This work researches the modal frequencies identification of a footbridge structure by output only technique. The structural system of the footbridge herein considered is a continuous curved girder, i.e. belongs to a frequently used one all around the world in urbanized areas. It is located on the Princeton University campus in the US and serves as an on-site laboratory for short- and long-term research and educational purposes. The investigation utilizes measurements collected by long-gauge fiber optic strain sensors installed along the main-span and south-east leg of the structure. The power spectral density of the distributed long-gauge dynamic strain response in terms of curvature is used for detecting the modal frequencies of the footbridge. Pedestrian loading represents the external excitation. However, detailed information on the load intensity, frequency and distribution are lacking. This aspect represents the challenge of the present research with the aim to obtain as much information as possible about the modal characteristics of the footbridge from very limited dynamic information (only on the structural response) and without knowing the input conditions. Previously published research serves as a guide for evaluating the effectiveness of the results and the effectiveness of the employed methodology. This preliminary step paves the path to further research, i.e., (i) to the implementation of a refined finite element model of the entire structure for dynamic analysis, and (ii) to the development of a new wireless sensor network, by using as a reference the results from the existing embedded fiber optic sensors.