The objectives of this research were to evaluate the
behavior of actual metal-plate-connected (MPC) tension-splice
and heel wood truss joints under seismic loads from the
Northridge earthquake (tension-splice joints only) and under
loads from a large artificial earthquake (1.0 g's and 0.67
g's maximum horizontal and vertical accelerations,
respectively). A proposed sequential phased displacement
(SPD) loading standard was used to determine dynamic
characteristics of MPC joints: energy dissipation, damping
ratio and cyclic stiffness. A sinusoidal loading function
was used to determine the effect of cyclic loading on the
strength of MPC tension-splice and heel joints.
Joints were constructed from 2x4 in. nominal Douglasfir.
The size of the metal-plate-connectors for the tension-splice and heel joints were 3x4 in. and 3x5 in.,
respectively.
Strength and stiffness of the MPC joints after the
seismic and SPD loadings were compared to those properties in
a control group of joints tested to failure under a static
ramp load alone. Strength degradation was not observed in
the tension-splice and heel joints as a result of the
earthquake loading regimes. Stiffness degradation was
observed in the heel joint as a result of the large
artificial earthquake loads and in both the tension-splice
and heel joints as a result of the SPD loading. The SPD
loading regime did not affect the ultimate strength of the
tension-splice joints, but did reduce the ultimate strength
of the heel joints.
Dynamic properties, determined from the SPD loading,
depended on the magnitude of displacement (displacement
increases in amplitude with time). The damping ratio and
energy dissipation tend to increase as the SPD loading
progresses, whereas, the cyclic stiffness decreases. For
design, damping ratios of 4.3% and 3.8% are recommended for
the tension-splice and heel joints, respectively.
Cyclic loading can have a significant effect on the
strength of MPC joints depending on the amplitude of the cycles. Cycles with amplitudes greater than 20% of the mean
ultimate strength (determined from the static loading control
group) for the tension-splice joints and greater than 35% of
the mean ultimate strength for the heel joints tend to reduce
the strength