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The amount of testing is considerably reduced – down to the 6 design
values, see Fig. 2. Tests with skew plates often give confusing results – the
need of these tests is eliminated.
19-7-7 B Norén
Design of joints with nail plates
Introduction
The following is a proposal for Annex to the CIB Structural Timber Design
Code. The principles agree with those applied in Paper 14-7-1. That
paper is referred to as the main paper.
21-7-3 T Poutanen
Nail plate joints under shear loading
Abstract
Some (15) shear tests with nail plate joints were conducted. The new thing
was that the stress distribution (i.e. the moment distribution) was measured.
It was found that the joint behaviour changes considerably if the
plate has a plastic deformation. The present nail plate design and testing
allows and utilizes steel plasticity: The characteristic values of the nail
plate are defined after the steel plasticity limit and the design assumption
for the stress distribution in the joint is assumed to be a plastic one. This
brings some advantages e.g. the design values for shear are high and it is
believed that the design is simple. But there are disadvantages too: the calculation
actually becomes more complicated because the analysis model
should take into consideration the degree of plasticity (or if plate plasticity
is not considered the calculation is inaccurate and even unsafe); further the
plasticity (at least in some common cases in practice) leads to an unfavorable
stress distribution and to an excess timber volume.
The paper also considers two phenomena in a nail plate joint: lock action
and gap constraint action and concludes that the lock action cannot be
utilized in spite of its potential benefits and the gap constraint action is
mainly harmful but in some cases it can be used for useful purposes.
16. In the author's opinion it is very obvious that linear joint behaviour
leads to a better total reliability, quality and economy than the present
concept based on the strength criterion and plastic joints.
Conclusions
1. Steel plasticity in the nail plate changes the joint behaviour considerably
and according to the test the eccentricity increased on average by 16
%. This produces an excess load increasing the csi by app. 50 %.
2. Lock action brings many benefits to the nail plate joint but it apparently
cannot be utilized in practice due to the requirement of a small gap or a
big initial slip.
3. Gap constraint action is mainly harmful in nail plate joints but it can be
utilized in some cases.
4. If steel plasticity occurs in a nail plate joint the degree of plasticity
should be taken into consideration and if not the calculation includes a
considerable error.
5. Nail plate tests should be conducted measuring the steel plasticity limit
and the characteristic values of the plate should be fixed to this limit.
6. Nail plate shear tests should be conducted without contact.
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.69