The simplest soil-structure platform model is considered in the paper. Soil foundation is modeled by six pairs of springs and viscous dampers. Impact of these soil dampers on the modal damping values is calculated by various methods and compared. First modal damping is calculated by direct approach, adding the contribution of the soil dampers to the modal damping value calculated for stand-alone structure model. Then alternative calculation is carried out using the substitution of the viscous damping by certain material damping — this approach makes it easy to obtain total modal composite damping for the further dynamic analysis. However it is demonstrated that for the first natural modes the alternative approach means the underestimation of modal damping, leading to the non-conservative results.