NC Science Challenge
Spaghetti Bridge Building Challenge
Instructor Information
Overview
The NC Science Challenge provides its challenges to teachers and instructors in
hope that they will use them with students. Instructor information is not a part of
the official NC Science Challenge. It intention is to provide background and other
useful information to instructors.
Most NC Science Challenges are specifically designed to use inexpensive
materials commonly available from home improvement and grocery stores and
minimize pre-activity construction and set-up. Yet, they incorporate and
demonstrate detailed sound scientific and engineering principles.
Background
Many bridge building competitions exists. In general terms, bridge building
competitions vary in the materials used for the bridge members and the method
of joining the bridge members. Typical bridge member materials are balsa wood,
popsicle sticks, toothpicks and pasta. Typical bridge member joining methods
include white glue (Elmer's or wood glue) or hot glue. Each material and joining
method has is pros and cons.
For general bridge building in a classroom or challenge setting, pasta (a
combination of spaghetti and fettuccini) is a great building material. First it is
very cheap and easily available. Pasta also can be precisely snapped to size
without the use of a knife or a razor blade which is important for safety reasons
and for incorporating diagonals and cross members into the bridge design.
Unlike toothpicks which are also often used, pasta does not have pointy sharp
ends that hurt particularly after long use. Furthermore, pasta is more brittle that
the other materials making it a more realistic bridge materials so that its strength
is on the order of the joints so that the pasta often breaks whereas other
materials, it is always the joints that break. Plus is also semi flexible so you can
see the modes of stress (bending, twisting, shearing) when a load is placed on
the bridge. You can partially load the bridge during construction to help students
physically see where their bridge needs cross members or diagonals to reinforce
the bridge. It is hard to do this with other rigid materials because the joints
typically break before being able to see the stress modes. By using both
spaghetti and fettuccini, you can simulate beams and cables. Beams are much
stronger when loaded along their thicker dimension. Spaghetti can be used
when the primary stress mode is tension to minimize bridge weight.
Popsicle stick bridges are great for younger students (less than 6th grade) as
they are sturdy and easy to work with for younger students who typically have
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less fine motor control and smaller hands. While popsicles sticks are difficult to
cut, two of them can be overlapped to make a longer diagonal.
For very simply bridges that very young students can build, use gumdrops and
toothpicks. They are good for very young students (under grade 3). The only
issue is that the ends are quite pointy and sharp.
Balsawood, a typical bridge competition material, is good for older students when
you are comfortable with them using a Exacto knife or razor blade to achieve
precise designs. Another benefit of balsawood is that you can teach different
type of joints: butt, lap, half lap, T, dado, box, dovetail, etc.
Mini low-temperature (250º) hot glue gules are great for rapid building as adheres
material quickly. Mini low-temperature glue gun are much safer than the larger
glue guns because they produce smaller volumes of cooler hot glue. This nearly
eliminates the chance of a serious burn.
Proper building surface is important for ease of construction. It is best to use a
surface that minimizes the glue adhesion. Hot glue adheres very well to
aluminum foil, plastic food wrap, and plastic sheets. Some aluminum cooking
sheets, glass and ceramic surfaces work well. Paper and paper towels work well
as the fibrous surface will peel away fairly easily and cleanly. Because paper
works well, a side view of the bridge can be drafted on graph paper and then
used as a template for construction. Building the sides of the bridges directly on
the graph paper greatly eases construction and aids in precise bridge
construction.
Learning Objectives
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Learn about the strengths of triangles compared to other shapes such as
squares.
Learn about forces that effect bridge design: compression, tension,
bending, torsion (twisting), shearing.
Learn about beam orientation and its effect on maximum loading.
Learn about various bridge designs such as Pratt and Warren trust
bridges.
Pre-Activity Presentation and Instructions
Introduction Multi Media
To capture students minds and entertain participants while waiting to start, show
videos on the Tacoma and other bridge collapses to demonstrate the importance
on structural design. The Tacoma Bridge video shows how strong winds can
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caused a bridge to oscillate weakening members of the bridge until the bridge
eventually collapses. The need for good bridge design and consideration for
other forces such as wind is visually demonstrated.
Tacoma Bridge Oscillation and Collapse
Bridge Collapse in Earthquake in Chile
Bridge Collapse: I-35W
Stress Modes and Basic Design Elements
Explain the modes of stress: compression, tension, bending, torsion (twisting)
and shear.
Compression is the force when an object is squeezed.
