Download Towering Toothpick Disaster: An Earthquake Kit Er Student

Towering Toothpick Disaster:
An Earthquake Kit Er
Student Guide
OVERVIEW
ln the following lab you will be issued a challeng-e:
_design and build a three-story "earthquake resistant,,structure
using only a set amount of glue, toothpicks,
sptints, Styrofoam@ and your own ingenuity.
once you have built your structule, you witiactually
"r"it "ti"x","*ooo"n
test its;ei.tnluar.e resistance,, by placing it on a specially
designed Shake Ratile and Roll Earihquake Boarcl.
Before you start designing your structure, you should be familiar
with the following terms and background informa-
tion.
GLOSSARY
converge
to come together from different directions; to meet.
-
earthquake
-
the natural shaking of the Earth due lo the release of energy
as rocks move along a fault.
earthquake focus
epicenter
fault
-
the location on the Earth's sudace directly above the focus of an
earthquake.
-
a break in the Earth's crust along which there is rock movement.
freguency
ioists
the point where an earthquake originates.
-
the number of wave cycles that pass a given point per unit of time.
-
parallel, horizontal beams set from wall to wall to
support the boards of a floor or ceiling.
-
'lithosphere
-
a layer comprised of the Earth's crust in combination with the rigid
upper man1e.
mantle
the region below the Jvloho (see definitio.n, below), consisting
of two major regions. The upper 100
- " --"
- rigid
meters is
and the lower 250 meteis is plastic_like .Moho
boundary between the Earth's crust and mantle that causes earthquake
waves to change speed.
- the
The Moho
layer is named after its discover, naonorovicic,
scientist.
natural frequencies
to vibrate.
plate
-
"-Vffir"rian
-
the frequency at which the least amount of energy is necessary to cause a
specific object
portion of the Earth's lithosphere that moves.
seismic
- term "ppii"o to shaking or vibrations caused by Earthquakes or movement of plates.
shearing
an occurrence where two parts of pieces slide onto each other
from opposite directions. Any strain or
- resulting
deformation
from this is referred to
as'shearing stress.
subduction
the downward thrust of dense oceanic crust into the mantle as less
dense continental crust is
- oceanic
forced over the
crust at zones of convergence.
BACKGROUND
The Earth's sudace is covered by a crust. This crust, together
with the rigid upper portion of the mange that lies
just below the crust, makes. up th'e lithosphere.
Ratherihan being one solid coating on the outside of the Earth,
the lithosphere is broken into approximatily is piec""
Each ptare is a different size and thickness.
Poftions of the plates are anywhere from 5'to 615 riro*rte[iJi"
iz mites thick). These tithospheric ptates ride
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atop a plastic-like portion of the mantle called the asthenosphere. lt is the interaction between plates as they
move over the surface of the Earth which produces earthquakes.
An earthquake occurs when energy is released, as rocks move relative to each other along a fautt. Usually,
earthquakes are not random events. While it may be difficult to predict when an earthquake will happen, scientists
can usually predict where an earthquake might happen. Although large earthquakes have occurred in the middle
of plates well away from plate boundaries, earthquakes or seismic altivity usually occur where plates come
together or converge. Seismic activity occurs in zones where convergenie, subduction, or grinding of plates
occurs.
Converging plate boundaries exist where continental crust meets continental crust. As these two.crusts are lorced
together, Iow density rocks pile up to form mountain ranges. ln the process, pressure and friction from the convergence cause the formation of metamorphic rocks, as well as new faults and fractures. The Himalayan mountains
of lndia and ribet are the direct result of continental crust convergence.
An example of subduction of plates can be lound off the west coast of South America. When thin, dense oceanic
crust meets thicker, less dense continental crust at a convergent plate boundary, the more dense oceanic crust is
forced under the continental crusl. As the solid rock is forced-to bend into tne asinenosphere, stress builds along
the faults in the subducted oceanic rock. This causes the rocks to move, releasing eneigy which results in an
earthquake.
