## 1 Introduction
In detail, this standard also prescribes the use of \(\lambda\)-factors values based on the experimental data provided by the specific manufacturer, and, in the case of unavailability of such data, it provides the Annex J with the same \(\lambda\)-factors values given by Annex JJ of the Eurocode 8 for bridges [19]. It is important to note that, nowadays, in both the national and international contexts, experimental data are often available only after the qualification tests of the devices, thus, resulting as unknown at the design phase. Moreover, type tests made during the qualification procedure do not always include all the necessary information to correctly evaluate the UBDPs and LBDPs. Consequently, the values of the \(\lambda\)-factors proposed by codes often become the only reference for structural engineers. The aim of this paper is to illustrate in detail such factors by making a historical reconstruction of their origin. In more detail, in the first part of the paper, the current European seismic codes are illustrated by highlighting that all the provided \(\lambda\)-factors derive from the second edition of the AASHTO (AASHTO 1999) [21]. Thus, such code is illustrated in detail by showing that most of the proposed values of the \(\lambda\)-factors derive from old and/or limited data that do not reflect the actual production of seismic devices, especially for HDRBs, and that were never updated in the subsequent editions: AASHTO 2010 [22] and AASHTO 2014 [23]. Moreover, in some cases, important clerical errors or omissions were made in the European standards, as was also outlined in [24]. This is the case, for example, for the temperature effect and the so-called \"scragging\" effect (or repeated cycles effect) of HDRBs. Concerning the temperature effect, indeed, a transcription error was made by passing from the American to the European standard. This error was corrected in the updated version of Eurocode 8-part 2 [25] but not in the EN15129 [20]. For the repeated cycles effect, instead, American values were not included in the European standards. In this regard, it is worth noting that the difference between scragged and unscragged properties suggested by the AASHTO code is remarkable, and this aspect has limited the use of this kind of bearings in the USA, especially for strategic structures, such as nuclear power plants and bridges [26; 27; 28; 29]. For this kind of structure, natural rubber bearings (NRBs) or lead rubber bearings (LRBs) are only allowed as alternative systems to friction pendulum (FP) systems. However, this paper explains that such a remarkable effect refers to old bearings (with incomplete curing) that could differ significantly from the current ones. For this reason, the values of AASHTO were not transposed by the European standards; however, alternative values were not provided, leaving a lack of data regarding this aspect. In the last part of the paper, more recent international regulations orientated towards new approaches [30; 31; 32; 33] are examined to provide a complete overview and to open new perspectives. In detail, the indications of ASCE 7 seismic standards for new and existing buildings [30; 31] are illustrated. These codes, which are also taken as a reference by high-risk countries other than the U.S., such as Chile, Turkey, and New Zealand, provide updated values of the \(\lambda\)-factors, which, however, must be verified by mandatory experimental tests. Finally, the approach of recent international regulations to elastomeric bearings, the ISO 22726 for buildings and bridges [32; 33], is also reported. However, these documents provide few values only, since, following the Japanese approach described in the design recommendations for seismically isolated buildings [34], they specify that the ranges of variation for most of the effects should be defined in agreement between the structural engineer and the manufacturers.