TECHNICAL ARTICLE Simplified Technique for Remote Monitoring of Deflection in Arch Structures I. Talebinejad, C. Fischer, and F. Ansari Smart Sensors & NDT Laboratory, Civil and Materials Engineering Department, University of Illinois at Chicago, Chicago, IL 60607 Keywords Optical Methods, Structural Testing, Nondestructive Testing Correspondence F. Ansari, Smart Sensors & NDT Laboratory, Civil and Materials Engineering Department, University of Illinois at Chicago, Chicago, IL 60607 Email: fansari@uic.edu Received: August 5, 2010; accepted: May 22, 2011 doi:10.1111/j.1747-1567.2011.00766.x Abstract This article describes a simplified technique to measure the vertical deflection of masonry arches in the presence of horizontal movements. One-dimensional laser Doppler vibrometer was utilized to measure the vertical displacement of the arch. A flat steel plate was mounted on the top surface of the arch crown to eliminate the measurement error due to the curvature of the arch. A reflective mirror was employed to guide the emitted laser beam normal to the surface of the steel plate. Vertical displacements recorded by laser Doppler vibrometer were compared to the readings of the mechanical dial gauge. Introduction The laser Doppler vibrometer (LDV) is designed to measure the vibration of structures based on laser interferometry. LDVs have been widely used for response measurement and modal testing of laboratory specimens 1,2 as well as applications in real structures. 3–7 LDVs have also been utilized for damage detection in structures based on vibra- tion measurements under both forced and ambient excitations. 4,8 – 10 LDV has been successfully used to measure the vertical deflection of some civil structures including girder bridges where horizontal (in-plane/surface- tangent) movement of the structure is negligible. 4,5 Because one-dimensional LDVs only measure out- of-plane (vertical/surface-normal) displacements, it cannot be used in situations where both vertical deflections and horizontal displacements are simul- taneously involved. Additional complications arise when it is desired to measure the deflection of curved instead of flat surfaces, such as in arches. In this arti- cle, a technique is introduced for the measurement of vertical deflections in a masonry arch subjected to horizontal motion based on a one-dimensional LDV. A hydraulic actuator was employed to apply horizontal support movements. The experimental technique separates the vertical deflection of the semicircular brick masonry arch from its horizontal motion. Laser Doppler Vibrometer A single-laser, one-dimensional LDV system was employed to measure the deflection of the masonry arch. The device includes a sensor head with an integrated scanning unit, a vibrometer controller, and a data acquisition system. Laser scanning and data processing are accomplished by the software package provided with the instrument. Details pertaining to the operational principle of the LDV are beyond the scope of this article. A brief description is provided here for completeness. LDV uses the Doppler effect to measure the vibration of the structure. It consists of a high-precision interferometer that splits the light into two, namely a reference and a measurement beam. The reference beam is pointed directly to the internal photodetector, while the measurement beam is incident on the test object where the light is scattered by the moving object. Depending on the velocity and displacement of the object, the backscattered light is changed in frequency and phase. Once analyzed, superposition of the backscattered light with the reference beam provides the instantaneous Experimental Techniques (2011)  2011, Society for Experimental Mechanics 1