Earthquake Essay, Research Paper

Earthquake Design

Benjamin Mallah

Geology is the study of the earth’s landmasses. The earth is constantly changing. These changes are to slow for one person to see in his or her lifetime. Forces cause different things to happen on the surface of the earth. Such as mountains growing and eroding. The earth is broken down into layers lithosphere, hydrosphere, and atmosphere. The lithosphere is the dense, solid layer that surrounds they earth. Which we call the ground. Scientists believe that the earth was formed by very dense elements such as iron and nickel. Which are in the core of the earth. The earth crust covers the mantle. The crust is made up of over two thousand different compounds called minerals. The crust is the outer most, thinnest layer. It ranges between four and seven kilometers. The mantel is sits right below the crust and is about two thousand nine hundred kilometers. As you go deeper into the mantel is becomes hotter and more pressure. The upper part of the mantel and the crust make up the lithosphere. There are major plates that sit on top of the earth’s mantle. These plates are very large rigid slabs of crustal rock that float on top of the mantel. The lighter plates carry the most landmass and the more dense plates carry less landmass. These plates are like large sheets of ice siting on top of a pond. Like the ice they do not sit still. Scientist do not know what moves these plates but we do know they move. There are nine major plates. These plates have boundaries. There are the types of boundaries. A divergent boundary is when two plates move apart from one another. A convergent boundary is when two plates move towards each other. A transform boundary is when two plates slide horizontally past each other. These boundries are also called faults.

Most earthquakes are caused by the sudden slip along geologic faults. The faults slip because of movement of the earth’s tectonic plates. This concept is called the elastic rebound theory. The rocky tectonic plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when pressure within the crust increases slowly over hundreds of years and finally exceeds the strength of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs, increase stress in the earth’s crust. Stress in the earth’s crust creates faults-places where rocks have moved and can slip, resulting in earthquakes. The properties of an earthquake depend strongly on the type of fault slip, or movement along the fault, that causes the earthquake. Geologists categorize faults according to the direction of the fault slip. The surface between the two sides of a fault lies in a plane, and the direction of the plane is usually not vertical; rather it dips at an angle into the earth. When the rock hanging over the dipping fault plane slips downward into the ground, the fault is called a normal fault. When the hanging wall slips upward in relation to the bottom wall, the fault is called a reverse fault or a thrust fault. Both normal and reverse faults produce vertical displacements, or the upward movement of one side of the fault above the other side, that appear at the surface as fault scarps. Strike-slip faults are another type of fault that produce horizontal displacements, or the side by side sliding movement of the fault, such as seen along the San Andreas fault in California. Strike-slip faults are usually found along boundaries between two plates that are sliding past each other. The sudden movement of rocks along a fault causes vibrations that transmit energy through the earth in the form of waves. Waves that travel in the rocks below the surface of the earth are called body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S, waves. The S waves, also known as shearing waves, cause the most damage during earthquake shaking, as they move the ground back and forth. Earthquakes also contain surface waves that travel out from the epicenter along the surface of the earth. Two types of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage to structures, as they shake the ground underneath the foundations of buildings and other structures. Body waves, or P and S waves, radiate out from the rupturing fault starting at the focus of the earthquake. P waves are compression waves because the rocky material in their path moves back and forth in the same direction as the wave travels alternately compressing and expanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about 6 to 7 km (about 4 mi) per second. P waves are followed by S waves, which shear, or twist, rather than compress the rock they travel through. S waves travel at about 3.5 km (about 2 mi) per second. S waves cause rocky material to move either side to side or up and down perpendicular to the direction the waves are traveling, thus shearing the rocks. Both P and S waves help seismologists to locate the focus and epicenter of an earthquake. As P and S waves move through the interior of the earth, they are reflected and refracted, or bent, just as light waves are reflected and bent by glass. Seismologists examine this bending to determine where the earthquake originated. On the surface of the earth, Rayleigh waves cause rock particles to move forward, up, backward, and down in a path that contains the direction of the wave travel. This circular movement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circular path onto a beach. The second type of surface wave, the Love wave, causes rock to move horizontally, or side to side at right angles to the direction of the traveling wave, with no vertical displacements. Rayleigh and Love waves always travel slower than P waves and usually travel slower than S waves.

Earthquake design in the Bay Area has become a pressing issue. We have seen the importance of earthquake safety in the bay area. The San Francisco Bay Area is part of a very complex plate boundary system between the pacific and the northern American plates. Near Hollister, the Calaveras fault branches off from the San Andreas Fault towards the north. The Hayward fault branches off from the Calaveras towards the northwest. At a much smaller scale many thrust faults run parallel and cross the San Andreas Fault. Although most of the present day seismic activity in the Bay Area comes from the major faults (San Andreas, Hayward-Mission creek, Concord-Calaveras, and the Antioch faults) ten percent happens in the minor and unmapped faults. Geologists and engineers use risk assessment maps, such as geologic hazard and seismic hazard zoning maps, to understand where faults are located and how to build near them safely. Engineers use geologic hazard maps to predict the average ground motions in a particular area and apply these predicted motions during engineering design phases of major construction projects. Engineers also use risk assessment maps to avoid building on major faults or to make sure that proper earthquake bracing is added to buildings constructed in zones that are prone to strong tremors. They can also use risk assessment maps to aid in the retrofit, or reinforcement, of older structures.

