## 5 Discussion and Conclusions  
A fast and easy-to-use tool for the preliminary assessment of the effectiveness of deck isolation for the seismic retrofit of existing bridges has been presented in this paper. The retrofit procedure under consideration consists of replacing the existing bridge bearings with seismic isolators, creating an isolation layer between the deck and the substructures. The procedure is feasible for bridges characterized by a static scheme with simply supported spans or a continuous deck on multiple supports. The objective of the retrofit is to isolate the mass of the deck and reduce the seismic force on the substructures in order to avoid plasticization or brittle failure of the piers. This intervention is advantageous both in terms of reductions in time and, consequently, in interruptions of bridge service, and in terms of costs, comparable to and even lower than those of other conventional strategies, such as strengthening [31; 32]. According to the procedure, two checks are performed: the first is that the piers are not able to resist the seismic force of the deck (need for retrofit), and the second is that  
Figure 14: Effectiveness matrix for continuous deck scheme. once the mass of the deck has been isolated, the piers are able to resist the seismic force associated with their own mass (effectiveness of deck isolation). If the second condition is not met, it is necessary to resort to alternative techniques, such as strengthening, or to the combination of deck isolation and pier strengthening. The procedure is characterized by an intrinsic simplicity, as it only requires performing an analysis of the pier section to calculate its capacity, which is then converted into the Elastic Resource acceleration and compared with the spectral acceleration of the bridge (As-Built acceleration) and with the normalized spectral acceleration of the pier alone (Equivalent Substructure acceleration). According to the procedure, the piers are analyzed separately, disregarding any coupling effects introduced by the isolators. In the second part of the paper, a parametric study is presented to show a practical application of the method. The study covers some typical characteristics of Italian bridges, highlighting how some configurations are characterized by an intrinsic weakness and lack of substructure resources, which cannot be resolved by introducing an isolation layer below the deck, but it is also necessary to increase the strength of the weak element. In particular, considering the simply supported deck scheme, the study reveals that seismic rehabilitation is always necessary to reduce the vulnerability of the piers, except for tall and flexible piers in bridges subjected to moderately low seismicity (zone 3 according to IBC [20]). However, it also emerges that circular section piers and frame piers are characterized by a serious structural deficiency if located in areas of high seismicity, as they are unable to resist the seismic force associated with their own mass. Rectangular hollow section piers perform better, and in this case, deck isolation is ineffective only in case of tall piers (20 m piers in the study). The high strength of the hollow rectangular section is also reflected by the fact that, in the case of moderately low seismicity, in many cases, the retrofit is not necessary at all. Moving on to the continuous deck scheme, results similar to those of simply supported bridges are observed. For the circular section pier, it is found that retrofitting is always necessary, except for tall piers in zone 3, but isolation is not effective in cases of tall piers in zone 1. The intrinsic structural deficiencies of the frame pier are less evident than in the case of the simply supported deck. Finally, for piers with a hollow rectangular cross-section, the retrofit is necessary in most cases in seismic zone 3.