By Ray Delaforce
Software Engineer, Intergraph
What is the basis of seismic or earthquake design? The principles were laid in the mid-17th century by the English natural philosopher, Sir Isaac Newton. He furthered the work of the great Galileo Galilei whose work on gravitation and force advanced the early renaissance science.
So, what was the basis for seismic design? Simply put, it was the discovery that a force was required to accelerate or change the velocity of an object. In fact, the relationship between force and acceleration is expressed in a very simple equation:
Force = mass x acceleration
But, what has acceleration got to do with earthquakes? This: When an earthquake occurs, the ground experiences an acceleration. Anything that sits on the ground also experiences that acceleration.
Let us consider the case of a tall vessel mounted on a foundation that undergoes this acceleration. As it experiences lateral (horizontal) acceleration, other forces come into play, such as inertia. Inertia is the resistance of any object to experience a change of velocity. Consider this simple diagram:
As the bottom of the vessel is accelerated to the right, a higher shell course tries to stay behind. This, of course, cannot happen as shown, because the two sections are welded together. This is what happens instead:
Longitudinal or axial compressive and tensile stresses are induced in the shell. Consider the situation where the force (acceleration) was constant. We could then fairly easily analyze the system as though it were a horizontal cantilever in a gravitational field like this:
That would not be too difficult. However, an earthquake does not deliver the constant force (acceleration). Learn how an actual earthquake acts in next month’s newsletter article, “Seismic Design, Part 2: Damping.”