In this paper, the structural behaviour of a long semi-integral HSRB, i.e., Unstrut Viaduct, under different traffic loads was studied by both SHM and static calculation using numerical simulation. The demanding requirements of semi-integral HSRB for both flexibility and rigidity were assessed by measuring the deformation of the superstructure and piers under quasi-static loads, as well as the structural responses under braking and high-speed loads, respectively. The results showed that the separating pier of the semi-integral bridge enables larger deformation freedom, which is beneficial to reducing the restrained stresses due to temperature, creep and shrinkage. The separated superstructure here would induce larger pier curvature and a greater warping effect under single-track loading. The coupling of continuous welded rail (CWR) at the bridge joint and the H-connection of the separating pier pair also caused the complex interaction of the two connected structural blocks. The rigidity of the semi-integral HSRB under braking and high-speed loads was also proven. The dynamic longitudinal stiffness under braking loads derived from measurements is unexpectedly 12 times that in the static calculation, showing the robustness and great load-bearing potential of the semi-integral bridge. The initial assumption in the static calculation, which treats the slab track as external loads without stiffness, is a conservative approach. However, incorporating both the mass and stiffness of the slab track into the model generally offers a more realistic prognosis.