what is a bridge? a structure across an obstacle to allow safe passage

What is a Bridge?

A Structure across an obstacle to allow safe passage

The Sault Ste. Marie International Bridge made traveling across the St. Mary's River and the Soo Locks much more desirable.

Mackinac Bridge The Mighty Mac

London Bridge

Confederation Bridge

Loads: Dead or Alive Two categories of loads try to destroy structures. They are

known as: Dead loads, and Live Loads

Dead loads are the permanent loads on a structure. For example, the self weight of a bridge

Live loads are all the variable loads, those that occur from time to time. For example, your weight in the classroom

Loads: Dead or Alive

List Loads Dead Live

Completion Of Bridge Loads

Self-weight Concrete, steel, asphalt Lights

Pedestrians Bikes Cars Trucks Trains Snow Wind Earthquake

Dead Loads Live Loads

Loads Create Forces

Types of Forces

Tension

Compression

Tension and Compression Activity

Drawings are used by architects and engineers to communicate important information.

Use your sketching skills to draw the wheel of a bicycle and a step ladder.

Note on the sketch which components are in tension and which components are in compression.

Types of Bridges

Beam Bridge2

When something pushes down on the beam, the beam bends. Its top edge is pushed together, and its bottom edge is pulled apart.

Sault Ste. Marie International Bridge

Types of Bridges

Arch Bridge2

The arch is squeezed together, and this squeezing force is carried outward along the curve to the supports at each end. The supports, called abutments, push back on the arch and prevent the ends of the arch from spreading apart.

Arch Bridge

Types of Bridges

Truss Bridge2

Every bar in this cantilever bridge experiences either a pushing or pulling force. The bars rarely bend. This is why cantilever bridges can span farther than beam bridges.

Truss Bridge

Types of Bridges

Suspension Bridge2

In all suspension bridges, the roadway hangs from massive steel cables, which are draped over two towers and secured into solid concrete blocks, called anchorages, on both ends of the bridge. The cars push down on the roadway, but because the roadway is suspended, the cables transfer the load into compression in the two towers. The two towers support most of the bridge's weight.

Suspension Bridge

Types of Bridges

Cable-Stayed Bridge2

The 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 Bridge

Building Materials

Need to pick the best material Wood, plastic, concrete, steel, aluminum

Compare forces that the material must resist to the properties of the material

Don’t forget Cost Materials Lab2

Elasticity

Bridges sway and flex under loads. But when the loads are removed the bridge

returns to its original shape. The property that the bridge possess to

return to its original shape is called elasticity.

Elasticity Demonstrated

Demonstrate, to yourself, the elasticity behavior of an elastic band by gently pulling and releasing the tension in it.

An eraser can easily substitute for the elastic band. But instead of pulling, slightly twist it.

Did you notice how both the elastic band and eraser returned to their original shape?

Elasticity

Bridge members don’t experience the extreme elongation or twist that you noticed with the elastic band and eraser demonstration.

They experience more moderate shape changes that are not easily detected by the human senses unless you pay real close attention.

Next time you are stuck in traffic on a bridge pay attention to the rocking motion of your vehicle as a heavy truck goes by in the opposite direction.

Contest Information

Refer to your handout on contest information and please feel free to ask questions. Make sure that you are clear on the following points:

materials to be used construction specifications judging categories testing procedure

References

1. Nova Online, Super Bridge, Build a Bridge

http://www.pbs.org/wgbh/nova/bridge/build.html

2. BUILDING BIG: All About Bridges

http://www.pbs.org/wgbh/buildingbig/bridge/

3. West Point Bridge Design Contesthttp://bridgecontest.usma.edu/