Seismology

Figure 2.1

Figure 2.1

By NGranderson (Own work) [CC-BY-SA-3.0], via Wikimedia Commons

Seismology is the study of earthquakes. Scientifically speaking, an earthquake is the collection of tremors that run through the ground as a result of a cracking in the earth’s crust. Stress concentrates in a point in the crust, causing a rupture and forming a fault line. This violent movement deep in the earth propagates elastic waves that, if large enough, cause destruction in surrounding structures. Seismology uses tools like the seismograph to detect and record seismic information. This data can be compiled as historical documentation and plotted to show earthquake-prone locations. Such a study is referred to as seismicity. As seen in Figure 2.1 we see that southern Alaska, Hawaii, the Western seaboard, and the area common to Missouri, Tennessee, Indiana, and Arkansas historically exhibit a tendency toward seismic activity. Below we will discuss a substantial earthquake that occurred in California which brought about significant changes to the steel and concrete design codes. With the use of the seismograph, each seismic event can be classified according to the Richter magnitude scale.

Seismograph

Seismograph

"Kinemetrics seismograph". Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Named after its inventor Charles Richter, the scale measures the energy released by each event with a ratio including the amplitude and period of the seismic waves. The entire system functions in a base-10 logarithmic scale which means that a magnitude 6 earthquake on the Richter scale is ten times larger in amplitude than a magnitude 5 earthquake on the Richter scale. Because of the severity of earthquakes of these sizes, structural engineers now follow updated codes using modern innovations to strengthen designs against seismic failures. Instead of simply preventing casualties, most up-to-code buildings must also survive with enough stability to remain habitable, particularly buildings such as emergency centers, which need to remain functional in order to respond to the disaster.

The physical reaction of structures to seismic waves may be best demonstrated by comparing it to a cube of gelatin. When the plate it sits on is subjected to any kind of horizontal vibration, the material also oscillates notably in the lateral direction. If special attention is paid, the base of the gelatin may be noted to move congruently with the plate while the upper portion oscillates unpredictably. This means that only the base of the gelatin moves with the same acceleration as the plate. Though buildings do not demonstrate the same level of exaggerated behavior, the similarity is notable. The firmly anchored base of the building must react with the same acceleration that the earthquake applies to the ground. Inertial forces inside the members of the building resist the movement, thus causing a delayed displacement recognizable as a wave. A shorter building may not behave with this wave pattern due to its decreased mass. Recall that the more mass in an object, the more it resists movement. This is due to the mathematical definition of force:

In this equation, “a” stands for acceleration. Therefore, the greater the mass, the greater the force must be to produce a large acceleration. Massive objects resist movement more than lighter ones. Thus the entire body of a short building travels with a similar acceleration as the foundation. Taller buildings do exhibit a quivering pattern, not because they possess any less strength than shorter ones, but because the rest of the building does not react instantaneously. Unlike other lateral forces that may be applied, such as wind load, seismic load is simply the force of the entire building acting on itself. Instead of an external force acting laterally, the mass of the rest of the building fights a displacement that occurs at the base. With so much mass resisting this change, the inertial force inside the building will often cause catastrophic failure.
 


Another important aspect of lateral design which is closely tied with seismic reactions is the concept of P-Delta.  This refers to the moment of a structure that occurs as a result of displacement.  A moment is the product of a force times the perpendicular length at which it is applied.  To use a classic example, a wrench applies a moment on a nut when a mechanic pushes down on the wrench.  The length of the wrench, known as a moment arm or lever arm, is at a 90-degree angle to the force applied by the mechanic.  That force causes a twisting which is referred to as a moment.  When a building stands perfectly upright, all of the weight acts directly down the building.  This makes the lever arm nonexistent:  the weight applies no moment to the system.  However, whenever a seismic load causes displacement, the weight is now applied across a perpendicular length.  This small lever arm produced by these oscillations causes a moment that adds significantly more stress to a steel system.  See Figure 2.2 for a graphical description of P-Delta.