In earthquake affected areas the speed and reliability in assessing the damage suffered by strategic structures, such as long-span bridges, is of paramount importance both for civil protection operations and for organization and coordination of immediate remedial measures for the structure safety. In this paper we present the results obtained by applying a damage identification method termed Interpolation Damage Detection Method to a numerical model of the Shimotsui-Seto bridge, a suspension bridge with a long span steel truss deck (940 m). The method allows to detect localized reductions of stiffness along the bridge deck on the base of accelerometric responses recorded on the main girder during a damaging seismic event, or during an aftershock following the onset of damage. This is possible as long as responses recorded at the same locations on the undamaged structure are available. The response of the suspension bridge, subject to seismic excitation, has been calculated in the ANSYS framework using a finite element model derived from the original design data. In order to reproduce real life conditions, the numerical results in terms of temporal responses are artificially modified by including a background noise characteristic of classes of Micro Electro-Mechanical Systems type sensors. The reliability of the Interpolation Damage Detection Method has been numerically verified by simulating the damage through a reduction of stiffness in one or more elements of the deck. Several different locations of damage have been considered in order to study the influence on damage location the results provided by this Structural Health Monitoring methodology.