# Image Analysis Applications for Building Inter-Story Drift Monitoring
## 5 Conclusions  
This work developed an image-based inter-story response measurement method and developed a small device prototype with a camera and a single-board computer, aiming to provide a cost-effective measurement method for structural health monitoring. With the aid of modern computer vision techniques, the developed method includes an inter-story motion model, a camera movement model, and an iterative procedure that estimates the inter-story responses. Instead of adopting triangulation that requires accurate time synchronization and a complicated calibration process, this method requires only a single camera installed at either the ceiling above or the floor below the target story for monitoring and is capable of capturing three degrees of freedom of the inter-story responses. Considering that images are sensitive to any slight camera rotation, this method estimates the camera rotations by monitoring reference points located at the same floor that the camera is fixed at.  
A series of experiments were performed to verify the developed method. A one-tenth reduced scale one-bay portal frame specimen was constructed and used for the experiments. In the static experiment that deformed the specimen through a cyclic uniaxial inter-story drift history up to 2% (i.e., the ratio of 8 mm to 400 mm), this method captured the drift with a maximum error of 0.025% (i.e., 0.1 mm). Two dynamic experiments were carried out that gave sinusoidal motions of 1-Hz and 3-Hz, respectively, at the bottom of the specimen. In the 1-Hz experiment, the inter-story responses were within \(\pm 0.45\%\) (i.e., \(\pm 1.8\) mm), and the measurement errors were approximately 0.075% (i.e., the \(\pm 0.3\) mm) over the transient response, gradually reducing to 0.025% (i.e., of 0.1 mm) during free vibration. In the 3-Hz experiments where the inter-story responses were up to 4.5% (i.e., 18 mm), the maximum measurement during the transient response was 0.5% (i.e., 2 mm), reducing to 0.1% (i.e., 0.4 mm) during the free vibration. The presence of image blurring induced by the rapid motion of tracking points was discussed, possibly resulting in significant measurement error. Technical issues induced by these effects should be seriously addressed. In addition, a realistic ground motion, which has sophisticated frequency contents, could also induce complicated imaging effects. It requires further study with improved analytical consideration and more realistic experiments to further improve the accuracy of the image analysis method. The rolling shutter effect was also discussed as possibly contributing to errors in the measurement. While this method using a low-cost device does not aim to replace conventional structural health monitoring, it provides fairly satisfactory accuracy for many practical applications in engineering, especially considering rapid developments in hardware performance and the resolution of digital cameras and imaging technology.  
Investigation, Y.-S.Y.; Supervision, Q.X.; Data curation, P.-Y.C.; Validation, J.-H.W., C.-H.L., C.-C.L. and C.-T.C.; Software, J.-S.C. All authors have read and agreed to the published version of the manuscript.  
This work is partially funded by the Sinotech Engineering Consultants Inc. (Project number RP19901) and the Ministry of Science and Technology (Project numbers: MOST 108-2625-M-027-001 and MOST 109-2625-M-027-002).  
The authors would like to acknowledge the National Center for Research on Earthquake Engineering in Taiwan for providing high speed cameras and experimental advices.  
The authors declare no conflict of interest.