A third type of seismically active zone occurs when plates grind against each other as they move past one
another' This movement may be at different rates in the same direction or, as in the west coast of North America,
in opposite directions. For this reason, the west coast of North America is a seismic hot spot. Scientists have
determined that the Pacific Plate (containing a portion of California) moves in a northerly direction while the North
American Plate (containing North America, including the rest of Caiifornia) moves in a southerly direction. As the
plates move in opposite directions, stress builds. When the stress builds beyond the strength of the rocks, the
rocks fail and an earthquake occurs.
ln all earthquakes, energy from the release of stress as the rocks fail is transmitted from the site of the earthquake
to surrounding locations. This energy takes one of two forms: body waves and surface energy waves. Body
waves move through the Earth's interior, traveling much more quickty than surface waves. Surface waves move
over the surface of the Earth.
Body Waves
Primary, compressional, or P waves are the first energy waves to arrive at an earthquake recording (or seismograph) station after an earthquake occurs. Primary waves travel through both solid and liquid layers oj ine eartn in
a push-pull, linear motion, similar to an earthworm. Compressional waves travel as fast as 5.5 (misecond, or
more than 12,000 miles per hour depending on the type of substance they are traveling through. Because P
waves travel in a linear motion, there is little displacement of Earth materials. Primary iaves ire the least damaging of all seismic waves.
Secondary, shear, or S waves are a second type of body wave that travel through the interior of the Earth.
Usually the most destructive of the two types of body waves, shear waves only piss through solids. Because they
cannot pass through the liquid outer core of the Earth's interior, secondary waves are not received at earthquake
recording stations that are more than 143 degrees from a quake's epicenier. Unlike the linear motion of P waves,
S waves travel in serpent-like motion. The energy of an S wave bounces to the Earth's surface and then back
towards the interior. Secondary waves travel slower than primary waves, reaching a maximum velocity o{ about 3
km/second.
As the distance between an epicenter and seismograph station increases, the difference in arrival times of P and
S waves also increases. Both types of body waveJ also become less intense as they travel from the site ol an
earthquake. High frequencies characterize both types of body waves. The high frequencies of body waves are
often similar to the natural frequencies of low Ouititings and structures. As th-e high'frequencies of 'body waves
approach the high naturalfrequencies of buildings, the buildings begin to vibrate. this increase in frequ-ency and
vibration causes buildings to sway. Frequency and sway increise towards the top of tall buildings, in a fashion
similar to the child's g_ame, "Crack the Whip" where the greatest amount of energy is felt at the 6nd of the line. A
poorly designed building with little flexibility often topplei or collapses as the resuit of the build-up of energy.
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,
Surface Waves
Love or L waves' named after Ehglish mathematician A.E.H. Love, arrive after secondary waves. Love waves
are horizontal, transverse waves that travel across the surface of the Earth. Like a snake, these type of waves
move fonvard as energy is distributed from side to side.
Rayleigh waves, named after Lord Rayleigh, an English physicist, are a second type of surface wave. Rayleigh
waves are the last to arrive at a location distant from the epitenter. Rayleigh waves travel in a retrograde or
backwards elliptical motion, much like the upwards uncoiling of a spring. T}e energy that is transmitt-ed as a
Rayleigh wave causes the surface of the Earth to appear tdrock and roll.
ln contrast to body waves which begin at the earthquake focus where plate movement occurs, surface waves
begin at the epicenter and usually travel greater disiances than body waves. Additionally, low fiequency sur{ace
waves have frequencies similar to the naiural frequencies of tall buildings and structures.
The intensity of vibrations and, therefore, the damage caused by both surface and body waves often depends on
the type of materialthat.a..building is situated on. eJitoings constructed on bedrock usually sustain litile damage
from an earthquake. Buildings, structures, and cities buill on unconsolidated or loose material such as sand, s'ilt,
and clay are often subject to devastating loss of property and human lives. Loose or unconsolidated Earth materials often magnify the intensity of seismic waves.
