What is a Bridge Exactly?

• A structure spanning and providing a passage over a gap or a barrier

• A supported, elevated structure

NYC Bridges

• As a table try to name and list as many NYC bridges as you can

Did you know?

• According to the New York City Department of Transportation there are 2,027 bridges in NYC

Major NYC Bridge Locations

• Which boroughs have the most major connections?

• Why are these bridges essential to the function of the city?

Traffic…

On an average weekday in 2004• 180,369 vehicles crossed the

Queensborough Bridge• 137,563 vehicles crossed the Brooklyn

Bridge• 110,528 vehicles crossed the Williamsburg

Bridge• 79,129 vehicles crossed the Manhattan

Bridge

What Makes a Bridge Stand Up?

• Individually, write a list of things you already know about bridges

• As a table generate three questions that you have related to bridges that you would like to have answered in this unit

Bridge Basics

• Every bridge, large or small, must withstand and overcome the forces or stresses to which it is exposed

• Can you think of some stresses bridges have to handle?

The Stressful Life of a Bridge

Bridges are designed to

• carry their own weight (dead load)

• carry people and vehicles (live load)

• resist natural forces (environmental load)

Forces Acting on a Bridge

• Tension (pulling force)

• Compression (pushing force)

• Torsion (twisting force)

• Shear (sliding force)

Basic Structures in a Bridge

• Beam

• Arch

• Truss

• Suspension

Beam Bridge

• What are the parts of this bridge?

• What forces are present in a beam bridge?

Bridge in Easton, Delaware

Column

Beam

__________Span______________

Model of a Beam• With a partner at your table create a beam using a

sponge• Note and sketch how the beam responds when you

apply pressure to the top of the beam• Next, shorten the span of your beam• Apply pressure to the top of your shortened beam and

sketch how it responds• Lastly, have your partner grip one end of the beam

while you apply pressure to the other end• Sketch how your beam responded• Label all of the forces acting upon both of your beam

bridges

Beam Bridge

• What are the parts of this bridge?

• What forces are present in a beam bridge?

Bridge in Easton, Delaware

Column

Beam

__________Span______________

Compression

Tension

LoadCompression

Cantilever

• A projecting structure, such as a beam, that is supported at one end and carries a load at the other end or along its length.

Frank Lloyd Wright Price Tower

Falling Water Pennsylvania

Famous Cantilever Bridge

• Firth of Froth Bridge: Scotland

• Sir Benjamin Jacob and Sir John Fowler

• Completed in 1890

Column Activity- “The Hold Up”

• Shape is an important property of columns

• Why do columns need to be strong?

• What would be the ideal shape for a column? Why?

Strong Columns

• The more sides to a paper structure the more weight it will support

• This helps to distribute the weight more evenly

• Which shape was the strongest? Which shape has the most edges?

Cylinder

• Ideal column because it moves the load away from the vertical axis

What makes a shape strong?

•The beams that make up the squares above are called members.

•For each square identify which of its members are in tension or compression.

•Which one is the strongest? Why?

Truss

Square Howe Truss Lattice Truss

A truss is a strong shape made out of beams.

Tension or Compression?

• Sketch and label the appropriate members that are in tension or compression.

Galloping Gertie• In July of 1940, the unique Tacoma

Narrows Bridge opened.• At seventy-two times longer than it

was wide, and without a stiffening truss, crossing the bridge was like riding a rollercoaster.

• The flexible bridge was quickly nicknamed Galloping Gertie and was a successful tourist attraction for 4 months.

Uh-oh!

• In November of the same year, a gale began to twist the bridge like a ribbon. The bridge was quickly cleared and the winds died down eventually. However, they died down to late. The concrete slabs began to drop out of the center span until the entire section fell with a great crash.

DISASTER

New Regulations

• After Galloping Gertie, it became a requirement that all suspension bridges be built with a stiffening truss.

Stiffening Trusses

Trusses into Towers Activity

• Dead load

• Live load

• Specifications

Parts of a Suspension Bridge• A suspension bridge suspends the roadway from huge

main cables, which extend from one end of the bridge to the other.

• These cables rest on top of high towers and have to be securely anchored into the bank at either end of the bridge.

• The towers enable the main cables to be draped over long distances.

Parts of a Suspension Bridge• Most of the weight or load of the bridge is

transferred by the cables to the anchorage systems. These are imbedded in either solid rock or huge concrete blocks.

• Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free

Forces in a Suspension Bridge

• The diagram below shows the tension in the cables of a suspension bridge. These cables are capable of withstanding tension but offer no resistance to compression

• TensionThe supporting cables, running between the two anchorages, are the lucky recipients of the tension forces.

• The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage.

• The anchorages are also under tension, but since they, like the towers, are held firmly to the earth, the tension they experience is dissipated.

• CompressionThe force of compression pushes down on the suspension bridge's deck, but because it is a suspended roadway, the cables transfer the compression to the towers, which dissipate the compression directly into the earth where they are firmly entrenched.

Arch Bridge

• The Bayonne Steel Arch Bridge in Bayonne, New Jersey is now the third longest steel arch bridge in the world.  With a 1,675 foot center span, it ranks just ahead of the Sidney Harbor Bridge.  Constructed between 1928 and 1931, it was designed by Othmar Ammann, chief engineer of the New York Port Authority. 

Arch Bridge

The downward forces acting on a simple arch bridge include the following:

• weight of the bridge

• weight of the live load

• gravity

Shape of an Arch

• Because of its curved shape the downward forces (compression) are spread out

• The forces are concentrated on the columns (end supports)

• An arch consists of two half arches joined by a keystone

Beam vs Arch Activity

Newspaper Challenge!

• You will have 10 pieces of newspaper and 1m of tape.

• You have to build a bridge than spans 60cm over 2 chairs

• You have to build the strongest structure that you can with the fewest materials

• Only uncut paper may be used• You may not tape the bridge to the chairs

or the table

Pre-Plan

• What features or structures will you think about in constructing your bridges?

• Spend 5 minutes sketching a design you would like to try

Bridge Analysis

• Where did the bridges start to break? Why?

• How can you make your bridge stronger?

Truss Bridge Building Challenge

• Will your team design and build the strongest truss bridge in the 6th grade?