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Conclusions
In addition to the semi-rigid anchorage stiffness, also the stiffness of the
plate in the joint line (plastification), the timber contact and the effect of
joint gap on them could be taken into account already in the determination
of the member forces and moments of the nail plate structure. However,
this would lead to a heavy iterative design procedure and the influence of
these factors on the member forces and moments would not be significant.
But very big differences would be observed in the force and moment values
acting on the plate if the analysis were done without taking account of
the timber contact and the plate plastification (allowed buckling). The effects
of these factors may be calculated theoretically with general plate
and wood properties and with certain allowed joint gaps. Based on the results
of the theoretical calculations it is possible to derive simplified equations
for the determination of plate forces and moments.
The results of the analysis of the effect of the timber-to-timber contact
in chord splices is shown and the general simplified method of determining
the force and moment components acting oil the plate in both tension
and compression loaded chord splices with the bending moment has been
derived. The simplified design method developed was verified with the altogether
148 bending and eccentric tension tests of different kinds of nail
plate joints. The simplified method corresponds well with the test results
of anchorage failure, and it is on the safe side when the plate capacity of
the joint line is the critical factor. The conservatism of the calculation
method is due to the higher plate tension stresses developed in bending
than determined in axial tension tests.
Eurocode 5 does not give any rules for the utilization of the timber
contact in bending and, together with the weak compression strength of
nail plates, this leads to a very conservative design results in bending
joints. The failure loads of the bending tests were 2,4..5,3 times higher
than the capacities calculated by EC5 with the mean strength values, and
the difference is further emphasized in combined compression and bending.
The very simple assumption that 50 % of the bending moment of joint
transfers through the contact couple and 50 % by the moment stresses of
the nail plate leads to much better results. However, EC5 is conservative
also in combined tension and bending, if the bending moment is so high
that there is contact pressure on the compression side of the joint, which
has been shown also by the analysis of the eccentric tension test results of
Wolfe (1990).
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.71