Engineering Marvels: Building Bridges, A, Way To Bridges The Gap
By
Gustavo A Sandoval Sr.
Introduction
A bridge is a structure built to span a valley, road, body of water, or other
physical obstacle, for the purpose of providing passage over the obstacle. Designs of
bridges vary depending on the function of the bridge, the nature of the terrain where the
bridge is constructed and the material used to make it. The first bridges were made by
nature itself as simple as a log fallen across a stream. The first bridges made by humans
were probably spans of wooden logs or planks and eventually stones, using a simple
support and crossbeam arrangement. Some early Americans used trees or bamboo poles
to cross small caverns or wells to get from one place to another.
Bridges may be classified by how the forces of tension, compression, bending,
torsion and shear are distributed through their structure. Most bridges will employ all of
the principal forces to some degree, but only a few will predominate. The separation of
forces may be quite clear. In a suspension or cable-stayed span, the elements in tension
are distinct in shape and placement.
A bridge's structural efficiency may be considered to be the ratio of load carried
to bridge mass, given a specific set of material types. In one common challenge students
are divided into groups and given a quantity of wood sticks, a distance to span, and glue,
and then asked to construct a bridge that will be tested to destruction by the progressive
addition of load at the center of the span. The bridge taking the greatest load is by this
test the most structurally efficient.
Orientation
There are more than half a million bridges in the United States, and you rely on
them every day to cross obstacles like streams, valleys, and railroad tracks. But do you
know how they work? Or why some bridges are curved while others are straight?
Engineers must consider many things -- like the distance to be spanned and the types of
materials available -- before determining the size, shape, and overall look of a bridge.
Bridges are some of mankind's grandest engineering works, but sometimes they
fall down. The most recent large-scale disaster came on Aug. 1, 2007, when the I-35W
Bridge over the Mississippi River in Minneapolis collapsed. The bridge collapsed at
6:05PM beneath bumper-to-bumper rush hour traffic confined to 4 of 8 lanes due to
resurfacing in progress. The NTSB said that undersized gusset plates, increased concrete
surfacing load, and weight of construction supplies/equipment caused this collapse.
Journal
Bridges need to hold the following obvious loads: cars, trucks, people, and bikes.
When a bridge is going to be constructed it must be approved by a professional
known as a civil engineer. During each phase of design and construction a
professional needs to sign off and stamp the drawings, calculations, observations, etc.
Describe how geometry and span contribute to the efficiency of a bridge and how
does the civil engineer’s understanding of span and geometry determine whether a
blue prints for a bridge get approved or denied for construction.
Assessment
This exam contains 10 multiple choice questions, each worth 2 points. Circle the correct
response for each question. Make sure that your answer is clearly marked. This exam is
worth a total of 20 points. Print your name at the top of this page in the upper right hand
corner.
1. Span is defined as?
a. Span is the distance between two bridges.
b. Span is the distance between two beams
c. Span is the distance between two cables
2. The beam or truss bridge is, in effect, a pair of girders supporting a deck spanning the gap
between two piers. Such a beam has to withstand what kind of forces?
a. Compression and Torsion
b. Shear and Tension
c. Compression and Tension
3. The longer the span of a bridge its structure will?
a. Will be able to support more weight.
b. Will be able to support less weight.
c. Will have no effect on the weight it can support.
4. A bridge's structural efficiency may be considered to be the ratio of?
a. Load carried to bridge mass
b. Type of materials used to type of terrain
c. Type of weather to number of vehicles
5. How are bridges classified? Bridges may be classified by how the forces of
a. How they handle weathering
b. How they handle traffic
c. How the handle forces
6. Which of the following is not a bridge joint?
a. Notched joint
b. Ball and socket joint
c. End Joint
7. Compression is what kind of force?
a. Pulling Forces
b. Pushing Force
c. Twisting Force
8. When does shearing occur?
a. When two opposing forces act on the same point
b. When two opposing forces act on different points
c. When two equal forces act on the same point.
