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24-7-1 B Källsner, J Kangas
Theoretical and experimental tension and shear capacity of nail plate
connections
Background
The strength of nail plates with respect to the plate material has been studied
at the Norweigan Institute of Wood Technology. This work served as a
basis for a Nordic proposal for a design method and was presented by
Norén in 1981.
In 1985 Bovim and Aasheim presented Paper 18-7-6 where they made
comparisons between measured and calculated values of plate strength.
For all the tests a Gang Nail 18 plate was used. Their conclusion was that
the design method presented gave a good prediction of the plate strength
of nail plates.
During 1980-81 the Technical Research Centre of Finland (VTT) made
an extensive investigation
of one type of nail plate to achieve an approval using different shapes and
sizes of the nail plates.
Purpose and scope
At the Swedish Institute for Wood Technology Research the design method
has been used for several years in connection with the evaluation of results
from testing of nail plate connections. Since it is possible to derive
rather simple expressions for the tension and the shear capacity one purpose
with this paper is to present these expressions and to explain where
they are valid.
It has often been questioned if the proposed design method can be used
for any nail plate. In order to elucidate this, the results from testing of 6
different nail plates are compared with the theory. All the tests have been
carried out at the Technical Research Centre of Finland.
Conclusions
The proposed design code is presented in a format easily applicable to results
from standard tests. An evaluation of 6 different types of nail plates is
made. The agreement between the measured and the theoretical capacity
values in tension is somewhat varying but mostly rather good. The capacity
values from the shear tests are often higher than the results from a theoretical
calculation assuming no contact between the timber members. Special
attention should be paid to the local buckling mode of the nail plate
edge in the shear test. In the case of tension shear it might be possible to
use a theory based on the assumption that there is contact but no friction
between the timber members if the length-to-width ratio of the plate is
high enough.
31-7-5 A Kevarinmäki
Timber contact in chord splices of nail plate structures
Introduction
In Eurocode 5 the contact pressure between timber members may be taken
into account to reduce the joint force F in compression provided that the
gap between the members has an average value not greater than 1 mm and
a maximum value of 2 mm. In such cases the joint should be designed for
a minimum compression force of F/2. However, EC 5 does not give any
rules for utilization of the timber contact in the bending of chord splices.
In the Nordic countries the moment capacity of nail plate joints of
chord splices with utilization of the timber contact has been used in practical
design since the 1970s. In compression the design value of the joint
normal force is 1/3 of the applied normal force of the chord, i.e. the timber
contact reduction is 2/3. The design equations are based on the force couples:
the moment, Md, is analyzed as a tension force of the plate and a
compression force of the timber contact located at a distance of h/6 from
the chord edge. The interaction between the normal force and the moment
is taken into account by the bi-linear capacity curve. Theoretically the design
method is clearly on the unsafe side with a big tension force, because
when the whole cross-section of the timber member is in tension there is
no joint contact and no force couple for the moment.
The aim of this study was to develop new design equations suitable for
use with EC5 for the chord splices with symmetrically placed nail plates,
when the main direction of the nail plate is combined with the grain direction.
The work may be done by theoretical calculations using previously
developed models of the moment capacity and rotational stiffness of nail
plate joints. The proposals for utilization of the timber contact in the chord
splices are verified with bending and eccentric tension tests on different
kinds of nail plate joints.
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.70