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distribution of load is prevented. Oversized and misaligned bolt holes or
split ring grooves tend to make the situation even worse: by causing differences
in initial slip of single fasteners which makes the load distribution
very uneven. This may lead to some single fasteners reaching their maximum
load while other fasteners just begin to carry load because of their
greater initial slip. In case of long-term or repeated loading, creep deformations
and residual plastic deformations after previous higher loading also
affect load distribution.
A realistic model to describe load distribution in multiple fastener joints
must therefore - apart from different elongations of the joint members -
take into account influences from fabrication tolerances and variable loadslip
curves within the joint. Wilkinson (1986) presented a model to calculate
load distribution in bolted joints. Knowing the different shape of loadslip
curves of different fasteners in the connection allows the calculation
of load distribution up to ultimate load. Using Wilkinson´s model, the following
steps are proposed in developing modification factors for multiplefastener
joints:
– Determine variation of load-slip behaviour in connections.
– Determine variation of initial slip due to fabrication tolerances within
joints.
– Extend Wilkinson's model to calculate load distribution taking into account
all important parameters.
– Simulate joints with different number of fasteners taking into account
correlation of load-slip behaviour within joints.
– Calculate maximum loads of simulated connections and determine
characteristic values of ultimate load for different number of fasteners.
– Derive modification factors by comparing characteristic values depending
on number of fasteners.
The objective of the present investigation was to develop a model taking
into account all important parameters influencing short term load distribution
in mechanical timber joints. To verify the model, tests were carried
out with specimens, which had been used in previous tests to determine
variation in load-slip behaviour within joints. Therefore, single-nail loadslip
curves of these specimens were already known.
Summary
An existing model to determine load distribution in mechanical timber
joints was extended to take into account the influence of plastic displace-
ments after previous higher load levels. To verify the model, tension tests
were carried out with nailed double-shear specimens made from spruce
and nine nails arranged parallel to load. The load-slip behaviour of ten
single nails was known for each specimen from a previous investigation.
The expected maximum load of the nine-nail tests was determined with
the model using the known load-slip data of the single-nail tests. Comparing
the expected with the actual maximum loads yielded very good agreement
on the average. A visual comparison of load-slip curves from -the
tests with those from the model also showed good agreement in most cases.
The maximum load per nail for the single-nail tests did not differ significantly
from the corresponding values from the nine-nail tests. Therefore,
the maximum load of a multiple-nailed joint can be estimated as the
sum of those for individual nails, provided joint failure is by nail yielding.
25-7-12 C J Mettem, A V Page
Load distribution in multiple-fastener bolted joints in European whitewood
glulam, with steel side plates
Introduction
Draft Eurocode 5 requires as a principle that account should be taken of
the fact that the load-carrying capacity of a multiple-fastener joint will frequently
be less than the sum of the individual fastener capacities. The use
of relatively thick metal plates, usually of mild steel, in conjunction with
bolts or plain round dowels, is common in forming connections in timber
engineering construction, particularly with glued laminated timber (glulam).
Designs normally involve double shear planes, and the plates may be
positioned as side plates, or as central inserts, or 'flitches'.
An accompanying paper presents a brief review of the current research
and code status in relation to the topic of multiple-fastener modification
factors for such joints. This paper describes recent tests, supported by a
linear elastic orthotropic finite element analysis. The work has been conducted
on both single and multiple bolted joints. An especially designed
test apparatus, which could accept thin central members of European
whitewood glulam, was used, to ensure failure in a pure embedment mode.
Strain-gauged mild steel side plates were used on this test jig, following an
established technique developed by previous researchers.
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.60