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more or less the same there is no agreement. For instance, the design rules
for Europe and Canada are both experimentally based. Nevertheless, nef
for Europe is two to three times higher than nef for Canada.
The last fifty years research have been carried out to determine the load
carrying capacity of multiple fastener connections.
Lantos, Cramerand, v d Put presented an analytical method for the calculation
of the load distribution among the individual fasteners based on
the assumption of identical linear elastic behaviour of all fasteners. Van
der Put extended his model by a plastic analyses.
Wilkinson developed a numerical model where fabrication tolerances
and variability of single fastener non-linear load-slip behaviour are taken
into account. The result of this model is a randomly non uniform load distribution
among the individual fasteners. Moss extended Wilkinson’s' research
by simulation of a large number of joints. Moss used a piecewise
linear approximation of the actual fastener load-slip curve measured by
Wilkinson.
Concluding remarks
For most configurations no significant difference has been found in the
load carrying capacity between tests carried out in compression or in tension.
The differences in individual hole clearances can be of some importance
if the end slip is rather low (smaller than 2 to 3 mm). In most
cases the end slip is higher. As a consequence, in most cases the individual
hole clearances does not affect the load carrying capacity.
The reduction factor R in load carrying capacity is calculated according
to R =nef/n.
The effective number of fasteners nef is calculated according to
nef �
characteristic result multiple fastener
characteristic result single fastener
This means that the results from tests on the single fastener connections
are compared to the results from tests on the multiple fastener connections.
The results from tests on single fastener connections are, however, for the
connections with rigid bolts significantly below the calculated values according
to the European Yield Model. Consequently, the reduction factors
for connections with rigid bolts decrease if the strength values for the single
fastener connections according to the European Yield Model are compared
to the results from tests on multiple fastener connections. This re-
sults in a more significant influence of the timber thickness (a slenderness
influence). This comparison has not been made, because this is not a comparison
between equal quantities. Consequently, it must be investigated
why the results from tests on single fastener connections differ so significantly
from the theoretical values.
It is quite remarkable that the load carrying capacity is almost linear to
the number of bolts. If this is true, the fact that the effective number of
bolts nef does not equal the actual number of bolts n is explained by the
spacing and the timber thickness of the middle member.
32-7-5 A Mischler, E Gehri
Strength reduction rules for multiple fasteners joints
Introduction
The load-carrying capacity of a multiple fastener joint is often significantly
lower than the strength of one fastener times the number of fasteners.
Therefore, strength reduction factors for multiple fastener joints had to be
introduced. But there are large discrepancies between these factors proposed
in structural codes such as the Canadian CSA 086.1 and the European
ENV 1995-1-1 and ENV 1995-2.
Failure modes of multiple fastener joints
The strength reduction of multiple fastener joints is mainly caused by the
failure modes which must not be the same as for the single fastener. In
multiple fastener connections which are commonly used in timber structures,
the failure is often caused by the timber parts and not by the fastener.
The following failure modes in the timber are possible:
– Splitting of the timber in a row of dowels
– Tensile failure of the timber in the reduced net section
– Combination of splitting, shear plug and tensile failure.
If premature timber failure is avoided by an adequate design of the connection
or by reinforcing the joint area, the failure occurs in the connection
itself. The load-carrying behaviour of a dowel type connection can be described
by the Johansen's theory. A very important condition in the Johansen
theory is that the joint allows large ultimate deformations in order to
reach the plastic failure modes. In multiple fastener connections even
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.62