36.14. Vertical Ground Motion Spectra

Vertical motions are critical for:

• Base-isolated structures
• Long-span bridges
• Nuclear facilities
• Usually less than horizontal spectra but can’t be ignored.

Vertical ground motion refers to the movements that occur in a vertical direction during seismic events, such as earthquakes. These motions are particularly important for certain types of structures, including base-isolated structures that are designed to move independently of the ground motion, long-span bridges that can experience heightened stress due to up-and-down movements, and nuclear facilities that require rigorous safety protocols against all forms of motion. Although vertical motions tend to be less intense than horizontal ones, they still play a significant role in the overall seismic response of these structures and should not be overlooked in engineering designs.

Imagine being on a seesaw in a playground. When your friend jumps down on one end, you experience an upward motion. This movement mimics vertical ground motion during an earthquake. Just like that seesaw can be affected significantly even by someone hopping on one end, buildings and bridges can also experience critical stresses due to vertical motions, impacting their stability and safety.

Used to scale vertical spectrum from horizontal. Typical V/H ratio ranges from 0.3 to 0.7, depending on site and magnitude.

The vertical-to-horizontal ratio (V/H ratio) is a scaling factor that helps engineers relate vertical seismic motion to horizontal seismic motion. In practice, seismologists observe that the intensity of vertical ground motion can be estimated using this ratio, which typically ranges from 0.3 to 0.7. This means that for a given horizontal motion, the vertical motion can be approximately 30% to 70% of that horizontal motion. Understanding this ratio is crucial for accurate structural design because it helps in predicting how buildings will respond to both types of motion during an earthquake.

Consider a person jumping on a trampoline. When they jump up (vertical motion), they also tend to sway sideways (horizontal motion). Now, if we imagine that the height of the jump represents the intensity of horizontal motion, the peak vertical height they reach can be understood as a portion of that jump, specifically between 30% to 70%. Just like we can predict how high someone can bounce based on how much they jump sideways, engineers can use the V/H ratio to scale vertical motions based on horizontal seismic activity.

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