Eggshell Dome Discrepant Event

Author(s): Peggy LeDuff & Nabila Jahchan
Discrepant Event - Teacher's Guide
SED 695B; Fall 2005

Handle eggs carefully!!!
We don't want any broken eggs.

If the arch or dome shape of an eggshell is so fragile,
why is the arch and dome shape used
for building all kinds of structures?
Perform an experiment, using eggs, to test the strength of the dome shape and effects of compression forces.

8th Grade Physical Science Standards:
Forces 2. b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.

d. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.

Investigation and Experimentation
9. a. Plan and conduct a scientific investigation to test a hypothesis.

 

 
Principles illustrated
  • Arches-even those made of eggshells--are strong because they exert horizontal as well as vertical forces to resist the pressure of heavy loads. The crown of an eggshell can support heavy books because the weight is distributed evenly along the structure of the egg.
  • An arch is a curved structure that supports or strengthens a building. Almost all arches span openings and support weight above them. Others are enclosed in walls.

    Most arches are made of stone, brick, concrete, or steel. Arches of stone or brick consist of wedge-shaped blocks called voussoirs. During the construction of most such arches, the blocks are supported by a wooden frame. The last block to be inserted is the keystone, the center stone at the top. The pressure of each side of the arc against the keystone supports the arch when the frame is removed. In addition, the arch is supported on both sides by masonry or by other arches to keep it from collapsing under the weight above.

    The first people to fully utilize the arch were architects of ancient Rome. During the 300's B.C., they began to use semicircular arches to build aqueducts and bridges. Later, they also constructed triumphal arches to honor their leaders. Pointed arches were developed during the Middle Ages. Medieval architects arranged arches in rows to form passageways called arcades. They also built arched roofs called vaults. Arches shaped like horseshoes are common in Islamic architecture.

    Sources:
      World Book Encyclopedia (1997)
      The Kids Science Book (1995)

Super EggShells: Dome Strength

Ever wonder why igloos are dome shaped and not square? Ever been amazed at how a suspension bridge can hold the weight of hundreds of passing cars with little or no apparent support underneath it? This experiment demonstrates how arches are used in architecture not only for aesthetic appeal but for a very useful and needed purpose...
OBJECTIVE: The student will perform an experiment, using eggs, to test the strength of the dome shape.
MATERIALS: 4 eggs, bathroom scale, towels, & textbooks per group
ACTIVITY
1. Share background material with students.
2. Provide time for students to write a hypothesis indicating the number of textbooks and total weight that the eggshells will hold before breaking.
3. Crack an egg, trying to leave more of the shell intact at the larger air cell end. Discard the contents of the egg and the small end. Carefully chip the large end of the shell until its edge is relatively even.
4. Fold the towel once. Place the eggshells on the towel, chipped edge down. Measure the height of the egg as it sits on the towel. Stack books on the towel across from the egg so they are the same height as the egg.
5. Stack books, one at a time, so their weight is evenly distributed between the stack of books and the egg. Do not press down on the books. Simply stack them one at a time.
6. Have students watch to see how many books you can stack on the eggshells before it begins to crack. Explain that the spiral pattern of the crack indicates the weight was distributed evenly over the dome shape.
7. Use the bathroom scale to weigh the books stacked on top of the eggshells. Have students record the weight before repeating the experiment. Have students record the weight of books each time you repeat the experiment and graph the numbers.
8. Make sure students wash their hands after handling the eggs.

 

     
 

Questioning Script
Prior knowledge, experience,
& common misconceptions
1. Eggshells are fragile.
2. Eggshells are structurally weak.
3. It does not take much pressure to crack an eggshell.
4. Students have handled eggs and have experienced cracking an egg on the side of a bowl or counter 		Two factors contribute to a structure's strength are material and shape.
The eggshell is weak in tension and strong in compression. Chicks are not strong, but by poking with their peaks from inside the egg, they are able to break out of their shells. They break the shell using tension.

To make a structure strong with a tension weak material, such as an eggshell, forces must create compression and avoid tension. Fortunately, the arch/dome shape of the eggshell fulfills these requirements.

For example, it is easy to crush an egg by squeezing it from the sides.

While it is difficult to crush an egg by squeezing it from its ends.


If an elephant is placed on the St. Louis Gateway Arch, the force of the elephant's weight is resisted by the compression along the steel arch. The ground pushes back and the whole system supports the elephant!

