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Received Sep 15, 2017; Accepted Dec 27, 2017
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1. Introduction
Traditional approaches for the seismic performance enhancement and retrofit of buildings by means of passive damping usually involve installing the dissipative devices within the building frame in either diagonal or chevron braces connecting adjacent storeys [1, 2]. This type of damping configuration has been extensively studied in the last decades both experimentally and numerically, with a significant number of works focusing on design criteria and performance assessment [3–11]. However, the use of dampers within building frames presents some disadvantages, especially when employed for the retrofit of existing buildings. The increase of internal actions in the nodes and columns adjacent to the dampers can induce premature local failures [10], strengthening the foundations is generally required, retrofitting operations cause remarkable downtime, and relevant costs must be sustained. For these reasons, there is an increasing interest on external passive control systems, characterized by minimized interferences with the existing frame, during the installation of the retrofit system and also during the building operation. External damping systems are available in different configurations [11–13], allowing high flexibility in controlling the structural behaviour, but their relative efficiency in terms of seismic performance has not been fully investigated to date. In fact, most of the studies in the literature focus on the most recurrent configuration involving the coupling of the structure to be protected with an external one by means of dampers placed horizontally at the storey level (Figure 1(a)). The external structure can be stiff, or it can be flexible, as in the case of adjacent buildings with different dynamic properties [14–18]. The dampers are activated by the absolute floor displacements in the first case and by the relative floor motion of the systems in the latter one. It is noteworthy that the first configuration can also be designed to achieve mass proportional damping, where the viscous damping constants are proportional to the floor masses [12].
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Recently, an innovative external retrofit configuration, denoted as “dissipative tower,” has been proposed. This consists of an external stiff braced structure...