Balsa Wood Bridge 8 th. How Does a Bridge Stay Up?

Balsa Wood Bridge8th

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)

TransferThe 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

• Dimension On graph paper

• Clear Lines Drawn to scale View of each side (3)• Top Bottom Side

Print each Side View Inventor

How the test looks likeBolt 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?