Usually, the seismic improvement interventions do not allow the use of the building during the execution of the works. The interruption of use in many cases is not allowed especially for buildings with strategic or public service functions. In recent years, an alternative approach to intervention has spread that allows to operate exclusively outside the structure through the use of a technology called structural exoskeleton. The exoskeleton in many cases consists of bi-dimensional or three-dimensional struss steel structure. The goal of this technology is to transfer the most part the seismic action to the external structure. This allows to avoid reinforcements of the existing structure. In order for the building-exoskeleton system to be effective against seismic action, the exoskeleton must be sufficiently stiffness and the coupling device between the two structures must not produce internal constraints and generate unwanted stresses for the existing structural elements. A critical aspect for the application of this technology is the transfer of the floor seismic forces from the existing structure to exoskeleton. The present work illustrates the study of an innovative building-exoskeleton coupling device to transfer the seismic shear forces of the floor from the existing structure to the exoskeleton without generating in the plane and out of the plane bending interactions. The study is performed on a real case study concerning the seismic improvement of a strategic building with external steel reticular shear walls. In conclusion, the results of the non-linear numerical analyses on the global models of the building-exoskeleton system and the results of the local numerical analyses of the coupling device are illustrated.