  Upon completing the upgrade from one to six degrees of freedom of the Outdoor
Shake Table at UCSD in 2019, forced vibration tests were carried out to
identify the dynamic characteristics of the reaction mass and soil system. This
report describes the motivation, execution, and results from such tests, which
independently excited the reaction mass in four degrees of freedom:
longitudinal, transverse, yaw, and vertical. The report discusses the frequency
response curves and deformation patterns from which the natural frequencies,
damping ratio, mode shapes, and rigid body motion were determined. The first
objective of the study was to investigate if the dynamic properties of the
system had dramatically changed after the upgrade by comparing the results to
those from forced vibration tests performed 20 years ago, during the
construction of the facility. In addition, most recent tests also contributed
with results from the vertical degree of freedom, which had never been tested.
The second objective was to obtain high-quality response data of the system
that will be used to develop a high-fidelity computational model of the
reaction mass in future research. A comparison of results showed a slight
difference of 0.5Hz in the natural frequency of 2 degrees of freedom. Moreover,
maximum displacements in the recent tests were overall larger than the previous
ones with few exceptions. The report thoroughly discusses the several sources
of discrepancy between the past and most recent results. Finally, test results
allowed us to estimate the system's response if the shake table actuators were
to be used at their maximum nominal capacity. Small displacement and high
damping results were consistent with those of previous tests and further
validated the design of the reaction mass.
