Fig. 6.38 Punched-metal-
plate fasteners, which form
combined nail-and-gusset
plate units, are used as the
fastening elements in these
trussed rafters.Timber structuressize of the connection may be greater when
bolts are used due to the larger spacing
requirements combined with the relatively low
load-carrying capacity of individual bolts in
timber.
Timber connectors are the mechanical
fasteners which can carry the highest loads and
there are three types of these: toothed-plate
connectors, shear-plate connectors and split-
ring connectors (Fig. 6.31). Toothed-plate and
shear-plate connectors are available in single
or double-sided versions. The former are used
for timber-to-plywood and timber-to-steel
joints and the latter for timber-to-timber joints
(Fig. 6.37). All timber connectors are used in
conjunction with bolts, which carry a propor-
tion of the load which passes through the
joint. The principal function of the connector is
to reduce the concentration of stress.
The detailed design of a mechanical joint
which is made with nails, bolts or timber
connectors involves decisions on the type of
fastener to be used, the number which are
required to provide the necessary strength, and
the spacing which must be adopted between
these. The choice of fastener type is affected by
constructional factors and considerations of
strength. Normally, nails and bolts (without
connectors) are used for structures of modest
size and timber connectors for the larger struc-
tures such as one-off trusses. The number of
individual fasteners which are required for a
joint depends on the load-carrying capacity of
the fastener chosen in relation to the load which
passes through the joint. This depends in turn
on the size of the individual fastener, the species
of the timber, and the dimensions of the timber.
Individual fasteners must then be positioned so
as to satisfy the minimum spacing requirements
and in some cases it is necessary to increase the
size of the element which is used in order to
accommodate the fasteners safely.
Punched metal plates (Fig. 6.38) combine
the functions of nails and gusset plates and
are used to make connections in trussed
rafters and trusses in which all sub-elements
are in a single plane. They are not suitable for
the largest trusses, however, and perform best
in elements of relatively short span (up to
12 m). A range of products of this type is avail-
able and load values for design must be
obtained direct from manufacturers.
Calculations are rarely performed for this type
of joint. Punched-metal-plate fasteners are not
suitable for joints which must be assembled
on site.6.6.6.3 Glued joints
A range of adhesives which are suitable for
making structural joints in timber and which are 213