Timber and wood-based products - ICE Manual of Structural Design: Buildings - page 362

(g) The embedment strength, f h,k , and yield moment, M y,k ,
should be calculated using d ef note there could be an advan-
tage in using the accredited values for yield moments given
in a manufacturer's European Technical Approval, for the
tensile strength is likely to be closer to 750 N/mm 2 than
the 600 N/mm 2 for nails.
(h) The spacings and edge distances for all diameters of screws
should be based on BS EN1995-1-1 Table 19.2 using the
thread diameter d for the same reasons set out for nails
in 8.4.1.
For screws in withdrawal the strength of the thread may be
readily calculated but EC5-1-1 refers to manufacturer's data
for head pull through and the tensile strength of the shank.
European Technical Approvals are now available from a num-
ber of manufacturers with this information which is very
important as head pull through is usually the critical parameter
in the characteristic withdrawal strength of a screw.

19.7.5 Joint slip
Slip is the local elastic deformation of a connection. This
deformation is additional to the elongation or shortening of the
members in a framework and any 'lack of fit' at the connec-
tion, for example, the 1 mm clearance usually provided when
drilling holes for bolts. Slip and lack of fit can significantly
increase the deformation of a pin jointed structure.
EC5-1-1 7.1 provides methods of estimating K ser in N/mm
for various types of fastener so given the axial force in a mem-
ber the local deformation at the connection can be estimate. An
iterative procedure is required to accommodate the effects of
the varying of member forces and with slip.
Ideally the computer software used for frame analysis
should be able to provide spring stiffness at the ends of mem-
bers. Where the program does not have this facility, then short
fictitious members have to be created at the ends of members
having an appropriate E × Area to give the required elongation
or shortening corresponding to the slip value.

19.7.4.6 Punched metal plate fasteners (PMPF) and nail plates
PMPF rely on manufacturer's test data for assessment of char-
acteristic design values. As a result it is difficult to run design
checks as one might do for other fastener designs.
The strength of nailed steel plates of various forms and
shapes, for example, tie straps, joist hangers, etc. can be calcu-
lated using the rules given in EC5-1-1 8.2 with due allowance
for the close spacing of fasteners.

19.7.6 Glued joints
Surprisingly EC5-1-1 does not mention glued joints. This
omission is corrected in PD 6693-1. The range of adhesives
now available is much greater than tabulated in BS 5268-2: for
example, moisture cured polyurethanes, cross-linked polyvy-
nyl acetate, melamine urea formaldehyde - used in laminat-
ing both as a conventional adhesive as a gap filling adhesive.
Note that for normal gluing the glueline thickness should not
be greater than 1 mm but there are situations where a modified
'gap filling' adhesive is required with a glueline thickness up
to 3 mm.
Environmental conditions are as critical as with glued lami-
nated timber with the temperature of the joint at fabrication
and during setting and curing being critical, together with the
moisture content of the bonding surfaces.
Shear strength of materials at the bonded surfaces are:
(a) timber to timber - the characteristic rolling shear strength
of softwood at an angle α to the grain is
f v, α ,k = f v,k (1-0.67 sin α) where f v,d is the design shear
strength parallel to the grain
Note that timber and wood-based products have fibres par-
allel to the grain and in very simple terms when a shear force is
applied at an angle to the grain there is a tendency for the fibres
to 'roll' over the fibres below hence the term 'rolling shear'.
(b) timber to plywood - the direction of grain alternates as the
veneer orientation changes by 90° through the plywood
thickness so rolling shear may be critical at the interface
of timber and plywood or at the first glueline into the ply-
wood depending on the layup of the plywood; the charac-
teristic rolling shear strength is taken as the planar shear
strength of the material c0 timber to OSB/3 - the rolling
shear stress is calculated at the timber/panel interface with

19.7.4.7 Connectors
A very large number of connectors are described in BS EN912
( Timber Fasteners: Specification for Connectors for Timber ).
Connectors familiar to the UK timber industry are

type A split rings
type B shear plates
type C tooth plates note the types commonly used in the UK
are C6, C7*, C8 and C9*. (* square sided)
EC 5-1-1 provides calculation procedures for connectors and
does not rely on accumulated test data as does BS 5268-2.
The mode of failure of types A and B is by either shearing
out the block of timber in front of the connector or if there is
adequate end distance by embedment (compression) failure
of the wood; the embedment failure may be either crushing or
splitting of the timber. Hence the timber dimensions, the diam-
eter of the connector, the spacing, end distances and the density
of the timber are critical to the shear strength of the connector.
Tooth plate connectors, type C, fail primarily in embedment
(compression) of both the teeth and the connector bolt. With
small end distances, splitting and shearing out of a block of
timber can occur. End distance is therefore critical with this
type of connector. It is usual to use a high tensile bolt at least
for the embedment process and then possibly replacing with
a mild steel bolt. The strength of the fastener is the connector
strength plus the bolt strength.