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CCEER-17-06: Large-Scale Experimental Verification of an Optically-Based Sensor System for Monitoring Structural Response
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Inter-story drift ratio (IDR) is a ratio of the relative translational displacement between adjacent floors (inter-story drift or ID) to the story height. It is an important engineering demand parameter and indicator of structural response during earthquake action. Current codes usually specify limits on IDR to ensure structural performance at certain level. Correct measurement of IDR can be used to: 1) evaluate the state of health of structures and allow a rapid post-event assessment, 2) improve/validate design performance measures (e.g., drift limits), and 3) validate the complex numerical models for high-rise buildings and/or flexible moment-resisting frames, where IDRs, as opposed to the usual strength-capacity ratio, often control structural designs. Based on the definition, IDR depends on accurate measurement of ID. Conventional techniques for measuring ID include accelerometers, GPS, displacement transducers, and video processing and so on. Each of these methods has limitations in their applications in terms of accuracy, cost and installation. As an alternative, a newly developed optically-based system was employed to measure ID. This system is a non-contact method, which utilizes lasers and recently developed Discrete Diode Position Sensors (DDPS). Its advantages are: 1) low cost, 2) easy installation, 3) elimination of rigid reference frame, and 4) transient and direct measurement of ID. The efficiency and accuracy of this system have been demonstrated by small-scale shake table tests and numerical simulations. However, its accuracy in the application to large-scale or in-situ buildings is still unknown. This report extend the validation of this system to the large-scale structure. To this end, shake table experiments on a 1⁄4 scale three-story steel frame were recently conducted in Earthquake Engineering Lab of University of Nevada, Reno. Details of the experiments including design, instrumentation, motion selection, test protocol, and installation are introduced. Displacement transducers (string pots) were installed horizontally along with rigid reference frames and diagonally spanning each bay as the ground truth instruments. The test frame is very stiff in one direction while much more flexible in other direction. As a result, only the response in the flexible direction is considered for the purpose of validation. The results show that the IDs measured by the horizontal and diagonal string pots almost overlap with each other throughout the records. Hence, the ground truth measurements can be used with confidence. In addition, it is found that with the correction of joint rotation, the IDs from the sensor system are in good agreement with the ground truth measurements throughout the records. This observation clearly reveals promise of the sensor system in directly measuring the dynamic IDs of large-scale structures during earthquake shaking. Therefore, it may be used for the critical facilities like high-rise building and nuclear power plants. Furthermore, numerical model of the test frame was developed in SAP2000 to reproduce its response during the tests. Comparison was made with the ground truth measurements for both inter- story drift and relative roof displacement histories. Good agreement was obtained for both quantities in terms of both frequency content and amplitude. These results show the confidence of the numerical model. Therefore, the validated numerical model can provide as an important tool to design the sensor system and assess its performance in multiple types of structural systems and configurations. Further large-scale experiments are needed to continue the validation of the sensor system for addressing issues such as biaxial effects and residual displacements.
Report No. CCEER-17-06