Quisi Bridge located in Benissa (Alicante) is an historic steel truss structure, which is investigated with different techniques to assess fatigue damages. A method based on vibration measurements, is illustrated to determine defect-induced reduction of stiffness, (combination of damage intensity and extension). Natural frequency change for a structural damage in a truss member is quantified through a classical FEM procedure in which a damaged truss element is defined and implemented. This model is used to produce pseudo-experimental response of a damaged truss which is dynamically excited by white noise. Stochastic Subspace Identification (SSI) has permitted to identify the main modal parameters related to both damaged and undamaged truss system. A damage index depending on Stiffness Reduction Factor (SRF) permits to determine the damage description based on measured quantities.