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precisely defined distances on the rigid L-frame. Dynamic calibration is achieved by sweeping a wand
with a marker mounted at each end through the desired viewing volume. For this exploratory study, the
system was statically calibrated to be within 1 mm positional resolution and dynamically calibrated
such that each of the camera's were within 0.5 mm precision of each other.
Once the system is calibrated, the entire environment can be instrumented with an arbitrary number of
passive markers. However, careful consideration must be given to the density of marker placement as
related to the resolution of the camera system. If makers are placed too close to each other it will be
difficult to distinguish them in the obtained image. In addition, if markers are placed behind an element
in the scene, they will be occluded during motion capture. This can be challenging since elements
within the scene move thus similar to conventional transducers, the magnitude and direction of
movement must be anticipated.
EXPERIMENTAL RESULTS
Two earthquake motions were used and multiple runs were conducted (with and without the roof
isolation system) to evaluate the accuracy and repeatability of the motion tracking system. In total,
seven shake table simulations were conducted. The first input motion was a modified version of the
SCT acceleration record from the 1985 Mexico City earthquake. The second motion was a modified
form of the Takatori acceleration record from the 1995 Kobe earthquake. Each of these motions were
tuned to the fundamental frequency of the model building (f = 2.375 Hz) (Villaverde et al. 2002) and
applied as a base excitation to the structure.
Figure 3 shows a comparison of inter-story drift ratios at the roof level of the model structure,
measured using the vision-based approach and the LVDTs. In this case, the difference in maximum