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and considering observations during testing an optimized screw for load
carrying joints with steel plates and screws under an angle of 45° can be
derived. This screw includes a strengthened shaft of about 3d length under
the head in order to minimize the curb effect of the steel plate’s edge.
Then an additional free length of about 1,5d is added to reach the optimal
embedment depth of 2d below the surface. Finally a thread length of about
18-20d is applied to the screw. As lower bound 18d are suggested because
the critical length that marks the transition of withdrawal failure to screw
failure is at around 16d (depending on steel strength). If the effect of embedment
is taken into account this length could even be reduced to 14 –
16d.
Figure 12: Optimized screw geometry for load carrying steel plate connections
Based on the research concerning moisture content and feedback from
practical use it is proposed to limit screws to applications inside the service
class 1 and 2.
42-7-2 H Krenn, G Schickhofer
Joints with inclined Screws and Steel Plates as outer Members
Introduction
The load-carrying capacity of connections with dowel-type fasteners and
steel plates as outer member of a single shear connection, determined on
the basis of Johansen’s yield theory, is limited by the embedding strength
of the timber member and the bending capacity of the fastener. The use of
self-tapping screws with continuous threads with the screw axis perpendicular
to the axis of the timber member is very inefficient because of the
limited diameter and the relatively small bending capacity of the screws.
The application of inclined screws that are arranged under an angle of
45° between screw axis and member axis was presented by Bejtka & Blass
as well as Kevarinmäki in 2002. They showed that the load-carrying capacity
and the stiffness of timber-to-timber connections in these joints is a
lot higher compared to common shear connections due to the high withdrawal
capacity of the screws. Going one step further, the additional use of
steel plates with special holes, incorporating the geometry of the screw
head, leads to a further increase in load-carrying capacity and stiffness of
this steel-to-timber connection. The ultimate load of these joints is then
mainly limited by the withdrawal capacity and the tensile capacity of the
screw as well as the friction between the steel plate and the timber member.
These parameters are relatively well known, whereas the influence of
the number of fasteners in such a connection is in question.
This paper will present the results of tests of joints with inclined screws
in glued laminated .timber and steel plates as outer members. Some aspects
of the influence of the number of fasteners will be discussed and a
proposal for design rules for single-shear joints with inclined screws will
be presented. General considerations about the joint geometry and the
screw arrangement will will conclude the publication.
Conclusions
The test results of this study showed that steel-to-timber joints with inclined
self-tapping screws yield a very high load-carrying capacity and an
even more impressive stiffness. A simple truss-model, incorporating the
axial resistance of the screw and the friction in the interface between timber
member and steel plate, is sufficient for reliable design of these joints
(although the dowel effect could be observed to some extent). In the ultimate
limit state, the effective number of screws shall be taken into account
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.78