Code Provisions About λ-Factors of HDRBs for the Upper and Lower Bound Analyses: Hystorical Review

Most of the current seismic codes on seismic isolation prescribe upper and lower bound analyses to account for the variability of mechanical properties of seismic isolators. This procedure should be based on experimental data provided by manufacturers during qualification tests. When such data are not available, codes provide the so-called property modification factors or λ-factors for different kind of devices, giving maximum upper and lower bound coefficients to apply to nominal properties of bearings. Different sources of variability are considered, from the variability due to the bearing production to environmental or behavioural effects (such as temperature, contamination, ageing or repeated cycles). However, the origin of these values is not straightforward: some of them are based on wide experimental data, whereas others are based on few and old indications given by technical literature. The aim of this paper is to illustrate in detail such factors, especially for HDRBs (High Damping Rubber Bearings). In particular, it is shown that in the European context, both the EN 1998—part 2 (on bridges) and the EN 15129 (on anti-seismic devices), provide values deriving (with some transcription inconsistencies) from the second version of the AASHTO guide specification, which in turn proposes values derived from the MCEER-99-0012 report written in 1999 and never updated. The aim of this paper is to illustrate in detail such factors and to make the reader aware that most of factors derive from limited and/or old data, especially for HDRBs (High Damping Rubber Bearings).