 Balsa Wood Bridge 8 th

How Does a Bridge Stay Up?

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?

Dealing with Forces  Design  Bridge design allows for the dissipation or transfer of forces. (shown in later slide) Transfer The movement of forces to a specifically designed area to account for force extremes.

Forces on a Bridge  Torsion  Twisting force on a material.

Forces on a Bridge  Tension  Pulling Force on a material

Forces on a Bridge  Compression  Squeezing force on  A material

Basic Bridge Designs

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.

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.

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.

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

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.

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.

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.

Forces on Suspension Bridge Forces act on Suspension Bridge Golden Gate Bridge

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.

Design Sketch will be done in CADD 3D Must meet the following Requirements.

Requirements  Opening will be 8 in.  Bridge must fit in box that’s - Height 5in  Width 5in  length 12in  

Materials  ¼ piece of balsa wood  Amount will be 7 lin feet ( 84 in ) Total

Sketch DimensionOn graph paper Clear Lines Drawn to scaleView of each side (3) Top Bottom Side

Print each Side View Inventor

How the test looks like Bolt Placement

Unrealistic

Not going to happen

Look the the cross members

What shape is common?

Point to carry load

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?