Base isolation in retrofitting decouples ground motion from the superstructure, resulting in what outcome?

• A. Base isolation decouples ground motion but can increase height and cost.
• B. Base isolation decouples ground motion, reducing transmitted accelerations and improving survivability during earthquakes.
• C. Base isolation increases transmitted accelerations.
• D. Base isolation has no effect on response.

Answer: (B)

Base isolation works by creating a flexible interface between the foundation and the structure, effectively decoupling the building’s motion from ground shaking. This shifts the building’s dynamic response to a longer natural period, so the accelerations the structure experiences from most earthquake frequencies are greatly reduced. With lower transmitted accelerations, inertial forces in the superstructure drop, leading to less damage to structural members and contents and a higher likelihood of remaining functional during and after an earthquake. While there can be cost considerations in retrofitting, the defining outcome is the reduction of ground-motion effects on the building, which is why this approach improves survivability.