In urban areas of the Bay Area, the seismic risk is greater in non-reinforced buildings made of brick, stone, or concrete blocks because they cannot resist the horizontal forces produced by large seismic waves. Fortunately, single-family timber-frame homes built under modern construction codes resist strong earthquake shaking very well. Such houses have laterally braced frames bolted to their foundations to prevent separation. Although they may suffer some damage, they are unlikely to collapse because the strength of the strongly jointed timber-frame can easily support the light loads of the roof and the upper stories even in the event of strong vertical and horizontal ground motions.

Seismologists have observed that some districts tend to repeatedly experience stronger seismic shaking than others do. This is because the ground under these districts is relatively soft. Soft soils amplify ground shaking. If you live in an area that in past earthquakes suffered shaking stronger than that felt in other areas at comparable distance from the source, you are likely to experience relatively strong shaking in future earthquakes as well. An example of this effect was observed in San Francisco, where many of the same neighborhoods were heavily damaged in both the 1906 and 1989 earthquakes. The influence of the underlying soil on the local amplification of earthquake shaking is called the site effect.

Other factors influence the strength of earthquake shaking at a site as well, including the earthquake’s magnitude and the site’s proximity to the fault. These factors vary from earthquake to earthquake. In contrast, soft soil always amplifies shear waves. If an earthquake is strong enough and close enough to cause damage, the damage will usually be more severe on soft soils.

There have been many major earthquakes in the bay area. In all of the major earthquakes there has been deaths in direct result of building design and earthquake safety laws. In 1868 October, 21 there was one of the most destructive earthquakes San Francisco has had. It ruptured from the southern end of the Hayward fault. It had a magnitude of 7.0. Know as the “Great San Francisco Earthquake” killing over twenty people and injuring over sixty people and having a destruction cost of $300,000 which was a lot of money back then. In 1906 April 18 the San Andreas fault ruptured with a magnitude of 7.7. This earthquake has set back San Francisco development a lest 10 years. Causing over three thousand death and near two hundred twenty five thousand injuries. The total destruction cost of this earthquake was over 400 million dollars.

The design of current buildings in California is highly complicated. Current laws state that new building built in earthquake zone must have up to date earthquake design. There are many types of earthquake design in the bay are. There is new technology that people are using like seismic isolation, which is the building sit on top of huge rubber bearing with steel sheets. Those rubber bearings are connected to the ground and the building. When the building is involved in a earthquake the rubber bearing act as dampeners and allow the building to move with they earthquake The most commonly used and cost effective is the lateral load resting system. These systems are shear wall, braced frame, moment resisting frame, diaphragm.

Diaphragms

Figure 1: Horizontal Diaphragm Action

Diaphragms are horizontal resistance elements, generally floors and roofs, which transfer the lateral forces between the vertical resistance elements (shear walls or frames). Basically, a diaphragm acts as a horizontal I-beam. That is, the diaphragm itself acts as the web of the beam and its edges act as flanges.

Shear Walls

Shear walls are vertical walls that are designed to receive lateral forces from diaphragms and transmit them to the ground. The forces in these walls are predominantly shear forces in which the fibers within the wall try to slide past one another.

When you build a house of cards, you design a shear wall structure, and you soon learn that sufficient card “walls” must be placed at right angles to one another or the house will collapse.

Figure 2: Shear Walls

If you were to connect your walls together with tape, it is easy to see that the strength of this house of cards would be immediately become greatly increased. This illustrates a very important point: In general, the earthquake resistance of any building is highly dependent upon the connections joining the building’s larger structural members, such as walls, beams, columns and floor-slabs.

Shear walls, in particular, must be strong in themselves and also strongly connected to each other and to the horizontal diaphragms. In a simple building with shear walls at each end, ground motion enters the building and creates inertial forces that move the floor diaphragms. The shear walls resist this movement and the forces are transmitted back down to the foundation.

Braced Frames

Braced frames act in the same manner as shear walls, but they may offer lower resistance depending on their details of their design and construction. Bracing generally takes the form of steel rolled sections, circular bar sections, or tubes. Vibration may cause the bracing to elongate or compress, in which case it will lose its effectiveness and permit large deformations or collapse of the vertical structure. Ducti1ity therefore must be designed into the bracing to create a safe assembly.

Moment Resistant Frames

Figure 3: Beam-Column Joint, MR Frame

When moment resistant frames provide seismic resistance, lateral forces are resisted primarily by the joints between columns and beams. These joints become highly stressed and the details of their construction are very important. Moment frames use, as a last-resort resistance strategy, the energy absorption obtained by permanent deformation of the structure prior to ultimate failure. For this reason, moment resistant frames generally are steel structures with bolts or welded joints in which the natural ductility of the material is of advantage. However, properly reinforced concrete frames that contain a large amount of steel reinforcing are also effective as ductile frames. They will distort and retain resistance capacity prior to failure and will not fail in a brittle manner.

Architecturally, moment resistant frames offer a certain advantage over shear walls or braced frames because they tend to provide structures that are much more unobstructed internally than shear wall structures, which facilitates the design of accompanying architectural elements such as exterior walls, partitions, and ceilings and the placement of building contents such as furniture and loose equipment. Nevertheless, moment resistant frames require special construction and detailing and, therefore, are more expensive than shear walls or braced frames.

The future of are children clearly is at stake. As we have seen earthquakes are not going to go away anytime soon. They are just getting stronger as we seen in San Francisco. If we continue to strive for the perfectly safe earthquake building then we are giving are children not only a better world but also a better chance of living. If we stop currently improving building old and new than we are doing nothing to help the future. We are only hurting our self. Millions of people have died from lack of earthquake building design. The future of not only the Bay Area is in play but the future of the world. Our generation is not only getting more technologically advance but smarter. If we put are resources together than we can guarantee our children a safe future.