 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.
It is also discussed how new information obtained during the operation
phase of the grading machine can be used for updating the model parameters
involved. Bayesian statistics constitutes the basis for these updating
schemes and a publication is plan which aims the illustration of typical
updating situations in timber engineering along some illustrative examples.
39-5-2 R Katzengruber, G Jeitler, G Schickhofer
Tensile proof loading to assure quality of finger-jointed structural timber
Introduction / Problem / Motivation
Timber as a natural growing raw material displays large variations in its
mechanical characteristics like strength and stiffness in comparison to other
materials such as e.g. steel. These variations can be considerable precise
with the beam-shaped product structural timber, characterised by lack of
homogenisation over the cross-section through gluing of individual components.
A statistical 'system effect' which can be considered for glulam or
bi- or trilam is not present for single sections. Although grading criterions
are defined in DIN 4074-1 with the currently common grading processes
strength reducing defects such as the global and local grain deviation,
compression failures, reaction wood, pre-broken timber or damage of treetop
are only with difficulty and often not economically ascertainable.
Rogues in the lowest quantile area of strength cannot be excluded for sure.
The grading process within the production of structural timber is therefore
still a challenge.
Even so, performance and minimum production requirements for finger
joints of structural timber are regulated in EN 385, a similar difficulty
comes up with the joining. This is because for internal and external quality
control only the bending strength and mode of failure of few randomly
taken finger joint samples are determined in destructive tests. This also results
in the fact that structural timber with features responsible for poor
finger joint strength can reach the customers.
Discussion and Conclusions
The completed cyclic stress tests, as described in point 4, confirm that a
low tensile stress not leading to failure, only minimally affects the strength
of structural timber. The evidence that the material is not significantly
damaged is herewith clearly adduced. The number of tested specimens
(4,886 #) or rather 39.000 # with the referred test length of 1.6 m of series
A in relation to the number of faults with slightly reduced strength charac-
CIB-W18 Timber Structures – A review of meeting 1-43 2 MATERIAL PROPERTIES page 2.97