λ-Factors for the Upper and Lower Bound Analyses of Base-Isolated Structures: Historical Review of Code Provisions for Elastomeric Bearings

The seismic response of base-isolated structures is notably influenced by mechanical properties isolation devices due to their essential role in structural behavior. Consequently, the variability such should be accounted for design process. current codes prescribe a simplified approach based on analyses two extreme situations resulting from upper and lower bound bearings (upper analyses). In case that experimental data are not provided manufacturers, provide so‑called “property modification factors” or “λ-factors”, i.e., coefficients applied nominal dynamic obtain properties. aim this paper historical review values factors main highlighting limits, as well some clerical errors, present codes. particular, European illustrated detail, Eurocode bridges (EN 1998-2) product standard anti-seismic 15129). Both these account different sources variability, production environmental behavioral effects. For all effects, same λ-factor codes, deriving second version AASHTO guide specifications (AASHTO 1999), which limited and/or old data, especially high damping rubber (HDRBs), were never updated successive versions. More recent standards also illustrated, providing perspectives deserve attention, even though they require further investigations practice.