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Conclusions
Splitting sensitivity of spruce
The clause 8.3.1.2(7) of Eurocode 5 (EN 1995-1-1) given for timber of
species especially sensitive to splitting is recommended to omit at all or to
limit only for the silver fir (Abies alba) and Douglas fir by National annexes.
With the Nordic spruce, the allowed minimum thickness of timber
for nailed connections without predrilled holes may be safely defined as
for the common species of timber. For the next version of Eurocode 5, the
clause 8.3.1.2(7) shall be corrected: spruce should be eliminated from the
list of species of sensitive to splitting and the real splitting sensitivity of
silver fir (Abies alba) and Douglas fir should be verified. The rules given
for "timber of species especially sensitive to splitting" seems to be overconservative:
the minimum timber thickness or the minimum edge distance
is required to double.
Nails in end grain
In National annexes, the clause 8.3.1.2(3) of Eurocode 5 (EN 1995-1-1),
"Smooth nails in end grain should not be consider capable of transmitting
lateral forces", should be recommended to replaced by clause 8.3.1.2(4).
The clause 8.3.1.2(4) with the limitation of the load-carrying capacity of
nails in end grain to 1/3 of the values for nails installed at right angles to
the grain may be safely applied.
Long-term lateral load-carrying capacity tests of nails in end grain
should be done for the future development of Eurocode 5. According to
the results of short-term tests, the lateral load-carrying capacity for nails in
end grain may be increased at least to 1/2 from 1/3 of the values for nails
installed perpendicular to the grain. Also the exceptional and unclear rule
for the restriction on use of smooth nails only for secondary structures given
in EN 1995-11:2004 should be cancelled from the next versions of Eurocode
5.
Withdrawal strength
It is proposed that the test standard for withdrawal strength EN 1382:1999
is modified so that the conditioning of wood is done to RH85% before
nailing and after it is conditioned to RH40% before testing at a temperature
of 20°C. This requirement could also be mentioned in the product
standard for nails EN 14592. In case the nail type specific withdrawal capacity
is determined according to EN 1382:1999 conditioned to RH65%,
the experimental withdrawal strength should be reduced by a factor of 0,4
for plain shank nails and at least by a factor of 0,7 for profiled nails.
The reduction of the short-term withdrawal strength due to the previous
long-term loading should be taken into account in design. The next version
of Eurocode 5 (EN 1995-1-1) could be supplemented for example as follows:
In case the share of the permanent and long-term loads is over 1/3,
the withdrawal capacity of nails is calculated with a modification factor
kmod equal or lower than 0,7.
42-7-8 E Gehri
Influence of fastener spacings on joint performance experimental results
and codification
Introduction
Engineer’s first aim is to connect timber pieces together in the most efficient
way; that includes performance, economy and reliability. Starting
from the known behaviour of a joint – for a given geometry and fasteners
arrangement – the performance can be established in relation to loadcarrying
capacity, stiffness and ductility.
For the evaluation of the above mentioned properties more or less
standardized procedures may be applied. Due to the complexity of such a
joint (different parameters involved, different failure modes possible) and
the lack of reliable strength models the analysis of the influence of fasteners
(and of fasteners spacings) is here quite impossible.
Therefore evaluation of fasteners starts mostly from the behaviour of a
single or individual fastener; in many cases test procedures are only directed
to a single property like the embedding strength (case for doweltype
fasteners) or the withdrawal strength (case for axially loaded screws)
for a determined species and wood density. In a second step the interaction
in a multiple fastener connection is than considered.
Basically such derived values are only valid for the test geometry and configuration
used. The
application for actual joint design may be problematic, if the behaviour of
the fastener in a joint does not follow the assumed behaviour of an individual
fastener.
The load-carrying capacities of fasteners are in the Codes generally
given in function of fasteners spacings, edges and end distances. In certain
cases they are furthermore linked to the number of fasteners acting in a
row or in a group. The stiffness is implicitly assumed to be independent of
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.85