###### Abstract
The influence of the SSI can be reduced to the backfill material's stiffening of the abutment walls and its effect on the eigenfrequencies. In addition, a significant increase in natural frequencies due to a stiffening ballast layer can be largely excluded according to the investigations in [13]. In addition, the damping characteristics react sensitively to the SSI modelling method. Structural damping is considered in European and German standardisation and is most comparable to material damping [3; 4]. Higher damping levels may only be used if further investigations, e.g., measurements, verify them. These further investigations probably include the consideration of the SSI, but this is not explicitly addressed. This methodology requires a heightened sense of caution and places significant responsibility on the design engineer. In practice, however, these circumstances often result in the unintentional neglect of the SSI and its additional damping mechanisms. The absence of these relevant damping effects leads to large capacities of the system response in the resonance region. Despite its complexity, modelling the SSI requires knowledge of the dynamic soil stiffness, which is usually described by the soil profile's shear wave velocity \(V_{S}\). The described relationships highlight the importance of understanding the dynamic interactions inherent in portal frame bridges and emphasise the need for practical modelling methods. In practice, the above-mentioned uncertainty in modelling is becoming apparent since the dynamic characteristics of short bridges, especially portal frame bridges, show a considerable discrepancy between numerically calculated and in situ-measured characteristics (e.g., [11; 14]). In particular, [PERSON] points out that the natural frequencies are underestimated by 30-60% [14]. In the case of modal damping, the discrepancies are particularly pronounced. Given that the dynamic properties used as the starting point for the analysis are subject to significant deviations, this circumstance can lead to an overestimation of the system response, particularly in resonance scenarios. This paper introduces a new numerical Calculation Model designed to incorporate application limits and thereby reduce the impact of shear-wave-induced structure-soil-structure interactions (SSSIs) on abutments. While the model is partly based on established principles [15], its innovation lies in applying a unique modelling technique complemented by considering the application limits associated with the SSSI. Another foundational aspect of the presented approach rests upon a numerical study, as presented in [12], exploring the core SSI effects inherent in the embedded frame. The performance of the Calculation Model was evaluated through a comparative analysis of the dynamic characteristics of four concrete portal frame bridges in Germany. The investigations were carried out using an operational modal analysis (OMA). An introduction to the principles of the SSI and its modelling methods, including the SSSI effect of adjacent foundations, is given in Section 2. This is followed by the proposed Calculation Model in Section 3. A description of the investigated bridge structures and the identification of the dynamic characteristics are given in Section 4. The results of the Calculation Model are compared with the in situ tests in Section 5. A final summary is given in Section 6.