Balsa Wood Bridge
8th

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
Cantilever
bridge Firth of
Forth
Every bar in this cantilever
bridge experiences either a
pushing or pulling force. The
bars rarely bend.
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
Golden Gate
Bridge
Forces act on
Suspension
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 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?