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Analysis,
hysteresis loops:
A
basic building block in the study of the dynamic
response of wood structures is an understanding of
the hysteresis loops for connections. These are
characterized by the nonlinear behavior of the
fasteners and the wood medium supporting them, and
the pinching which develops as slackness develops
during cyclic loading. Since the process is
history dependent, it must be adjusted to
different excitations. Characterization of
hysteresis loops is normally done by adjusting a
numerical model to a test result for a connection
under a selected experimental history. However,
the approach taken in this project is to calculate
the hysteresis loops from basic properties of the
fasteners and the wood medium, and embeding this
calculation into the dynamic analysis of the
complete structure. Although more computational
intensive, the approach permits automatic
adjustment to any excitation history, as well as
to changes in connection geometry and mechanical
characteristics.
A
general hysteretic subroutine, HYST, has been
developed for the calculation of the loops for a
single fastener in one or more layers of wood
medium. In the case of multiple layers, some of
these can displace relative to the others. Input
for HYST requires elastic and plastic properties
of the fastener and the embedment characteristics
of the wood medium.
The
following figures show examples of calculated
loops for 1) a single fastener with a enforced
displacement history at the head, and 2) a single
fastener joining three layers, where the middle
layer displaces with respect to the outside ones.
An
extension of HYST, including friction between the
shank of the fastener and the surrounding wood
medium, required for pull-out predictions, has
also been developed under this project.
HYST
model details can be found in:
Foschi,
R.O. 2000. "Modeling
the hysteretic response of mechanical
connections for wood structures",
Proceedings, World Timber Engineering
Conference, Whistler, B.C. Canada, July
31-August 4, 2000.
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