- Engineering analysis of heavy timber structures under dynamic loads
is generally lacking. Some literature suggests that heavy timber structures
are stiffer than expected, but popular engineering opinion is to the contrary.
In addition, prior research has shown that passive friction dampers can be
used in timber structures for the intended purpose of increasing energy
dissipation under seismic loadings. This study was conducted based on the
hypothesis that heavy timber structures require bracing for seismic design
and further that heavy timber performance under seismic loads will benefit
from the addition of passive dampers.
A computational study was designed to assess the performance of a
two-dimensional model heavy timber structure when it is subjected to a
seismic ground motion. Five building systems were included in the
investigation; all were two story, single bay timber frames with 1-percent
inherent material damping. The frames were differentiated by connection
characteristics, the presence or absence of bracing, and the presence or
absence of friction damper devices. The five building systems were: (1) moment-resisting frame (rigid connections, no bracing); (2) semi-rigid
frame (semi-rigid connections, no bracing); (3) pinned braced-frame (pinned
connections, concentric braces); (4) braced damped pinned-frame (pinned
connections, cross braces with dampers); (5) semi-rigid damped frame
(semi-rigid connections, concentric bracing with dampers). By subjecting
these five structural configurations to the same ground motion, the effects
of the connection rigidity, bracing, and dampers can be resolved as
measured by story drift and member forces.
The ground motion used was a corrected, unscaled, USGS Loma
Prieta record with maximum acceleration of 1.05 g. The study frame was
designed with access floor loading with computer use for a geographic
location near San Francisco following the Uniform Building Code and the
National Design Specification for Wood Construction. Beams and columns
were glulam. The semi-rigid connections were designed as a dowel circle
connection following DIN 1052, which is widely used in Europe.
The software used for the investigation was DRAIN-2DX. Semi-rigid
connection behavior was implemented using the Florence Model. The
DRAIN-2DX software does not have a dedicated damper element. A
damper element would produce a rectangular displacement-force
hysteresis when subjected to a fully reversed cyclic load; the key features of
this hysteresis behavior are brace stiffness followed by nearly zero stiffness
at a defined force. A brace-damper superelement was created by using two
compression/tension link elements in parallel with an inelastic bar element.
This combination of elements produced the necessary behaviors, and the
performance was verified in a single story, single bay frame subjected to a
piecewise forcing function and demonstrated in a two story, one bay timber
The results of the analyses for the five building systems are specific
to the Loma Prieta seismic ground motions. The semi-rigid moment-resisting
frame remained within allowable drift limits, which suggests that the dowel circle connection produces satisfactory moment resistance. The
braced pinned-frame and the braced-damped, pinned and semi-rigid frames
responded similarly to each other because the large cross-section timber
brace in the low rise, low mass system results in an extremely stiff
structure. These structures are so stiff that the damper does not function.
Evidence suggests that a rotational damper may be functional in semi-rigid
connections of unbraced timber frames, but computational development of
the rotational damper is needed.