Arches and domes are structurally strong and statically stable.

This is a microscopic view of an eggshell. The chicken eggshell is composed
of three layers of calcium carbonate.

Notice the lattice structure.

The staff members at the Ontario Science Centre in Toronto were successful with one unbroken egg supporting a 90 kg person! That's a 200 pound person.

www.uiowa.edu/.../archive/ sem/large/Eggshell.gif

 

 

Root Questions

What is compression force?

What is tension force?

Why are arches and domes strong structures?

Why is the end of an egg able to withstand greater
force than its sides?

Target answers are located on this web page.

 

 

 

www.legoeducation.com/

An Egg-ceptionally Egg-celent Egg-periment
Testing the Influence of Morphological Design
Frank E. Fish, Professor of Biology, West Chester University, PA

The development of the amniotic egg is considered one of the major evolutionary events, heralding full independence from water and the conquest of land by vertebrates. Although first evolving with the reptiles, it is in the form of the avian or more specifically chicken egg with which most people are acquainted. From a mechanical point of view, the avian eggshell is an impressive example of natural engineering. It provides the outer capsule of an almost fully self-contained environment that supports the mass of the developing chick (Ar et al. 1979). In addition, the shell forms a protective barrier that prevents the egg from being crushed during incubation.

The shell is a composite of a biological ceramic, calcite, and 2-4% of organic fibers (Vincent, 1990). The calcite component in the shell has greater strength and stiffness compared to structural proteins and insect chitin, but calcite is more brittle. The distribution of calcite crystals is not homogeneous throughout the shell. The shell is porous permitting respiratory gases to pass through it. The morphology of the shell materials is highly complex and mechanically enigmatic (Vincent, 1990). The shell is arranged in layers starting externally with a cuticle, crystal layer, palisade layer, cone layer, outer membrane, and inner membrane. As a result, the physical properties of the shell vary through its thickness. Hardness is lower in the center of the shell compared to the inside and outside of a domestic hen's egg (Tung et al., 1968). However, the particular geometry of the eggshell makes it remarkably resistive to external loading.

 

The mechanics of an arch permits great structural strength with an economy of materials. A dome is an arch rotated about its vertical axis and, as is familiar to architects, large roofs can be supported without internal bracing. Domes are stiffer than arches, because of their three-dimensional structure. Arches and domes are statically stable. They can support large loads, because their walls are mainly under compression.

However, the magnitude of forces necessary to stabilize an arch varies with its geometry. The pointed and parabolic arches are more vaulted and require lower stabilizing forces when subjected to a compressive load than a simple semicircular arch. Similarly, a highly vaulted dome (i.e., low radius of curvature) is stronger than a flatter dome (i.e., high radius of curvature).

 

www.legoeducation.com/

www.legoeducation.com/

Set Up: eggs at
corners of books _____________ First Book ________

_______Second Book______________Third Book_________

_____Fourth Book____________Fifth Book_________

______Sixth Book, and they all come tumbling down with only 27 lbs on them!

This was NOT a successful attempt.
The eggs will normally hold many more books.
The results definitely depends on the preparation of the eggs.
When the eggs are trimmed, any cracks created will result with fewer books being able to crack the eggs.
Also, using a towel under the eggs will allow the eggs to withstand
more pressure.

Use this chart to record the results of a class.

Use this link to download file as pdf.

The record for my classes was 16 textbooks with a total weight of 76.8 lbs. However, a student was able to sit
on a stack of books for 6 seconds before the eggshells broke. The student weighed approximately 135 lbs.

The downward arrows show the force of the books acting on the eggshells.
The upward arrows show the force of the eggshell acting against the books.
This activity demonstrates compression forces.

 

References & Links:

http://www.design-technology.org/page1.htm

http://www.sicb.org/dl/rtfview.php3?file=30.1.rtf

http://www.spartechsoftware.com/reeko/Experiments/ExpEggShellArches.htm

http://www.polial.polito.it/cdc/Mercurio/Polymer/mercurio_mechpro/05MP_Concepts.htm

http://www.physicsmyths.org.uk/

http://www.diydoctor.org.uk/projects/forces.htm

http://www.legoeducation.com/content/item.aspx?CategoryID=43&art=26&ap=2

http://www.loblaws.ca/en/science_exp2.asp

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