AbstractThe interstory drift ratio (IDR) is a key engineering parameter utilized by several codes and standards to quantify structural performance and correlate with potential structural damage following a seismic event. A number of methods have been explored for measuring building interstory drift, including approaches that rely on accelerometers, linear variable differential transformers (LVDTs), global position systems, combinations of cameras and optical sensors, and, most recently, on machine learning surrogate models. Nevertheless, limitations related to accuracy, cost, and challenges associated with data processing and interpretation still hinder the broad utilization of such methods for rapid post-event damage assessment. This paper presents a recently developed optical sensor, termed a biaxial discrete diode position sensor (2DDPS), which is able to directly measure earthquake transient and residual interstory drift in the two horizontal directions and transmit structure observables to offsite locations in near real-time. The sensor performance is herein evaluated through a series of shake-table tests carried out on the single 2DDPS to provide a statistical basis for sensor performance evaluations and on the 2DDPS installed on a 3D two-story frame subject to realistic earthquake biaxial excitation to evaluate system performance under realistic deployment conditions. The experimental campaign design and execution has been supported by advanced computational models. Results of the investigation demonstrate that the 2DDPS can reliably measure transient and residual bidirectional interstory drifts with an accuracy of the order of a millimeter. A critical review of the observed error and strategies to reduce it are also provided. Evidence from the presented study demonstrates that the 2DDPS has the potential to be utilized as a key tool for rapid, postevent decision-making and field inspection prioritization.