Abstract: By interposing a properly chosen flexible layer between superstructure and his foundation the base-isolation technology can shift the natural period of structure away from the dominant period of earthquake ground motion and thus avoid the destructive effects given by the system resonance.
As a result of flexibilization the natural period of the past fixed-base structure undergoes a jump towards the longer new natural period. Certainly, this “period-shift” depends on non-linear strength and damping characteristics of the materials or devices from this isolation layer. In addition, all the site soils materials has the well-known non-linear mechanical characteristics which affects the dynamic structural response. Thus, in the period-shift evaluation the non-linear behaviour of both isolator and site layers must take into account.
Two is the main objectives of this paper. First is to present a method for necessary period-shift determination using the dynamic magnification functions. Then, using a non-linear one-degree-of-freedom model for superstructure-base system by numerical simulation one present the effects of the soils and isolator layer non-linearity on shape and resonant magnitude of the magnification functions.
In terms of non-linearity kind – hardening or softening – can arise the different period-shift modifications. Thus, the isolator layer which can have both type of non-linearity lead to shortening shift in case of the hardening non-linearity or can lead to the increasing shift if the isolatory materials or devices has softening behaviour. Also, the site degradable materials have softening non-linearity, therefore in this case can arise only increasing period-shift.
Neglecting these period-shift variations due to non-linear effects by using linear assessment of necessary period of the base-isolation structure the main purpose of the base-isolation technology – the drawing out of structure from dangerous resonant zone - can be compromise.
 

Brief Biography of the Speaker: Dinu BRATOSIN graduated the Faculty of Structural Engineering (Bucharest Civil Engineering Institute, 1971) and has a Ph.D. in Technical Sciences (Mathematical methods in solid mechanics, National Center of Physics, 1984). His research activities focused on solid mechanics, soil mechanics, soil dynamics, earthquake engineering. He is the author or co-author of more then 50 published papers, 4 books, 3 book chapters and more then 60 research projects in these fields. He received in 1996 Anghel Saligny award of the Romanian Academy for group of papers: Dynamic viscoelastic nonlinear model for soils.