Displacement-based design procedures for rigid block isolation

When subjected to earthquakes, many objects or structural elements behave like rocking rigid blocks. Computer servers, medical shelves, art objects, statues, and electrical transformers are frequently included in this category. Protection of these objects is an important task, considering that their value could be inestimable or their operation crucial during earthquakes; base isolation technology has been proven to be a viable option for this purpose. Initially, the dynamic model of a rocking rigid block placed on a base isolation device is reviewed. Then, two equivalent-static displacement-based procedures for designing the isolators for these types of objects are proposed, and the main steps are illustrated. The first procedure aims to determine isolator characteristics to prevent the initiation of rocking motion during the code-level earthquake event. The second procedure is aimed at designing isolators that allow a specified maximum rotation of the block during seismic events. The proposed procedures are validated by means of time-history analyses for a suite of spectrum-compatible accelerograms. The first displacement-based procedure appears particularly suitable for objects of small to medium size. The validation of the second procedure demonstrates that the equal displacement rule can be applied for this kind of systems, despite their softening. The results also indicate that the approach is particularly effective for medium to large structures/objects, if small oscillations are acceptable. The controlled rocking procedure offers a significant advantage by allowing for a reduction in the maximum displacement and period of the isolator, compared to situations where rocking motion must be prevented entirely.