9. Beam bridges were design to do what?
a. Span long distances
b. Span short distance
c. Span any distance
10. Truss bridges use what geometrical shape?
a. Triangle
b. Rectangle
c. Square
Glossary
Arch - A typically curved structural member spanning an opening and serving as a
support
Arch bridge – a bridge whose main support structure is an arch. Additionally, the bridge
may be termed a through arch, which is simply one where the roadway appears to go
through the arch.
Beam - A horizontal structure member supporting vertical loads by resisting bending. A
girder is a larger beam, especially when made of multiple plates. Deeper, longer members
are created by using trusses
Beam Bridge - A bridge built of beams, either classified as a short-span or long-span
beam bridge, which is supported on beams whose ends rest on piers or abutments.
Compression - is a pushing force
Dead Load - the static load imposed by the weight of materials that make up the bridge
structure itself.
Force - Any action that tends to maintain or alter the position of a structure
Joint - A device connecting two or more adjacent parts of a structure. A roller joint
allows adjacent parts to move controllably past one another. A rigid joint prevents
adjacent parts from moving or rotating past one another.
NTSB – National Transportation Safety Board
Shear – is a sliding force
Span - The horizontal space between two supports of a structure.
Structural efficiency – is a ratio of load carried to bridge mass.
Tension - A stretching force that pulls on a material.
Terrain - General term in physical geography, referring to the lie of the land
Torsion - An action that twists a material.
Bridging the Gap: Building Bridges 101, It Is Time to Get To Work
Objectives:
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Be able to define in your own words what your bridge must achieve
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Design your bridge
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Build the bridge
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Test the bridge
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Evaluate of the bridge
Background:
There are many different types of bridges. Some are simple bridges that have
beams underneath to support the bridge deck like the type used for highway overpasses.
This type is usually used for spanning things like roads or small rivers. Other bridges use
truss design and can "span" wider roads or rivers or to support heavy loads like trains.
Another type of bridge is a suspension bridge. Suspension bridges are used for very long
spans. The word "span" means the distance between the supports that hold up the bridge.
Purpose:
The purpose of any bridge is to crossover to a new land. People make them so they don't
have to go the long way, or if the bridge is over water that means simply people do not
want to swim in the water. A bridge must withstand different forces such as: tension,
compression, and torsion. Beside forces a bridge must have good structural efficiency
which is, is a ratio of load carried to bridge mass.
Materials:
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Spaghetti noodles – can be purchased at any supermarket
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Spaghettini: very thin spaghetti
Spaghettoni: Thick spaghetti
Bucatini: is a thick spaghetti-like pasta with a hole running through the
center
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Tools
- Hot Glue Gun
- Glue Sticks
- Exacto Knife
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Construction paper
Scale
First you must read and understand the parameter:
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The bridge must be constructed using only spaghetti noodles use glue sticks that
are used for glue guns.
The minimum length of the bridge is 8 ¼ inches. This is to ensure that the bridge
is long enough to fit into the testing frame and tested at a span of 5 5/8 inches.
The maximum height of the bridge is 2 inches to 2 ½ inches and is measured from
very the top of the bridge to the very bottom. This is to ensure that the bridge fits
into the testing frame.
The total outside width of the bridge should be a maximum of 2 ¾ inches. This is
to allow a testing block to be placed across the bridge deck to test the strength of
the bridge.
Procedure:
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First you must decide on the shape and overall design of the bridge just as an
engineer does.
Sketch out the shape of the bridge to full size on a piece of paper. The sketch will
tell you where each spaghetti noodle will go when you start gluing them together.
The sketch will include the top and bottom view of the bridge and the cross
bracing in between the top and bottom.
Glue the cross bracing between the top and bottom cords.
You have now completed only one truss. This has to be repeated for the second
truss. This picture shows two completed trusses.
You will need to put some supports across the bottom cords of the two trusses to
support the bridge deck, these are called deck beams. Making sure that the trusses
are no more than 2 ¾ inches apart.
The next step is to add the bridge deck.
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Let the bridge sit 15 minutes before testing the bridge.
Weigh the bridge before testing and record this number.
Place the bridge on the testing frame.
Once you have place the bridge on the testing frame you made weight to the
bridge. You can use steel beams make sure they are the same length and weight.
Once the bridge breaks you can test the efficiency of the bridge by using the
following equation efficiency = total weight held/weight of bridge