Geology Blog #9: Seismic Waves and Building Design Principles

In Geology lab this week, we have begun constructing buildings that will (hopefully) withstand various types of seismic waves. Therefore, I would like to use this weeks blog post to review the various types of seismic waves and introduce a few design principles recommended by the National Institute of Building Sciences. (Hence, the A team will win this building competition) 🙂

First… the types of seismic waves!

“Regardless of the type of plate interaction that causes an earthquake, the result is the same. Vibrations from the release of energy as the rocks fail are sent out from the site of the earthquake to surrounding locations in the form of body and surface waves” (Shake, Rattle, and Roll Earthquake Board). Body waves are seismic waves that travel through the Earth interior; body waves travel much more quickly than surface waves and start at the earthquake focus. Surface waves start at the epicenter and move over the Earth’s surface.

These two main categories of waves are then farther divided into subcategories. Body waves are further divided into: P waves and S waves. Surface waves are subdivided into L waves and Rayleigh waves.

P waves (also known as Primary compressional waves) “are the first energy waves to arrive at a seismograph station once an earthquake occurs ( Shake, Rattle, and Roll Earthquake Board). These waves have the capability to travel through both solid and liquid layers of the Earth in a “push-pull, linear motion;” due to this type of motion, little displacement of Earth materials actually takes place (Shake, Rattle, and Roll Earthquake Board). These waves can travel more than 12,000 mph depending on the type of material they are traveling through.

S waves (also referred to as Secondary shear waves) are usually much more destructive than P waves; S waves move in an up- down motion. Interestingly, these waves only pass through solids— this characteristic limits which seismographs can register the earthquake occurrence. In regards to speed of travel, S waves reach a “maximum velocity of 3 km/second” (Shake, Rattle, and Roll Earthquake Board).

As previously mentioned, surface waves are subdivided into L waves and Rayleigh waves. L waves– or love waves– arrive after S waves and are “horizontal transverse waves that travel across the Earth” (Shake, Rattle, and Roll Earthquake Board). The motions of these waves are similar to that of a snake. Of the four types of waves caused by an earthquake, love waves are the most destructive. Lastly, Rayleigh waves. These waves travel in a “backwards elliptical motion” (Shake, Rattle, and Roll Earthquake Board).

Now…. the design principles to help us mitigate the effects of these waves.

Just to provide context as to why this topic is important…the statistics below are an excerpt from the National Institute of Building Sciences.

EXCERPT: “About half of the states and territories in the United States—more than 109 million people and 4.3 million businesses—and most of the other populous regions of the earth are exposed to risks from seismic hazards. In the U.S. alone, the average direct cost of earthquake damage is estimated at $1 billion/year while indirect business losses are estimated to exceed $2 billion/year.”

The website offered wonderful explanations of seismic design factors that must be taken into account such as: damping, ductility, strength, stiffness, and building configuration. Moreover, the website discussed a few strategies that the A team could take advantage of:

1) Moment-Resistant Frames: Column/beam joints in moment-resistant frames are designed to take both shear and bending. These frames look like triangles in the corners of walls. Our stucture will integrate this feature.

2) Base Isolation: “This seismic design strategy involves separating the building from the foundation and acts to absorb shock. As the ground moves, the building moves at a slower pace because the isolators dissipate a large part of the shock” (Seismic Design Principles). Due to the fact that we are using Popsicle sticks and toothpicks, this may be difficult to achieve… but we will nonetheless make an attempt!

Works Cited

“Shake, Rattle, and Roll Earthquake Board.” Lab Handout.

“Seismic Design Principles.” WBDG. National Institute of Building Sciences. Web. 06 Apr. 2013. .