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43-21-1 J Munch-Andersen, J D Sørensen, F Sørensen
Estimation of load-bearing capacity of timber connections
Introduction
Experimental determination of the characteristic value of the properties
used to calculate the load-bearing capacity of timber connections involves
3 steps. There is a standard which covers each step:
1. Selection of timber specimens (ISO 8970 = EN 28970)
2. Performing the tests according to the standard relevant for the property
3. Deriving the characteristic value (EN 14358)
This paper is discussing step 1 and 3 which are general for estimation of
all properties. The standards are not referring to or in accordance with the
principles in EN 1990, Basis of design.
Examples are given for withdrawal of threaded connector nails, where the
tests should be carried out according to EN 1382. It is assumed that the
withdrawal load-bearing capacity can be expressed as
�� �0�
c
ax �
tr
F b dl
where
d nail diameter
lthr length of the threaded part of the shank
ρ timber density
ρ0
a reference density
c a power determining the dependency on the density
b the withdrawal strength fax at density ρ0
The parameters b and c should be estimated from the test results.
Conclusions
The present method for estimating strength properties of fasteners based
on ISO 8970:1994 suffers from several drawbacks. It aims at determining
the characteristic strength for timber with the characteristic density, which
is too safe. For Method 1 it is quite difficult to find test specimens with
sufficiently low density, and for Method 2 it is not clear which power
should be used to correct for variations in density. Both methods are quite
dependent on how the specimens are chosen, even though the standard
gives no guidance except for the density.
The new ISO 8970:2010 focuses on determining the strength property
at the mean value of the density but allows the test specimens to have the
same density, so the influence of variation of the density is ignored. This
causes unsafe values when estimating the characteristic strength as before
using EN 14358.
Instead it is proposed to select test specimens so their densities are
evenly distributed over the relevant range of densities, perhaps even requiring
that timber from different sources is used. The observed values
then have to be corrected to a reference density proposed as 420 kg/m 3
(mean value for C24). The correction should preferably take place using a
power c fixed in a standard, but if c is to be determined from the observations,
the estimate will be much better when the observations represent a
wide range of densities. If fixed values of c are used, they shall of course
be identical in the test standards and in Eurocode 5 when the strength for
another strength class than C24 is determined by calculation.
There is no single safe choice for c. When shifting to a higher strength
class a lower bound is the safe value, but when shifting to a lower class it
should be the upper bound. That might be usilized when sufficient information
to fix a single value is not available.
When a model for the mean value of the load bearing capacity is established,
the parameters including the variation of the model error can be estimated
using Annex D in EN 1990. The Annex also offers a method to include
the effect of the natural variation of the density (and other parameters
such as dimensional tolerances). Simplified equations are presented
and their use illustrated for two test series with connector nails.
The examples suggest that the proposed method is quite robust but the
selection of test specimens still is important. The characteristic withdrawal
strength becomes higher when using the present method but lower if the
new ISO 8970 is used together with EN 14358. This is desirable as the old
and new ISO 8970 were believed to be too safe and unsafe, respectively.
It is demonstrated that a constant withdrawal strength fax for different nail
lengths can be obtained only if it is determined using a threaded length not
including the point. The tests also suggest that the minimum length of
threaded nails required by Eurocode 5 should be taken as the real penetration
length, not the threaded length as it might be read.
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.76