What is the phenomenon called when the natural period of a structure coincides with a dominant frequency of the ground motion?

The phenomenon where the natural period of a structure coincides with a dominant frequency of the ground motion is known as soil-structure resonance. This occurs when the frequency at which the ground moves matches the frequency at which a structure tends to oscillate. When this alignment happens, the amplitude of the vibrations can significantly increase, leading to greater potential for damage or failure.

In the context of engineering and geology, understanding soil-structure resonance is crucial because it helps engineers design structures that can withstand seismic forces. Structures that are designed without accounting for this resonance may suffer catastrophic failures during earthquakes due to the amplified motion.

This understanding contrasts with the other concepts mentioned. The seismic design category relates to how structures are classified based on their seismic risk but does not directly refer to the resonance phenomenon. Site class pertains to the geological and soil conditions at a specific site but does not specifically address the interaction between ground motion and structural frequency. The term "sufficiently active" does not refer to a recognized phenomenon in the context of seismic design and is therefore not relevant here.

Thus, recognizing the critical nature of soil-structure resonance helps ensure that buildings can appropriately respond to seismic activities without suffering excessive damage.