An innovative seismic protection system for existing buildings: external dissipative towers

A common technique for the retrofit of existing structure is based on the installation of dissipative devices connecting adjacent storeys of buildings in either diagonal or chevron brace configurations. This type of damping system may present some disadvantages like the increment of axial forces in columns, which may leads to premature local failures, or some feasibility limits on the strengthening of the existing foundations at the base of the bracing system. In addition, indirect costs related to the interruption of the building utilization during execution of the retrofit can be very demanding, in particular for strategic buildings, such as hospitals or schools. Most of previous problems can be overcome by placing the dissipative bracings and the relevant foundations outside the building; systems with external dampers can be grouped into three main categories, depending on the kinematic behaviour of the system, which is a function of the arrangement of dampers and bracings. Recently, some applications have been developed by proposing a new configuration exploiting the rocking motion of a stiff external tower. This paper deals with this innovative system for seismic protection of existing buildings, especially strategic ones, patented as “Dissipative Towers”. The protection system is based on the structural coupling of the building with new steel truss towers constructed externally and then rigidly connected to the building floors by means of steel elements; the towers are erected over a rigid r.c. thick base plate that is restrained to the foundation cap with a spherical hinge located in central position of the base slab. The towers are equipped with dissipative devices connecting the corners of the two plates; the effectiveness of the dampers is enhanced by the use of articulated quadrangles which amplify the vertical displacements of the devices. The efficiency of the system is so high that usually it is designed to satisfy the immediate occupancy limit state even for high intensity earthquakes. The above system is applied to the retrofit of an existing school building in Italy, constituted by different blocks made of reinforced concrete frames. The seismic rehabilitation is obtained by suitably positioning external dissipative towers and eliminating expansion joints between adjacent blocks. The towers allow a high level of seismic protection at the ultimate limit state, with a significant reduction of horizontal displacements and accelerations. Moreover also the shear actions resisted by the existing frame are significantly reduced by the "Dissipative Towers". The seismic protection is achieved with a moderate economic impact due to the elimination of indirect costs related to the arrangement of internal spaces, interruption and/or relocation of activities.