Earthquake Towers Reading Packet

Earthquake Towers Reading Packet

You have been learning that the earth is a very dynamic planet. There are many forces within the earth that bend and strain the very thin crust we call home. When the rock bends and then snaps this action is called an earthquake. Earthquakes can cause tremendous changes in the crust and result in catastrophic destruction. Engineers have been trying to understand the effects of earthquakes on structures for many years. Scientists have been studying how and why these tremors cause such tremendous damage. Working together, scientists and engineers are looking for ways to construct buildings that can withstand earthquakes.

Earthquakes

The tremors that cause the shaking are called seismic waves. The study of earthquakes is called seismology. The name comes from two Greek words: seismos meaning “earthquake” and logos, meaning “study”. Scientists who study earthquakes are called seismologists. Seismographs are instruments that measure ground vibrations and record the movement on a pendulum bob. (a heavy weight that hangs free)

There are several types of seismic waves that can effect buildings. Primary Waves or P waves travel at great speed. P waves expand and compress material in their path with a push pull action. They travel through gases, liquids, and solids. P waves move buildings back and forth. Secondary waves or S waves travel more slowly than P waves but often cause more damage to structures. S waves cannot travel through liquids or gases. S waves shake buildings up and down and side to side.

The place where an earthquake starts is called the focus. The focus can be a small or large fault and it can be located far below or very near the surface. The point on the surface of the Earth directly above the focus is called the epicenter. The strongest shaking often occurs at this point. The magnitude of the earthquake is its strength which is measured by the amount the ground moves as a wave passes by it. In 1935 Charles Richter developed a scale, called the Richter scale to measure earthquake magnitude using numbers between 0 and 10. Because it is a logarithmic scale, each whole number represents a 10-fold increase in released energy. Thus, a level 6 earthquake moves the ground 10 times more than a level 5 earthquake and releases 30 times more energy.

Forces on buildings

Engineers design structures with enough strength to withstand the forces or loads placed on them. Structures must contend with two types of loads-live loads and dead loads. Dead loads are permanent loads that do not change. The weight of the structure itself is a dead load. Live loads are changing loads such as the number of people in a building, the weight of the furniture, wind and earthquakes. Stress is the measure of the amount of force placed on an object. When an object is placed under stress it will change in shape or deform. . There are five types of stress. Compression is the tendency to push or squash a material. Tension is the tendency for a material to be pulled apart. Shear occurs when a material is divided by two opposing forces. The tendency of a material to be twisted is called torsion. Bending can be shown when a load is resting in the center of a horizontal beam. The beam will bend down, placing the top of the beam in compression and the bottom in tension.

In earthquakes shearing forces can knock buildings off their foundations which can cause them to collapse, trapping people inside. This happens when the ground moves back and forth sideways while the building tends to remain still due to its mass. In earthquake zones houses are bolted to their foundations and the bolts need to have a very high shear strength.

The Importance of Earthquake-Resistant Buildings

Whether an earthquake causes damage to structures depends on many factors. Most deaths in earthquakes have been the result of faulty building construction. For example, one earthquake in Agadir, Morocco was not strong enough to be recorded by seismographs at a distance more than 1,000 miles away, but the quake completely destroyed the city, killing more than 12,000 people. It wasn’t the severity of the quake, but the poor construction of the buildings that killed so many people.

Engineers have learned a great deal from studying past structures. The best known Greek temples were constructed between 480 B.C.E. and 323 BC.E. Many of these temples were built on foundations designed to be resistant to earthquakes. Several layers of marble were joined with iron beams embedded in lead. China, Japan, and Greece endure frequent earthquakes. Ancient builders used timber post and beam construction with flexible joints. During an earthquake, this type of structure would shake and the internal wall fall, but the building often remained standing. In the aftermath of the 1906 earthquake in San Francisco, the downtown was littered with collapsed stone buildings, but most of the wood-framed and steel-framed structures survived with little damage. Everyone realized that this type of construction was superior in resisting the strong lateral forces of an earthquake. Another big earthquake hit Tokyo in 1923, and engineers drew the same conclusions.

An earthquake produces a series of waves that move horizontally across the ground causing buildings to sway from side to side. A rigid structure, such as one built out of unreinforced stone, can withstand only minimal shaking. The best defense is to build a building that will move with the earthquake. By bending with the wave, a structure can absorb much of the wave’s destructive energy. Steel and concrete reinforced steel are the best answers to balancing flexibility and load-bearing capacity for large structures. The value of earthquake-resistant buildings can be shown by comparing two earthquakes with a similar magnitude and different results. The Loma Prieta Earthquake in San Francisco in 1989 reached a magnitude of 7.1 on the Richter scale and killed 62 people. The 1988 earthquake in Armenia reached a magnitude of 6.7 and killed 25,000. There none of the buildings were earthquake resistant.

Until recently, engineering buildings to withstand earthquakes has not been a priority because few people choose to spend money to prepare for something that they thought would never happen. With a better understanding of Plate tectonics public awareness has increased and people expect there to be earthquake-resistant designs in areas of high seismic activity along plate boundaries.