Tension is the pulling force when an object is stretched (it is the opposite of
compression).
Bending the curvature deformation of an object when a load is applied
perpendicular to an axis of an object such as a beam. When a object bends, the
top side of the material undergoes tension and the bottom undergoes
compression.
Torsion is the twisting of an object due to an applied torque (rotational force).
Shear is the stress that occurs when opposite parallel or tangential to a face are
applied.
Demonstrate tension and compression on spaghetti. Ask the participants which
of the two modes is the spaghetti the strongest and which the weakest. Then
demonstrate that spaghetti is strong when the force is applied as tension but
weak when applied as compression. Tension pulls on the spaghetti forcing it to
stay straight. Compression leads to bending which breaks the brittle spaghetti.
Explain this is in part how suspension bridges work. Suspension bridges rely on
the ability of cables to support great tension loads. Point out the cables have
essentially no ability to support compression loads (i.e., you cannot push on a
string as there is no resistance but you can pull on a string).
Beams, oblong pieces of lumber or metal, are often used in bridge construction
because they can support great weight as they resist bending along their long
cross sectional axis. Beams increase strength and reduce weight when the force
on the beam will primarily be along the long axis.
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Demonstrate the effect beam dimension and orientation on the bending mode
using a fettuccini noodle. Notice how little force it takes to bend the fettuccini
when the force is applied against its thin side. Now apply the force against the
top of the thick side of the fettuccini and notice that it take much more force to
bend it along this axis.
When the load is mostly in one direction, a flat beam can be used to give the
structure great strength with minimum weight. Using two or three beams
oriented 90 degrees from each other forms a beam that is strong in two
directions while minimizing weight. This is the basic principle behind an I-beam.
In simplistic terms, an I-beams is composed of two horizontal beams on the top
and bottom of a vertical beam. This allows an I-beam to resist bending in two
directions: vertically and horizontally. An I-Beam minimizes weight but is very
strong both on vertical and horizontal bending. This makes them great for many
construction purposes.
Explain what shear force is. Shear force is applied when forces parallel or
tangential to a face of an object is applied. Demonstrate shear force with a
square made from a straw or gumdrops and toothpicks that a square is not
strong against shear forces. A square in theory could support strong loads if
there were no shear forces. Realistically, any shear force on the square
collapses it.
Then show that a triangle is very strong against shear force. In fact, triangles are
also strong against compression and tension forces. This is why triangles are
used on most trust bridge designs.
Demonstrate that a square with a single diagonal makes two triangles which
makes the structure very strong because it can handle compression and shear
forces. Squares with diagonals are also commonly used in structure design
because of their strength.
Torsion is the twisting of an object due to an applied torque. Demonstrate torsion
various pasta shapes.
Demonstrate torsion on a square tubular bridge. Show how it twists along the
long axis with ease. Adding triangles or diagonals reduces the ease at which the
bridges twist.
You can have the participants manually apply these modes of stress to spaghetti
and fettuccini pasta to feel the different in the modes and the ease or difficulty
each mode will break the pasta.
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Show various simple bridge designs such as the Pratt and Warren trust bridges
and various modifications of these designs and ask then why these designs are
so strong yet light weight.
Materials and Equipment:
Pencils (drafting bridge)
Mini hot glue gun
Mini hot glue sticks (clear all temperature craft hot glue)
Spaghetti and fettuccini
Testing platform (see Testing Platform Design)
Load block, rod and weight hanger (see Testing Platform Design)
30 lbs. of River Rocks ($3.00 - the most economical weight material but must
weight with a scale), 2 inch washers (less expensive weight alternative -$50) or
weights set: 0.1, 0.25, 0.5, 1-lbs weights ($200)
Hanging weight scale if calibrated weights are not used (i.e., rock or washers) (http://www.americanweigh.com/product_info.php?cPath=46&products_id=485)
Handled pale for holding weights
Extension cable for weight pale for larger bridges
Calipers for measuring pasta to specification if standard pasta is not provided
Safety glasses (worn when building and when testing bridge)
Digital scale for bridge weight
Extension cords and power strips for hot glue guns
Optional:
Digital Camera - photos of bridges
DVD player or Computer monitor - video of bridges
Testing Platform
Test Platform
Any two strong flat surfaces such as tables that are steady and about 2-3ft off the
ground are suitable as a test platform.
