## 5 Conclusions and Future Directions  
In this paper, the design of advanced structural vibration control systems for the seismic protection of adjacent multi-story buildings has been investigated. The proposed approach considers multi-actuation schemes that combine interstory and interbuilding force actuation devices implemented at different locations of the structure. Using an advanced static output-feedback \(H_{\infty}\) controller design methodology, active and passive vibration control systems have been obtained for a multi-story two-building structure equipped with a selected set of linked and unlinked actuation schemes. After studying the corresponding frequency and time responses, the following positive points can be highlighted: (i) The proposed design methodology allows dealing with a wide variety of actuation schemes. (ii) The obtained control systems provide a significant reduction of the frequency response in the main and secondary resonant modes. Moreover, they also produce a significant reduction of the maximum interbuilding approaches and the absolute interstory drift peak-values in both buildings. (iii) The obtained control systems produce positive results in reducing the buildings acceleration responses. (iv) Control configurations with interbuilding linking devices provide a higher protection against bounding events and produce lower control-effort peak-values. (v) Lower interstory drift peak-values are attained in buildings equipped with interstory actuation devices. (vi) A remarkable performance level is achieved by the passive control systems. (vii) In general, more effective and better balanced results are obtained with a larger number of actuation devices. Additionally, the following negative aspects can be pointed out: (viii) The design procedure can fail due to feasibility issues in the associated LMI optimization problem, specially in the constrained passive designs. (ix) The controller \(H_{\infty}\)-norm is a suitable index to obtain effective controllers, but it cannot be used to identify optimal configurations of the actuation system. (x) Due to the computational cost and complexity, the proposed design methodology is only effective for structures with a moderate number of stories.  
In summary, the observed results indicate the convenience of using multi-actuation systems that combine interbuilding linking devices and interstory actuators implemented in both buildings. The good behavior exhibited by the obtained passive control systems is a fact of singular relevance, which certainly deserves a deeper investigation. Further research effort should also be invested in studying the acceleration response of linked buildings, and in performing more realistic numerical simulations, which should include the effect of bounding events and other nonlinear aspects. Finally, it is worth mentioning the important open problem of finding a suitable methodology to determine the optimal configuration of distributed multi-actuation systems in large-scale structures.  
This work was partially supported by the Spanish Ministry of Economy and Competitiveness under Grant DPI2015-64170-R/FEDER.  
**Author Contributions:** All the authors contributed to the modeling and the scientific elaboration. Moreover, [PERSON] and [PERSON] designed the centralized controllers; [PERSON] designed the decentralized controllers and computed the frequency responses; [PERSON] and [PERSON] performed the numerical simulations; [PERSON], in collaboration with the other authors, wrote the paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.