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Balsa Wood Bridge8th
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How Does a Bridge Stay Up?
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Bridges don’t always Stay Up
Forces act on a bridge
Engineers must design bridges that account for these forces.
Forces can come from many sources.
Can you think of any? Cars Bridge materials Wind? Earthquakes?
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Dealing with Forces
Design Bridge design allows for
the dissipation or transfer of forces. (shown in later slide)
TransferThe movement of
forces to a specifically designed area to account for force extremes.
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Forces on a Bridge
Torsion Twisting force on a material.
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Forces on a Bridge
Tension Pulling Force on a material
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Forces on a Bridge
Compression Squeezing force on A material
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Basic Bridge Designs
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Beam Bridge
Beam Bridge……
Consists of a horizontal Beam supported at each end by piers. The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 Feet.
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Forces on Beam Bridge
When something pushes down on the beam. The beam bends it top edge is pushed together, and its bottom edge is pulled apart.
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Truss Bridge
Truss bridge……
Consists of an assembly of triangles. Truss bridges are commonly made from a series of straight, steel bars. Ridged arms extend from both sides of two prier's. Diagonal steel tubes, projecting from the top and bottom of each pier, hold the arms in place.
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Forces on Truss Bridge
Every bar in this cantilever bridge experiences either a
pushing or pulling force. The bars rarely bend.
Cantilever bridge Firth of
Forth
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Arch Bridge
Arch bridge…. Has a great natural strength. Thousands of years ago, Romans
built arches of of stone. Today, most arch bridges are made of steel or concrete, and they can span up to 800 ft.
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Forces on Arch Bridge
Ancient Roman Aqueduct
The arch is squeezed together, and this squeezing force is carried outward along the curve to the supports at each end.
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Suspension bridge
The suspension bridge… Can span 2,000 to 7,000 ft why farther than any other type of
bridge. Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.
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Forces on Suspension Bridge
Forces act on Suspension
Bridge
Golden Gate Bridge
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Cable- Stayed Bridge
Cable stayed bridge….. Like the suspension bridge, supports the roadway with massive
steel cables, but in a different way. The cables run directly from the roadway up to a tower, forming a unique A shape.
Cable stayed bridges can be build much faster then a suspension bridge and can be built faster.
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Design
Sketch will be done in CADD 3D
Must meet the following Requirements.
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Requirements
Opening will be 8 in.
Bridge must fit in box that’s - Height 5in Width 5in length 12in

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Materials
¼ piece of balsa wood
Amount will be 7 lin feet ( 84 in ) Total
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Sketch
• Dimension On graph paper
• Clear Lines Drawn to scale View of each side (3)• Top Bottom Side
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Print each Side View Inventor
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How the test looks likeBolt Placement
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Unrealistic
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Not going to happen
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Look the the cross members
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What shape is common?
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Point to carry load
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Enrichment
How might a natural disaster change your bridge design?
Discuss:
How bridges are designed differently around the world?
Why some bridge materials are used instead of others?