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tion of the yield capacity of dowel type fasteners with thin diameters like
nails and staples. Whether this is correct is discussed in this paper.
Conclusion and suggestion for code modification
Since the values for the yield capacity obtained directly from tests are
higher than the values obtained whichever equation mentioned in this paper,
it is not on the unsafe side to use Eq. (1) for small diameter dowel
type fasteners as well.
Driven by the idea to keep it as simple as possible, we like to suggest to
keep Eq. (1) into Eurocode 5 for all diameters.
41-7-1 M Snow, I Smith, A Asiz, M Ballerini
Applicability of existing design approaches to mechanical joints in
structural composite lumber
Introduction
Despite recent increase of Structural Composite Lumber (SCL) consumption
in construction, there is little knowledge about how to make efficient
connections using such material. From a design perspective, it is essential
that failure characteristics in SCL connections be recognized and understood.
To date a very conservative approach based on an assumption of
equivalency between performances of SCL and solid wood lumber connections
has been adopted in day-to-day design practice in North America.
The purpose of this paper is to investigate applicability of existing design
approaches to mechanical joints in SCL based on comparison with test data
collected at the University of New Brunswick .
Experimental work investigated failure characteristics of mechanical
joints constructed using the types of SCL known as Laminated Veneer
Lumber (LVL), Parallel Strand Lumber (PSL) and Laminated Strand
Lumber (LSL). These materials are manufactured as proprietary products
in nominal sizes similar to those for dimensional lumber, and their mechanical
responses are highly dependent on manufacturing process variables.
LVL, PSL and LSL corresponded to the most diverse alternative
commercially available SCL products at the time of the investigation.
Thus tested joints were surrogates for establishing how mechanical responses
of all mechanical joints in SCL might differ from mechanical responses
of similar joints in sawn lumber. Joints were constructed using
commonly available dowel-type metal fasteners like bolts, nails and
screws. Centre members were either SCL or sawn lumber, with the lumber
alternative being the benchmark situation. Side members were made from
sawn softwood lumber, SCL, steel plates or a high strength transparent
plastic. Single or multiple fasteners in single or double shear arrangements
were loaded in joint configurations.
General discussion
Mechanical joints, especially those made using multiple dowel-type fasteners,
are complex systems exhibiting a range of complex failure mechanisms.
Therefore it is extremely difficult, and arguably impossible, to
achieve robustly accurate predictions of which failure mechanisms will
govern for particular joint designs. Similarly it is very difficult to predict
strengths of joints accurately. Further, if it is desired to predict design capacities
of joints using simple models, whether they are explicitly empirical
or semi-analytical, it should not be surprising if they fail to make robust
predictions of governing mechanisms or of failure loads. Introducing
SCL as alternative types of structural members in lieu of products like
sawn lumber will not in general alleviate the conundrum in general, but
could in specific cases. For example, as discussed in detail by Snow, using
a material like LSL instead of easily splitting wood products largely eliminates
the need to consider brittle mechanisms and simple EYM type design
models yield good results. More generally the message to be gained
from this paper is that particular design models will only yield good predictions
under well defined circumstances (i.e. for those of which they are
intended and calibrated.
Conclusion
The existing Canadian and Eurocode 5 design methods for joints in wood
products made using dowel-type fasteners did not perform consistently
well for joints with SCL members. The same is true for proposed alternative
models that more explicitly address the possibility of brittle failure
mechanisms governing the design of joints. It is imperative therefore that
existing models and code rule be used with caution especially in an environment
where the available range of of structural wood products is evolving
rapidly.
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.27