Amplitude is a physical measure of the extent of a vibration. Vibrations of very high amplitudes translate to very
intense or strong earthquake waves. A seismograph ls an instrument which recoids the intensity of an earthquike
by measuring the shear wave of the largest amptiiuOe. Many seismographs include a free weight suspended
from a wire attached to a support which is anchored to the giounO. A
ieri is attached to the weight and paper is
positioned below the weight. As vibrations pass through thjground, tire pen records any large displacement
of
the free weight' lntensity recorded in this way is then 6onvertid to a Richter magnitude to a[proximate the
strength of the earthquake. The Richter magnitude scale operates on a system 6f tens. An earthquake registering
a magnitude 6 is_10 tiTg:
:lrgn.ger than magnitude 5, 100 iimes stronger than magnitude 4, i o00 times sirongerthan magnitude 3, and 10,000 times strongel than magnitude 2. Magn'itudes greatEr than 7 are classified as major
earthquakes, capable of causing mass destruction and'death.
Actual earthquakes rarely last more than 60 seconds. In these 60 seconds, the ground moves violently, surface
cracks form and rocks are released. The shaking caused by earthquakes often darses poorly designed buildings
to collapse. Populations living in earthquake prone areas aie at greatest risk not from the shiking 5t tfre ground
itself, but from the collapse of structures caused by the earthqua-ke. Architects and engineers em-ptoy sevleral
strategies in an attempt to "earthquake proof" buildings and structures such as bridgeJ and overpasses in earthquake prone areas.
.
On? strategy employed by_ e_ngineers to earthquake proof a building is to drill through loose earth material and
anchor the base of the. building into underlying bedrock using pillar! or pilings and i reinforced concrete slab, as
shown in the figure below..A properly designed and reinforcJd pile foundati5n resists shearing and bending forces
even as loose earth material shifts horizontally around the foundation.
Pilings
A second approach to.building earthquake resistant structures uses a series of base isolators which help to
cushion and disperse forces between a physical structure and its loundation. Base isolators can consist of sheets
of rubber bonded to thin steel reinforcing piates, as shown in the figure below.
Rubber
Steel
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L
Another way to cushion and deflect energy is to use large springs between a building and its foundatiorr.
ln addition to foundation designs, the design of the building itself plays an important role in how it withstands
destructive forces produced by seismic waves. ln general, the most earthquake resistant building designs are
symmetrical and use materials that are strong yet somewhat flexible (or ductile, meaning bendable).
The best way to describe the term symmetrical is to think of a mirror image. Think of the mirror as an imaginary
axis or Iine. Except for the fact that they face each other, the shapes on either side of that axis are exactly the
same. The same is true for symmetrical building designs. lf you drew an imaginary line through the middle of the
design, both sides would look exactly the same, except that they would face 6ach-other. Symhetry (as shown in
the following figures) helps a building maintain its balance in the event of an earthquake. ln buildings that are not
symmetrical, mass is unevenly distributed, and horizontal lorces produced by earthquake waves cause twisting
and distodion of the structure. Once this occurs, the building may become unsafe and eventually collapse.
Pillar
Sides
Symmetrical forms
Ductility is a term that refers to how well a material can be fashioned into a new form, usually by hammering it
thin or stretching it. Usually, when you stretch a material, it becomes more flexible, and is therefore, able to
absorb energy. The key of course is not to stretch a material too much, or it may break. (Think, for instance, of a
piece of taffy; it becomes more flexible when you stretch it; if you stretch it too much, though, it breaks.) While
strong, concrete is not at all ductile. Steelis a very strong material; it is also a very ductilehaterialthat maintains
its strength long after it has been bent or olherwise deformed. Because of this, steel rods are often used to
strengthen and reinforce building materials such as concrete. Concrete walls that are reinforced with steel rods
are known as shear walls.
