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and numerical results were found to be in good agreement for all three
joint types. This paper furthermore shows how the probabilistic method
has to be formulated in the framework of current standards to predict characteristic
values of joint capacities. The proposed method has immediate
application for the design improvement of the investigated joints and can
be extended to other joints; e.g. dowel type connections.
Discussion and Conclusions
The capacity prediction of timber joints is difficult due to the anisotropic
and brittle nature of the material, the complex stress distribution as well as
the uncertainties regarding the associated material resistance. This paper
describes a probabilistic method to predict the capacity of timber joints
under brittle failure modes. The method considers the statistical variation
and the size effect in the strength of timber using a Weibull statistical
function. The design method presents an explanation for the increased resistance
of local zones subjected to high stress peaks as it takes into account
not only the magnitude of the stress distributions, but also the volume
over which they act [26]. The method, besides yielding accurate predictions,
has the additional benefit of relying solely on objective geometrical
and mechanical parameters, excluding any empirical input.
The method was applied to three different types of joints: (i) adhesively
bonded double lap joints; (ii) CNC fabricated rounded dovetail wood-towood
joints; and (iii) linear friction welded joints. The paper reports on
experimental and numerical investigations to determine the failure modes
and capacities of these joints.
The statistical distributions of the material strengths were obtained with
small scale specimens. The mechanical parameters can be determined using
standardized test specimens exhibiting different shapes and material
volumes, and subsequently have to be brought into a coherent mechanical
form, i.e. volume. An increase of accuracy can be expected by using a less
disparate set of tests; using samples that are more comparable in their geometry
and volumes, for example a set of off-axis tension tests that allow
to formulate the failure criterion more straightforwardly.
The experiments for the adhesively bonded and the welded joints were
carries out on high quality almost defect free timber. The authors are
aware that this leads to consider an idealized situation, since in practical
applications such a selection is unlikely to occur. However, using less
strictly selected timber will in first instance only increase the scattering of
material strength, without altering the principles behind the dimensioning
method subsequently developed.
Finite element analyses were applied to determine the stress distribution
and provide input data for the capacity prediction. The subsequent application
of the probabilistic strength prediction method proved to be sufficiently
accurate, as it has been demonstrated for all investigated joints. The capacity
determination of systems, including joints, exhibiting brittle failure
has, for a long time, been considered difficult, and often solved using empirical
methods. This paper furthermore shows how the probabilistic
method has to be formulated in the framework of current standards to predict
characteristic values of joint capacity. Since brittle failure tends to be
well described by extreme value probability density functions distributions,
e.g. Weibull, such statistics also lead to good agreement between
experimentally and numerically determined characteristic values of capacities.
This paper offers a new approach by implementing probabilistic concepts
in an engineering context. Firstly the method overcomes the difficulties
raised by the timber’s inherent brittleness and strength variability; secondly,
it offers an alternative to much more complex fracture energy based
methods, for which the correct input data is complicated to generate. Its
implementation proved to be straightforward, and sufficiently accurate to
predict joints capacities of the investigated joints under brittle failure over
a large set of parameters. The proposed method has immediate application
for the design improvement of the investigated joints and can be extended
to other types of timber joints.
43-12-1 S Aicher, G Stapf
Fatigue behaviour of finger jointed lumber
Introduction
Solid lumber is increasingly replaced in engineered timber structures by
finger jointed lumber used as studs and beams. There are several reasons
for this, being i) the significantly increased yield of higher quality/strength
lumber by cutting out the knots and other strength reducing timber defects,
ii) a highly increased production flexibility (drying, storage, sizes, delivery)
and iii) a considerably higher dimensional stability of sticks of longer
length, being a prerequisite of today's wood machining technology. At
present, finger jointed lumber is almost exclusively used in constructions
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.45