The theory of operation of a fiber-optic-based strain sensor system, suitable for long structures, is described. The proposed sensor design is composed of several hundreds of sections separated by reflectors in order to monitor structures of more than 1 km of length. A quasi-distributed sensor structure accomplishes the best condition monitoring, where each section of the sensor integrates the measured magnitude, obtaining a value associated to the average shortening or elongation in this section. Tools for the design of the optical reflectors of the sensor are also provided, including a study of the optical noise due to multireflection within the system.