Seismic Design of Base-Isolated LNG-Storage-Tanks

The efficiency of seismic base isolation by High Damping Rubber Bearings (HDRB's) is based on the capability of large elastic deformations of the bearings as well as on the dissipation of energy by hysteretic damping. A reliable dynamic analysis of such systems requires an accurate modelling of the overall structure and an adequate description of the non-linear characteristics of the HDRB's. This report studies these using, as an example, a storage tank for liquefied natural gas (LNG). For the computational simulation of HDRB's the following possibilities are discussed: direct numerical description of the hysteretic loops (1), combination of a multilinear elastic and an elastoplastic spring (2), combination of an elastic spring and a velocity-dependant damper (3). Time-history analyses on lumped-mass models (1) yield the best results from a scientific point of view. However, the most common approach (3) is acceptable, given suitable parameters. The different types of energies occurring during an earthquake, i.e. the input and return of energy from and to the subsoil, the potential (spring) and kinetic energy and the dissipation of energy by HDRB's are defined and quantified for the example under consideration. The results confirm the beneficial effect of the dissipation by hysteretic damping for the global behaviour of the dynamic system. However, the random characteristic of the different types of energy also becomes apparent. Therefore, it seems impractical to base a seismic design concept on these energies.