This paper presents a new seismic damage index feasible for automatic damage detection approaches. The definition is based on a new approach to damage detection based on the comparison between the accuracy of a given shape function in interpolating the response of a structure, in the damaged and the undamaged (reference) states. The comparison is carried out in terms of an error function defined as the difference between responses recorded on the structure and responses calculated via the spline interpolation. The variations of the error with respect to the reference value report the existence of damage in the region of the structure close to the locations where the higher changes occur. The method has been tested on the numerical model of a multistorey frame where damage has been simulated as a storey stiffness reduction. Results relevant to several damage scenarios show that the method allows both the detection and the localization of damage with a level of accuracy that increases with the intensity of damage. The application of the method requires a densely instrumented structure and the availability of responses recorded before and after a damaging event. The main advantages of the proposed method are that it does not require a numerical model of the structure nor heavy post-processing of recorded data. Hence, after a damaging earthquake, it can provide, nearly in real time, reliable information about the location of damage. These characteristics make the method a potentially useful tool for automated post-earthquake damage assessment.
