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perimental data to characterize the statistical variation in timber material
properties which facitilitates the formulation of probabilistic models for
the behaviour of timber structural members. The resistance factors for
members are calibrated employing probability based calibration techniques.
Therefore the design of structural members is based on the same
consistent foundation as for other building materials as concrete or steel.
Codes for timber connections have been adjusted in recent years to reflect
additional data and information on connections, but reliability based design
for joints is not yet implemented in the code formats. LRFD for joints
is in general still soft-converted from the traditional ASD format.
Explanations for this imbalance in advancement of design provisions
for members and connections can be found in the relative simplicity of
characterising mechanical behaviour of members, as compared to connections.
A diversity of connections types is used in practise and these types
have infinite variety in arrangement. This usually precludes the option of
testing large numbers of replicas for a reliable quantification and verification
of statistical and mechanical models.
For commonly used connections, a distinction is made between carpentry
joints and mechanical joints that can be made from several types of
fasteners. The mechanical joints are divided into two groups depending on
how they transfer the forces between the connected members. The main
group corresponds to the joints with dowel type fasteners. Connections
with dowels, nails, screws and staples belong to this group. The second
type includes connections with fasteners such as split-rings, shear-plates
and punched metal plates in which the load transmission is primarily
achieved by a large bearing area at the surface of the members.
In the course of this paper it is discussed how a probabilistic model for
connections can be derived. It is focused on connections with dowel type
fasteners, more precisely on timber to timber double shear parallel loaded
connections with single dowel type fasteners.
Summary and conclusions
A probabilistic modelling framework for the load bearing capacity of single
dowel type fastener connections is derived based on the design framework
utilized in the present version of the Eurocode 5. Reliability analysis
for connections with different fastener slenderness ratio is performed and
the following conclusions can be drawn:
– The reliability index is an order of magnitude too high.
– System model assumptions have minor consequence on the result of the
reliability calculation.
– The relative importance of the basic variables is quantified by sensitivity
factors. The model uncertainty and the variable load are found to be
most relevant.
Based on these findings the model uncertainty is quantified based on test
data of connections. The following can be observed:
– The parameters of the model uncertainty are considerably different for
cases where different Johansen failure modes are relevant.
– Especially the model bias is different for different failure modes.
Model verifications are proposed, whereas the introduction of an additional
spitting mode and the utilisation of an alternative embedding strength
model where derived independently of the considered data set. Friction
factors for Johansen modes II and III are introduced. It is found that:
– A significant model improvement is reached by introducing the splitting
mode.
– A minor improvement is reached by using the alternative embedding
strength model.
– The improvement which is obtained by introducing friction factors has
to be seen conditional on the data base which is considered.
43-7-1 T Tannert, T Vallée, F Lam
Probabilistic Capacity Prediction of Timber Joints under Brittle Failure
Modes
See: 4.5 Glued joints
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.75