Past earthquakes indicate that pounding between inadequately separated structures may cause considerable damage or even lead to collapse of colliding structures. Intensive study has been recently carried out on mitigation of pounding hazards. The assessment of damage due to structural pounding, or its prediction under a particular ground motion, requires the knowledge of the maximum impact force value expected during the time of earthquake. The aim of the present paper is to consider the concept of impact force response spectrum for two closely-spaced structures, which shows the plot of the peak value of pounding force as a function of the natural structural vibration period. The spectrum can be used as a practical tool to assess the magnitude of the expected pounding-induced damage and, if necessary, to apply some damage reduction techniques. In the analysis, both interacting structures have been modelled by single-degree-of-freedom systems and pounding has been simulated by the non-linear viscoelastic model, which accounts for the energy dissipation during impact. The examples of response spectra show that the appropriate selection of the separation gap between structures as well as the dynamic structural parameters, such as the natural vibration period, mass and damping, might have a significant influence on the intensity of damage due to earthquake-induced pounding.