In traditional earthquake design, mitigating seismic effects is achieved through high strength or high ductility properties. However, an alternative approach known as base isolation uses a system of flexible (possibly dissipative) devices at the interface between the foundation and the structure’s base. This system directly reduces seismic forces by decoupling the structure’s motion from the ground.

For a given strength level of the isolated structure, the level of earthquake protection substantially increases by assuming an ever-increasing value for the "isolation ratio" αI = TI / TF, where TI and TF are the fundamental vibration periods of the isolated structure and the fixed-base structure, respectively. This implies that inserting a very flexible base-isolation system is generally favorable regarding seismic protection.

However, under wind loads, the considerable increase in deformability of a base-isolated structure compared to a fixed-base structure can lead to displacements and accelerations at floor levels so significant that serviceability limit states, particularly the discomfort threshold, must be carefully checked.

The above considerations suggest the need to compare the dynamic responses of a base-isolated structure subjected to strong earthquakes (ultimate state) or wind loads (serviceability state) to achieve an optimal design of the base-isolation system.

In Alfonso Vulcano's recommended study, a numerical investigation compares the dynamic responses of base-isolated reinforced concrete-framed structures subjected to earthquakes or wind loads.

The results obtained for the test structures subjected to earthquakes show two different kinds of behavior depending on the strength level:

The structure behaves elastically when the strength level is relatively high regarding the earthquake intensity.

Otherwise, the ductility demand of the isolated structure in ever-stronger earthquakes can increase so much that it becomes even more significant than that of the fixed-base structure.

This study provides detailed insight into balancing seismic protection and comfort under wind loads. It offers a comprehensive approach to designing base-isolation systems in structures. This study is a must-read if you are interested in structural design and earthquake engineering.

Reference

Comparative study of the earthquake and wind dynamic responses of base-isolated buildings, Journal of Wind Engineering and Industrial Aerodynamics, Volumes 74–76,1998