Alternately, a simple gorge can be assembled with 2-1x4x4 ($2-4ea) and 21x4x2 ($2-4) inch precut pine lumber and C-clamps ($0.99ea) as shown below.
If willing to cut lumber the cost can be further reduced. The boards can be hot
glued into position to save the cost of C-clamps but disassembly, adjusting gorge
size, and storage is more difficult.
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Another suggestions is a appropriate size piece of 3/4 inch plywood with the
gorge cut out with a jig-saw and placed between tables or saw horses. This
helps keep the gorge size consistent compared to using two tables for the gorge.
However, this gorge is non-adjustable.
Load Block, Rod and weight hanger
Loading the bridge with weight to test its strength has many issues. Should an
expensive weight set be used or should cheap materials like rocks or sand be
used in combination with a scale. The other issue is how to distribute the weight
and where to place the load.
Scientific slotted weights are costly, generally well over $100 but provide a way
to calculate weight without a scale or other weighing apparatus. Another
disadvantage is that the weight can only be added in increments of the smallest
weight. Others have proposed a relative weight system by using a standard
weight such a metal washers or bolts or other heavy consistent size objects.
Inexpensive electronic hanging scales used to weight luggage ($5-$25ea) are
now widely available and measure in very small increments. This allows for
accurate load measurement and allows for the use of inexpensive materials like
sand and river rocks or pebbles. Sand is easy to scoop and flows smoothly, but
it can be messy to work with and clean up. Smooth river rocks or pebbles are
easy to work with and easy to clean up; hence, are the recommended load
material for the average classroom.
The load must be evenly distributed over some portion of the bridge. This is
often done with a small piece of wood with a metal hook and attaching a pale
where weight is added to produce a load on the bridge. This works if there is a
opening large enough to get the block into the bridge. If the bridge is an open
design (objects can pass through) then this is not a issue. However, building a
open designed bridge is more complex and requires more thought and is better
suited for older students with sufficient time. Often students will build a bridge
that they later realize there is no opening for a wooden test block and then
testing their bridge requires other methods or the bridge must be disqualified.
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Other approaches use u-bolts as the load apparatus. The flat plat is often easier
to fit in the openings of a bridge and the u-bolt attached with the screws. Then a
pale is attached with an s-hook.
The long length of the wooden block or the u-bolt's plate is typically placed along
the long axis of the bridge. If the bridge design allows, the load plate can be
placed across the width of the bridge. This can minimize the need for high
strength decking and place more of the weight onto the trust structure of the
bridge. While real life bridges need a decking systems, this add another
dimension of complexity to bridge building; hence, this needs to be factored into
the age and time appropriateness.
When instruction and building time is limited and the competition will involved a
wide range of ages, we recommend using a more simple approach. Bridges can
be load tested via 1/4' polyester or polypropylene rope across the width of the
bridge or a short section of decking in the center of the bridge. This simple load
approach allows for testing a wide range of bridges that students might build
without having to disqualify a bridge.
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Budget
Studnets
per
Session
(Classes)
12
Item
Spaghetti
Fettucinni
Mini hot glue gun
Mini hot glue sticks
Testing platform
Rope, U-bolt or load rod
Load block, rod and hanger
Bag of river stones or pebbles
Calipers
Safety glasses
Total
#
1
1
1
5
1
1
1
1
1
1
Units
Box
Box
each
sticks
each
each
each
each
each
each
Number of
Sessions
8
# of
Unit
Studnets
Cost
Served
I or R
$3.00
5
R
$3.00
5
R
$5.00
1
I
$0.05
1
R
$20.00
100
I
$2.00
100
I
$2.00
50
I
$5.00
100
I
$10.00
100
I
$1.00
1
I
(I) Initial
Expense
Total w/ Options
Total First Time
Expense w/ Options
(R)
Reoccuring
Expense
$57.60
$57.60
$60.00
$24.00
$20.00
$2.00
$2.00
$5.00
$10.00
$12.00
$111.00
Total First Time Expense
Optional:
Digital Camera
DVD player
Total
Total
Studnets
96
$139.20
$250.20
1 each
1 each
$200.00
$100.00
300
300
I
I
$200.00
$100.00
$300.00
$0.00
$411.00
$139.20
$550.20
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Participant Name Tag and Bridge Documentation for Photographs
(optional)
(fold here)
NC Science Challenge - September 25, 2010
Name Code:
Bridge Weight:
Grade:
Raw Max Load:
Out of Spec. Penalty:
Scoring Max Load:
Load/Weight Ratio: