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small fabrication tolerances lead to an uneven load distribution among the
fasteners. A certain amount of balancing of these unequal forces is possible
by plastic deformations. Therefore the ultimate deformation capacity
of a multiple fastener joint has to be even larger than the capacity of a single
fastener connection.
As the timber fails in a brittle way, these deformations are typically only
possible if the failure occurs after significant plastic deformations of the
steel dowel. This failure mode (Type III, according to Johansen) can be
reached when fasteners of effective slenderness ratio bigger than a limit
slenderness ratio are used.
Conclusion
The tests on steel-to-timber connections show the influence of
– dowel slenderness ratio
– dowel strength
– end distance and spacing
– fabrication tolerances
on the strength reduction factor of multiple fastener connections.
The only difference to the research of Jorissen is caused by the fact, that
his values for the single fastener connections with rigid dowels are 30 %
lower than predicted by the Johansen theory. The Johansen theory has
been validated by many researchers for single fastener connections. As
long as there are differences of 30% in the resistances of the single fastener
connection it is not possible to establish reliable rules for multiple fastener
joints.
35-7-6 P Quenneville, M Kasim
Effect of row spacing on the capacity of bolted timber connections
loaded perpendicular-to-grain
Introduction
When a wood member is loaded perpendicular to the direction of grain by
a bolted connection, failure of the joint may result either by yielding of the
bolts, crushing of the wood under the bolts or by splitting of the wood
member. The first two failures are considered ductile, while splitting or
fracture of the wood is brittle and may lead to catastrophic collapse. Most
researchers agree that the bolt or line of bolts furthest from the loaded
edge initially carries the biggest share of the load, and, therefore, splitting
or fracture is more likely to occur at this level. The aim of any connection
design is to ensure that brittle failure does not happen prior to yielding of
the bolts or the wood, making it necessary to predict the splitting strength
of the wood when the joints are subjected to a force perpendicular to grain.
The Canadian design standard for wood structures (CSA 086) lays
down minimum requirements for the bolt spacing, edge distance and end
distance that define the layout of bolt groups. Tests conducted by many researchers
showed that these minimum requirements do not guarantee ductile
failure especially in multiple-bolt connections. In bolted connections,
the manner in which the load is transferred from the bolts into the wood
members affects to a great extent the capacity of the joints. Overlapping of
the load paths of adjacent rows of bolts in a joint, for instance, can reduce
the effectiveness of the bolts in the group, so that the joint capacity is less
than the sum of the capacities of the individual rows.
Cluster Principle
A load transferred to a wood member perpendicular to grain by a bolt
causes compression in the local area under the bolt. The ability of the
member to spread the concentrated load and the manner in which the load
is spread over the remaining depth can have a significant effect on the capacity
of joints. Overlapping of the load path from the bolts in a row with
those in adjacent rows within the same joint is not considered in design
standards. However, the concept of fastener cluster has been used by many
researchers, especially in Europe.
Foschi used the concept of cluster to predict the ultimate capacity of
riveted joints. Later, other researchers used and applied the same concept
to different types of fasteners. Görlacher used the concept of cluster in his
design proposal for the new Eurocode 5. He recommended that several
groups of fasteners can be considered as one cluster with one load carrying
capacity if the distance between the groups is less than 0.5h (h being the
depth of the member). On the other hand, they can be considered separate
groups if the distance between the groups is more than 2h. The assumed
angle of distribution with the grain direction is therefore less than 45° (depending
on the unloaded edge distance). A reduction factor is proposed if
the distance is between 0.5h and 2h. The 2h limit differs from that used by
Ehlbeck, Görlacher and Werner where only h was assumed. The latter,
however, gives a more consistent load distribution with the approach used
for interior connections.
CIB-W18 Timber Structures – A review of meeting 1-43 4 CONNECTIONS page 4.63