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32-7-4 I Smith, P Quennevile
Predicting capacities of joints with laterally loaded nails
Abstract
Capacities of joints with laterally loaded nails may be predicted using "European
yield" type models (EYM's) with various levels of complexity.
EYM's presume that a nail and the wood on which it bears exhibit a rigidplastic
stress-strain response. Consideration is given in this paper to the
'original' model published by K.W. Johansen in 1949, an empirical approximation
proposed by L.R.J. Whale and coworkers in 1987, and a curtailed
and 'simplified' model proposed by H.J. Blass and coworkers in 1999. Predictions
from the various EYM's are compared with experimentally determined
ultimate capacities of single shear joints. Experiments covered a
fairly broad range of combinations of 'head-side' and 'point-side' member
penetrations. The impact of modeling assumptions is illustrated in the context
of the Canadian timber design code. Suggestions are made regarding
the necessity level of complexity for nailed joint models used in design.
Conclusions
European Yield Model type calculations give accurate predictions of the
maximum load capacities of nailed single shear timber-to-timber joints. A
'simplified' model based on a proposal by Blass et al is the most appropriate
choice. Two model equations should b used in conjunction with a restriction
on the minimum head-side member thickness. Calculations are
valid across the range of head-side and point-side nail penetrations that are
of interest to designers. The reference capacities for laterally loaded nailed
joints tha tare specified in the Canadian timber design code should be revised.
33-7-5 H J Blass, A Bienhaus, V Krämer
Effective bending capacity of dowel-type fasteners
Introduction
The load-carrying capacity of connections with dowel-type fasteners like
bolts, dowels or nails may be determined on the basis of Johansen. According
to Johansen the load-carrying capacity depends on the geometry of
the connection, the bending resistance of the dowel and the embedding
strength of the timber or wood-based material. For the bending resistance
of the dowel, Johansen assumed the elastic moment capacity of the
dowel's cross-section, the possible increase due to plastic deformations
was disregarded. The design equations in Eurocode 5, which are derived
from Johansen's work, are based on a rigid-plastic behaviour of both, the
dowel under bending moments and the wood under embedding stresses
and take into account the plastic moment capacity of the dowel. According
to Johansen, three different failure modes are possible for timber-timber -
connections in double shear. Failure mode 1 corresponds to the embedding
failure of the middle or side member, respectively, where the embedding
strength according to EN 383 is defined as "an average compressive stress
at maximum load in a specimen of timber or wood-based sheet product
under the action of a stiff linear fastener". In failure modes 2 and 3, apart
from the embedding strength of the wood the bending capacity of the fastener
is reached. Failure modes 2 and 3 of dowels loaded in double shear
correspond to identical failure modes of dowels loaded in single shear.
According to EN 409 the yield moment of a nail is determined at a
bending angle of 45°. For such a large bending angle, the whole crosssection
of the fastener is assumed to be under plastic strain.
For bending angles below 45° only the outer areas of the cross-section
of a fastener are deformed plastically. In this case, the reserve capacity can
only be partially used and the fastener bending moment lies between the
elastic and plastic bending capacity of the often circular dowel crosssection.
Effective bending capacity of dowel-type fasteners
For different types of fastener the bending angle α(d) at ultimate load was
determined, resulting in minimum values of the bending angle α. For this
purpose, the governing parameters were conservatively chosen, resulting
in maximum values for the steel tensile strength and minimum values for
the characteristic density. For connections with bolts or dowels, the tensile
strength was chosen as 1000 N/mm 2 and the characteristic density as 350
kg/m 3 .
A limiting deformation of 15 mm was assumed resulting in the following
approximate expression for the effective bending capacity of bolts and
dowels:
y, k u, k
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.25
2,7
M � 0,24 f d Nmm
where
f uk , fastener tensile strength in N/mm 2