Abstract:
In recent years, log structures have been marketed as an alternative
to conventional light-frame wood structures. Log structures are constructed
using round timbers (or manufactured timbers) that are stacked vertically
and have interlocking corners. Thru-rods and lag screws are used to
provide inter-log connections. This project was conducted to evaluate the
lateral force resisting pathways that are developed by anchor bolts, thrurods
and friction.
An experimental study was used to investigate inter-component
friction, force-displacement behavior and yield mode of sill log-foundation
details. A family of finite-element models was developed to assess the
force-displacement response of log shear walls without the corner
connection for a set of construction variables that included foundation
connectivity, aspect ratio, wall perforations, and thru-rod hole diameter. The experimental study used test specimens that represented
common construction details for sill log-foundation anchorage. One detail
had the sill log on the floor diaphragm and the other detail had the sill log in
direct contact with the sill plate. A sinusoidal cyclic testing protocol was
used to assess friction between the sill log and plywood surfaces. It was
shown that a reasonable value for the coefficient of friction is 0.4. The sill
log-foundation details were tested statically and then with a fully reversedcyclic
quasi-static test protocol. The force-displacement curves showed an
initial stiffness, slip, and post-slip stiffness and capacity. The open shape of
the hysteresis diagrams suggests that energy dissipation occurs primarily
through friction rather than bolt yielding and material damage. Connection
details were shown to have capacities at least 4.8 times greater than that
needed for an upper bound on design base shear as calculated following
the Uniform Building Code.
A finite-element model using ANSYS 6.0 was developed for a
representative unit log shear wall and a sill log-foundation assembly. The
wall model was eight logs high (2438 x 2438 mm) and included foundation
anchorage, friction, and thru-rods. Linear springs were used to model loglog
normal contact, while nonlinear springs were used to model log-log
friction and thru-rod behavior in oversized holes. The wood materials were
given linear elastic, planar isotropic properties. The force-displacement
behaviors of the wall model and the sill log-foundation model were verified
with test data. These models were then used in a parametric study to evaluate the lateral force resistance response of a log shear wall given a
range of slip force and design alternatives. It was shown that log-log friction
affects the initial wall stiffness and slip force. A 50 percent loss in thru-rod
tension decreases the log-log slip force by the same amount. Oversized
thru-rod holes can be the source of increased lateral displacement; an
increase in hole size of 13 percent can increase the wall displacement at
the plate log by 31 percent. Window and door openings are accompanied
by additional thru-rods, and the additional thru-rods improve wall forcedisplacement
response relative to walls with minimum thru-rod hardware
and no openings.
This research did not address the three-dimensional system
behavior that is expected to develop in a box-like structure with integral
corner connections. Further research is needed to assess the role of
integral corner connections in three-dimensional response to lateral loading.
