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