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Earthquake Towers Reading
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.
Section 1: 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
affect 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 10fold 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.
Section 1 Review Questions:
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?
Section 2: 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.
Section 2 Review Questions:
1. What’s the difference between dead loads and live loads?
2. What are the five types of stress and how do they affect materials?
3. What kind of stress is one of the biggest problems during earthquakes?
Section 3: 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 B.C.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 would 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 steel reinforced concrete 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. In Armenia, none
of the buildings were earthquake resistant.
Until recently, engineering buildings to withstand earthquakes has not been a priority. 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.
Section 3 Review Questions
1. What have Engineers learned from studying the foundations of ancient buildings?
2. How can engineers lessen the damage and number of deaths from earthquakes?
Section 4: Earthquake Resistant Methods
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 is called a joint. Joints
play 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 ends of the columns and girders will overlap. The
weakest joints are butt joints where the ends of columns and girders
are joined without overlapping.
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 pieces
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.
Section 4 Review Questions:
1. Columns-
2. Girders-
3. Joint-
4. What is the difference between a lap joint and a butt joint?
5. What two factors are the reason earthquakes cause so much damage?
6. How do Engineers design structures to resist racking?
7. Truss-
8. Central core-
Section 5: The Effects of Resonance
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. The mere weight of an enormous
building can help a building resist resonance, but the building’s load is often not
enough.
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 begins 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.
Section 5 Review Questions
1. What is the period of a tall building?
2. What does resonance cause buildings to do?
Section 6: How to 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.
Section 6 Review Questions
1. The number of cycles of vibrations per second a building sways is a building’s natural frequency. Can
this be changed? Explain.
2. base isolators-
3. mass dampers-