Earthquake Resistant Methods

Besides the materials the building is made of, The shape of a building’s frame can minimize the amount of tension or compression that every individual beam and column bears. Let’s look at a very simple frame with four joints, or corners. If a lateral (sideways) force, such as wind or an earthquake, acts on this frame from the side, the top and bottom would slide past each other, causing shear. A tall tower may start to bend as shear increases. Remember that bending occurs when one side of an object experiences tension and the other side experiences compression. When rectangles or squares have lateral forces on them they

tend to flatten. This is called racking. When bending or racking becomes too great the structure is in danger of toppling to the ground. Engineers want to minimize the effects of bending and racking as much as possible. They do this by adding diagonal members forming a truss.

A truss is a triangular arrangement that increases a structures rigidity. When a shearing force pushes on the side of the frame, the truss holds the joints in place so they cannot slide apart. The entire structure resists the force, not just each individual column.

Steel frames and truss systems are quite strong, but they are not the only way to make a tall building stand up to the forces of nature. Nowadays, many buildings are constructed around a strong central core that supports the structure like a spine. This spine acts like a tall, wide column held together with trusses or walls. In many buildings the core is constructed around the elevator shafts because they must be very straight. There could be multiple elevators, with a wide core composed of many solid vertical columns. The floors of the building then span from the core to the columns, held up by tension from the core. Often the central core and perimeter columns are supported from deep underground in underground columns called piles. These piles act like roots of a tree, adding even more stability.

The Effects of Resonance

The mere weight of an enormous building can help a building. But the building’s load is often not enough. When the wind blows or the ground shakes with exactly the right speed, they can intensify a building’s natural swaying motion, causing it to rock back and forth dramatically. This great increase in movement of an object due to matching its frequency of shaking is called resonance.

If you have ever been to the top of a sky scraper, then you may have felt it swaying because of wind. The building was purposely designed to be flexible and to move a little with the wind and the movements of the Earth. That’s because very small movements at the base are translated to larger movements at the top. In an earthquake a flexible building rides the waves of the shaking Earth almost like a surfer. Actually, a smaller, more rigid structure that does not sway is more likely to fracture and collapse. It may seem unlikely, but you may be better off in a well designed skyscraper than in a two-story building during an earthquake.

Every tall building sways from one side to the other and back again in a set number of seconds. The time it takes for one full cycle is called a period. You may also hear of this idea called frequency which is the number of cycles in a second. The Sears Tower in Chicago, for instance, has a period of 7 and ¾ seconds. The Burj Dubai will have a period of eleven seconds. If the wind pulling on the edges of the building begin pulling the structure from side to side in rhythm with the natural period of the building, the building will begin to sway a greater and greater distance from its vertical position. To understand how this happens, think of how you move back and forth on a swing. When you kick your feet in rhythm with the motion of the swing, each kick propels the swing higher and higher. When timed correctly, the relatively small force of kicking results in increasingly higher swings. Similarly, at certain speeds, the small forces of the wind may cause a building to start rocking violently.

How to Prevent and Reduce the Effects of Resonance

All skyscrapers are designed to compensate for these forces. Engineers can “tune” a building to make sure that it has a period that is unlikely to become synchronized with the side-to-side pushing of the wind. By

analyzing data from wind currents we can determine the safest period for the structure. Engineers tune a tower by moving a weight of the structure higher or lower. The higher the weight, the longer the building’s period.

The base isolation technique supports the entire building on bearings made from alternate layers of rubber and steel that act like springs. The base isolators are placed between the foundation and the building so the structure floats in isolation and dampens (reduces by absorbing) the vibrations. This results in the building vibrating at a lower frequency/period so earthquake damage is reduced. Some buildings use massive weights called Mass dampers that swing back and forth to counterbalance the building’s movements. .

Joints are critical

One of the most important aspects of making a tower strong is the quality of the joints. When a joint fails the entire structure is weakened. Total failure of the structure will follow. For additional strength horizontal pieces called girders between the vertical columns can be added below the floors. If you study pictures of various open towers you will notice many different joints, bracing and other construction methods that could provide a stabile quake-resistant structure. You can do your own testing to see if diagonals with wide angles or those with narrow angles would work better. One of the most important aspect is the way you make your joints between two beams. It is very helpful to identify whether a beam is in compression or tension so you will know which way it will likely move. The vertical components of a tower are called columns. They transmit the load to the ground. Beams between columns are called girders. The connection between the girder and column at the joints plays a crucial role in the overall strength of the frame. It is the most critical aspect of making the tower strong. The strongest joints are lap joints where the side-grain of the material joined. The weakest joints are butt joints where the end-grain of the material is joined.

Notes topics

1. Earthquake

2. Seismology

3. Seismograph

4. What’s the difference between p-waves and S-waves?

5. Focus

6. Epicenter

7. How much greater is a level 7 magnitude earthquake compared to a level 6 magnitude earthquake?

8. What’s the difference between dead loads and live loads?

9. What are the four types of stress and how do they affect materials?

10. Which type of stress causes crushing and buckling of beams?

11. What kind of stress is one of the biggest problems during earthquakes?

12. What have Engineers learned from studying the foundations of ancient buildings?

13. What two factors are the reason earthquakes cause so much damage?

14. How do Engineers design structures to resist racking?

15. Truss

16. Central core

17. What does resonance cause buildings to do?

18. The number of cycles of vibrations per second a building sways is a building’s natural frequency. Can this be changed? Explain.

19. base isolators

20. What must be taken into consideration about making joints where beams and girders join together?.