LAB OBJECTIVES
.'
'
To design an earthquake resistant structure, using only specified materials and specific building guidelines
To test your building design
To observe the effect differences in architectural design have on building strength
DESIGNING YOUR EARTHQUAKE RESISTANT STRUCTURE
Now that you know about different types of earthquake waves and building designs, it's time for you and your lab
partner to put your knowledge to use. Together you should start designing a buiiding that you thihk will withstand
earthquake testing. Keep in mind that you will only be able to use tne following materials ih tne actual construction
of your building.
Up to 200 craft sticks Up to 200 toothpicks
Up to 200 wood splints Titebond@ glue
ln addition, your building must meet the following requirements:
Buildings must be 45 cm tall
Buildings must have 3 stories
Each story must be 15 cm high
Each story must have a floor
Buildings must have roofs
Buildings cannot have solid walls (instead, the structures should be more like scaffolding)
Building bases (footprints) must be 22.5 x22.5 cm
Also keep in mind that you have the option of designing and building a base isolator for your building. You may
construct the base isolator from any materials you select and supply (Styrofoam@ is proviUeO;. -
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When you are designing your walls, remember that they must be strong enough to hold up the ceilings, walls and
roof above. Likewise, floors and ceilings, which are horizontal, must belesigrieO to support both veri-ical forces
(for.example, the people and eqdipment that will be placed on the lloor) as iell as horizontal forces (wind. motion.
etc.) You can increase the strength of these structuralcomponents by adding joists or support beams. Remember, however, that you also want the floors lo be flexible, in order to a6sorb s6me of the energy from the earthquake.
The following diagrams show examples of various structural components. you may want to use these components, as shown, in yqYI design, or you might want to revise the structures, or completely ignore them, and' make
up your own designs. Whatever you do, remember the requirements listed on the previous page; make sure that
your design meets these requirements and that it uses only those materials which are allowed.
Sample horizontal support:
Sample vertical support:
:
Sample wall support:
Sample trusses:
Trus.ses lend support and stability to walls. In the following diagrams, the sample trusses have been applied to
sections of wall.
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Sample floors with corner bracing or cross beams:
Sample base isolator (optional):
(Please note that, other than the Styrofoam@, materials for building a base isolator are not provided with this kit).
o
o
C
o
o
C
Top view (above): This example uses tvvo squares of plywood or Styrofoam@, with matching holes drilled in both
squares, for placement of bolts. The figure above shows iuggested piacement of holes for thJee different base
isolator designs.
end of
threaded
bolt shaft
spflng
bottom plate
bolt head
of
Styrofoam@
or plywood
Side view (above): Bolts are placed through holes of one piece of wood or Styrofoam@. A spring is then placed
on each bolt shaft and the second piece of wood or Styrofoam@ is placed on top of the unit. The first story of the
building can then be glued or otherwise attached to the base isolator.
After you have come up with your design, draw a diagram of the building on graph paper. All diagrams must be
drawn to scale. Along with your drawing, you must provide written support of your design, outlining the qualities
and characteristics that will make the building resistant to earthquake damage. All designs must be approved by
your teacher.
BUILDING YOUR STRUCTURE
Now it's time to actually start constructing your building. Here are a few hints for you to keep in mind while you
build:
'
'
For the tightest hold, apply glue to both surfaces being glued; then press the two surfaces together.
Allow the glue to completely dry before adding more stress or weight to a structure.
Most peo-ple begin by constructing the vertical and horizontal supports (also known as walls, ceilings, floors, and
beams!) for each story. Once the walls are complete, they add any necessary trusses, and then glue the walls for
olly ? single story together, until they have three 22.5 ci X 22.5 cm open boxes. tf desired, the flrst story is then
glued or bolted to the base isolator. The floor/ceiling is then placed atop each story, and the three stories are
stacked and glued on top of one another. The roof is usually the last structural piebe to be placed on the building.
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