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38-7-7 B Murty, I Smith, A Asiz
Design of timber connections with slotted-in steel plates and small diameter
steel tube fasteners
Introduction
Finding combinations of member materials and fasteners that produce ductile
timber connection responses is challenging and research toward that
end is being conducted by the authors. Others have pursued the same aim
but their techniques resulted in solutions that are labour intensive and also
expensive in other ways, e.g. involving localised reinforcement of the
members and grouting the fasteners in with epoxy. The approach taken
here is to use 3 mm thick steel plate link-elements that slot into ends of
wood members and circular cross-section steel tubes of relatively small
external diameter (up to 12.7 mm), Figure 1. Solid spruce and Laminated
Strand Lumber (LSL) were used as representative diverse 'wood' member
materials. LSL is an advanced type of engineered wood product common
in North America. While spruce is splitting prone, LSL is not. Tested connection
arrangements had one or two slotted-in link-elements and one or
four tube fasteners. Specimens were subjected to axial tensile static load
until failure. Ductile load-deformation responses were expected to occur
through combinations of wood crushing beneath fasteners, bending induced
plastic hinges in fasteners, and plastic distortion of fastener crosssections.
Distortion of fastener cross-sections was the result of them being
hollow, with the extent of those distortions controlled by the ratio of inner
and outer tube diameters and the yield stress. Especially in cases where the
fasteners were not slender, that mechanism compensated for loss of the
other sources of ductility.
The remainder of this paper is focussed on test and results, examination
of the acceptability of closed form Johansen type yield models for design
level predictions of strengths of connections with small diameter steel tube
fasteners, and the format of design rules. Yield models, or European Yield
Models (EYM) as North Americans like to call them, are very widely accepted
for making strength predictions for connections with dowel fasteners.
However, EYM as currently implemented in many national and model
international timber design codes, can over predict a connection's capacity
by a considerable margin. Errors occur mostly when failure is due to splitting
of the wood member(s), rather than creation of plastic hinges in fasteners
and localised crushing of members by fasteners. Further, it is erroneous
to suppose that if codes specify use of large fastener spacings and
large fastener end distances then splitting of members can be reliably
avoided.
Conclusions
Small diameter steel tube fasteners are an effective means of achieving
strong and ductile structural wood connections. This is especially true if
steel tube fasteners are used in conjunction with slotted-in steel plate linkelements,
and join members manufactured from one of the newer generation
of engineered wood materials (e.g. Laminated Strand Lumber).
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.83