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where F is the fundamental matrix (Faugeras 1993). The 3D position of the markers can then be
computed from the acquired marker sets using the intrinsic and extrinsic parameters of the camera
system (e.g. position, focal length). Positional information (x,y,z) on a per marker basis and orientation
(yaw, pitch, roll) is available through groups of markers aligned along two different axes.
EXPLORATORY STUDY
An exploratory study was conducted to investigate the applicability of a vision-system for capturing
the response of a specimen mounted on UCIs bi-axial shake table. In this case, a scale 5-story steel
moment frame structure with a dual damping system (Villaverde et al. 2002) subjected to uni-axial
seismic motions provides a simple structure for evaluation of this new technology. The frame structure
is 2.44 m in height and mounted with a roof isolation system. Figure 2(a) shows an elevation of the
specimen mounted on the shake table with strategically placed spherical tracker elements (which show
up as bright reflections in the photograph). The tracker elements (markers) are 12.5 mm diameter
Styrofoam spheres wrapped in reflective tape. Compared with the mass of the specimen and the
conventional transducers, the mass of the tracker elements is negligible. Four high-resolution
(1280x1024 pixel) CCD cameras (Vicon, Oxford Metrics, Oxford, England) with maximum capture
rates of 250 Hz were mounted on tripods and strategically placed facing the frame structure. The
cameras were rigidly mounted with light emitting strobes.
The model building is instrumented with a total of six LVDT displacement transducers, one attached at
each fluid viscous damper (brace element) and one at the roof damping system. Five accelerometers
were installed at the individual floor levels. In total, 21 spherical tracker elements were mounted at
floor levels and at several locations on the base of the shake table for reference, as shown in Figure
2(a). The use of the mass less spherical trackers allows multiple measurements for comparison and
potential averaging. Figure 2(b) shows a detailed photograph of the roof level including selected
placement between the roof and isolation system of the trackers. It took approximately 30 minutes to
array the trackers in strategic positions on the frame.
It is important to note that the LVDTs at the damper level provide an additional resistance to the
damper element and overall system response, which is large relative to the scale of the model
specimen. Although it may not be evident from the photograph, a roof isolation system is being studied
in this work (Villaverde et al. 2002), thus the desired results are a comparison between two systems
(with and without roof isolation), where each includes engaging the damping at the braces. Therefore,
for these particular experiments, the additional resistance due to placement of the LVDT in-line with
the brace is not important.
The cameras were evenly spaced (at approximately 2 meters on center) along the line of action of
lateral input seismic motion. They are aligned such that at least three of the cameras can observe any of
the desired points at any given time. The closest camera was approximately 5.5 m, and the farthest
camera was 6.0 m from the center of the specimen.
Since positional information is computed by triangulation of the image data obtained by each camera,
careful calibration is required. Calibration should ideally occur prior to running the tests but can be
reapplied to the acquired raw data should a better calibration be required at a later time. Two phases of
calibration were conducted: (1) static calibration and (2) dynamic calibration. Static calibration is
achieved by placing a rigid L-frame within the static viewing volume. Markers are mounted on