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Traditional sensors, such as accelerometers and displacement transducers, are widely used in laboratory and field experiments in earthquake engineering to measure the motions of both structural and nonstructural components. Such sensors, however, must be physically attached to the structure and require cumbersome cabling and configurations and substantial time for setup. For reduced-scale experiments, these conventional sensors may substantially alter the dynamic properties of the system by changing the mass, stiffness, and damping properties of the specimen. Moreover, it is very difficult with traditional sensors to capture the three-dimensional motions of light or oddly shaped components such as microscopes, computers, or other building contents. In this paper, the methodology of light-based motion tracking is applied to the measurement of the three-dimensional motions of various types of equipment and building contents commonly found in biological and chemical science laboratories. The system is comprised of six high-speed, high-resolution charge-coupled-device (CCD) cameras outfitted with a cluster of red-light emitting diodes (LEDs). Retroreflective (passive) spherical markers discretely located in a scene are tracked in time and used to describe the behavior of various types of equipment and contents subjected to a range of earthquake motions. Results from this study show that the nonintrusive, light-based approach is extremely promising in terms of its ability to capture relative displacements in three orthogonal directions and complementary rotations. CE Database subject headings: Tracking; Digital techniques; Earthquakes; Ground motion; Shake table tests.

Unmanned aerial vehicle (UAV) imagery has recently emerged as a promising alternative for operational condition inspection and postdisaster damage assessment of civil structures (e.g., bridges and buildings). However, the use of such sensing techniques for quantitatively tracking the subtle (centimeter-level) variations of the responses of structures has been limited. This is largely due to the difficulties related to obtaining accurate location measurements of the cameras aboard small UAV platforms. To address this research gap, we propose a video analysis methodology for tracking the displacement response of buildings subject to dynamic loads using camera-equipped UAV platforms. The movement of the image sensor on-board the UAV platform is corrected to allow image-by-image natural feature detection and tracking. In this methodology, the image processing procedure does not rely on the camera position and orientation. As such, the approach first corrects for image distortion introduced by UAV drift and subsequently extracts the dynamic displacements of the building by tracking its natural features at the pixel level. Motion-tracking errors are investigated by analyzing the building displacements using pre- and postevent videos. The proposed methodology is validated by monitoring the dynamic response of a full-scale building during a shake table test program. Uniquely, global positioning system (GPS) displacement measurements are independently utilized to validate the proposed UAV video-based method and assess its effectiveness for capturing the dynamic responses of full-scale structures with a level of precision that is sufficient for engineering applications (less than 2 cm root-mean-square errors). [ABSTRACT FROM AUTHOR]