To understand the Seismic Zoning method and how it pertains to the Monolithic Dome, we must first understand what effective peak ground acceleration means and how it is measured against gravity.
First let’s discuss what is meant by ground acceleration. It’s not unlike the feeling we have when a car takes off at high speed, or when we ride roller coasters at Disneyland.
In order to measure the acceleration of an earthquake, it must be measured against Gravity (or 1.0g). Gravity is the rate at which an object falls when dropped from being at rest in a vacuum. It’s quite a high rate of acceleration, approximately the same as a car traveling 100 meters from rest in just 4.5 seconds.
There are studies that show that much of the damage done in earthquakes is, perhaps, due rather to the velocity of the back and forth movements of the earth, rather than to the ground acceleration. However, the mean and peak ground accelerations do have much to do with the intensity of damage a building may have to withstand. Consequently, engineers and designers rely a great deal on the measure of the peak ground acceleration, as compared to gravity, to determine how strong an earthquake force a new building may have to withstand.
Instruments called accelerographs measure ground acceleration against the value of gravity (acceleration in g/10). These values are gathered from all parts of the nation to create a seismic-risk map, which is used by engineers and builders when designing earthquake-resistant structures for different parts of the country.
Attenuation is another important detail that is factored into plotting a seismic-risk map. Attenuation is, basically, how far earthquakes’ waves are felt, and what is the duration of the earthquakes. This is very different in various parts of the nation.
Next, the values on the seismic-risk map are figured this way: If you live in seismic zone 4, you have a one in ten chance that an earthquake with an active peak acceleration level of 0.4g (4/10 the acceleration of gravity) will occur within the next fifty years. Likewise, if you live in zone 1, you have a one in ten chance that an earthquake with an active peak acceleration level of 0.1g (1/10 the acceleration of gravity) will occur within the next fifty years.
The Monolithic Dome is designed for zone 4. Dr. Arnold Wilson, a leading engineer in thin shell concrete construction, has said, “It is easy to see that earthquake forces do not even approach the design strength the Monolithic Dome is built to withstand under normal every day usage. It would take an external force much larger than an earthquake to approach the design strength of the concrete itself.”