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This paper deals with the seismic protection of building frames by means of external dissipative systems. Dampers and external framing system can be arranged in several configurations, involving different kinematic behaviours and seismic performances. This study analyses a recently-developed solution called "dissipative tower", which exploits the rocking motion of a steel braced frame, hinged at the foundation level, for activating the dampers. This system aims at controlling both the global response and the local storey deformation of the frame, by using a reduced number of viscous dampers. A state space formulation of the dynamic problem is presented in general terms, together with the solution of the seismic problem via the modal decomposition method. A parametric study is carried out to evaluate the influence of the added damping and of the braced frame stiffness on the modal properties and seismic response of a benchmark reinforced concrete frame retrofitted with the external dissipative towers. It is shown that the addition of the towers yields a regularization and reduction of the drift demand along the building height, but it may induce significant changes, not always beneficial, in the distribution of internal actions of the frame and in the absolute storey accelerations.

The use of buckling restrained braces (BRBs) represents one of the best solutions for retrofitting or upgrading the numerous existing reinforced concrete framed buildings in areas with a high seismic hazard. This study investigates the effectiveness of BRBs for the seismic retrofit of reinforced concrete (RC) buildings with masonry infills. For this purpose, an advanced non-linear threedimensional model of an existing building in L'Aquila is developed in OpenSees, by accounting for the effect of infill walls through an equivalent strut approach, and by using a recently developed hysteretic model for the BRBs. The seismic performance of the building before and after the retrofit with BRBs is evaluated by performing both non-linear static analyses and incremental dynamic analyses under a set of real ground motion records. Seismic demand hazard curves are built for different response parameters before and after the retrofit, by accounting for and by disregarding the contribution of the infill walls. The study results shed light on the effect of the BRBs and of the infill walls on the seismic performance of the various components of the system, and on the effectiveness of the retrofit with BRBs for a real case study.

This paper investigates the seismic design of fluid viscous dampers connecting adjacent structural systems. A simplified dampers design strategy is proposed, which relies on a linearized reduced order model of the coupled system. A stochastic linearization technique is adopted with the aim of extending the design method to non-linear viscous dampers. The effectiveness of the design method and of the coupling strategy are assessed via numerical analysis of two adjacent buildings with shear-type behavior connected by linear or non-linear viscous dampers and subjected to Gaussian stochastic base acceleration. Different dampers locations are analyzed. The accuracy of the reduced order model is assessed, by comparing the relevant response statistics to those provided by a refined multi degree of freedoms model. Finally, a parametric study is performed to assess the effectiveness of dissipative connection for different values of seismic intensity and dampers parameters (i.e., viscous coefficients and velocity exponents).The equation of motion of two adjacent MDOF systems, A and B, coupled by viscous dampers (Figure 1), is:Frontiers in Built Environment | www.frontiersin.org

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This paper deals with the seismic retrofit of existing frames by means of external passive dissipative systems. Available in different configurations, these systems allow high flexibility in controlling the structural behaviour and are characterized by some feasibility advantages with respect to dissipative devices installed within existing frames. In particular, this study analyzes and compares the performances of two external solutions using linear viscous dampers. The first is based on the coupling of the building with an external fixed-based steel braced frame by means of dampers placed horizontally at the floor levels. The second is an innovative one, based on coupling the building with a "dissipative tower," which is a steel braced frame hinged at the foundation level, and activating the dampers through its rocking motion. The effectiveness of the two solutions is evaluated and compared by considering a benchmark existing reinforced concrete building, employing a stochastic dynamic approach, under the assumption of linear elastic behaviour for the seismic performance evaluation. This allows efficiently estimating the statistics of many response parameters of interest for the performance assessment and thus carrying out extensive parametric analyses for different properties of the external systems. The study results provide useful information regarding the design and the relative efficiency of the proposed retrofit solutions.

This paper analyses the new Research Centre designed for the University of Camerino and entirely financed by the national Civil Protection Department (DPC), following the seismic events in Central Italy in 2016. The building has been designed to guarantee speed of execution as well as a high level of safety, especially regarding seismic actions. The structural solution was to create an isolated system with a steel braced superstructure with pinned joints and r.c. sub-structures able to adapt to the complex morphology of the area. As described in the first part of the paper, design choices have been made to achieve a high level of resilience and robustness, i.e., to limit damage to structural and non-structural components and equipment under moderate and design seismic actions and to avoid disproportionate consequences in the event of extreme actions, larger than the design ones. In the second part of the paper, specific risk analyses have been carried out to evaluate the real performance of the building under increasing intensity levels, with reference to both serviceability and ultimate conditions. To this purpose a site-specific hazard study was first conducted, then non-linear analyses were performed using a hazard-consistent set of records with return periods ranging from T R = 60 years to T R = 10000 years. The main demand parameters of both the isolation system and the superstructure were recorded and capacity values corresponding to different ultimate and damage limit conditions were defined. The results obtained in terms of demand hazard curves show that the building performances in terms of robustness and resilience are very high, confirming the efficacy of the strategies adopted in the design.

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