Download Earthquake Towers Reading Packet You have been learning that

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 EarthquakeResistant 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.
weights called Mass dampers that swing
back and forth to counterbalance the
building’s movements. .
How to Prevent and Reduce the
Effects of Resonance
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.
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
Joints are critical
Notes topics
1. Earthquake
2. Seismology
3. Seismograph
4. What’s the difference between pwaves 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?.