Do Now: **Please make sure you have the following definitions copied** down on your Vocabulary sheet.  Tension- is the pulling force exerted by a string, cable, chain, or similar solid object on another object. Tension is the opposite of compression.solidcompression  Compression- is the result of the subjection of a material or structure to compressive stress, which results in reduction of volume as compared to an uncompressed but otherwise identical state. compressive stressvolume  Force- is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects.  Mean- The sum of a set of data divided by the number of data.  Median - The middle value, or the mean of the middle two values, when the data is arranged in numerical order. Think of a "median" being in the middle of a highway.  Mode- The value ( number) that appears the most. It is possible to have more than one mode, and it is possible to have no mode. If there is no mode-write "no mode", do not write zero (0).

Marshmallow Tower Discussion

Introduction to Bridges

Bridges For hundreds of years, people have been obsessed with taking the shortcut over valleys and rivers instead of going around them. To do that, we use bridges. In this unit, you will have the chance to become a bridge engineer and design and build your own. You will also learn about some of the different types of bridges and how to decide which one to use in different situations.

We build different bridges for different needs

Beam Bridge

Bridges are characterized by how they support themselves. The simplest type of bridge is the beam bridge. This type of bridge has a single horizontal beam across two supports. You have probably made this bridge a number of times yourself. Any time you have placed a log across a creek, or have created a balance beam by placing a long piece of wood on two bricks, you have created a beam bridge. Beam bridges are fairly common. Beam bridges are also common for highways going over other highways or over small creeks. Beam Bridge

The simplest and least expensive Beam Sometimes called a girder

Beam Bridge Can be made of steel

Beam Bridge Concrete

Beam Bridge or even wood

Beam Bridge Not the best for long or high spans

You Try  Position two stacks of books 12 inches apart.  Try building a bridge that connects one stack of books to the other by using the toothpicks and small marshmallows given to you. The first thing you will need to do is build a bunch of units that act as the basic building blocks we will use.

Try supporting it on a book at each end so that the middle has nothing but air underneath it. Put some moderately heavy object on the middle of your bridge, or gently push down on the middle with your hand. What do you notice? Here's what happened when we did it: You Try

This is not good. What you should notice is that beam bridges are not good at supporting weight if they are too long. If you make a beam bridge too long, it starts to sag in the middle. It might even break! You Try

Truss Bridge

Suppose we want to build a bridge across a wide space. We don't want to sink supports in the middle, so we were hoping to build a beam bridge with one very long span. But, we know that a long beam bridge will sag too much in the middle. What do we do? One approach is to still build a bridge that is supported just at the two ends, but use cross members to make the bridge stronger so it doesn't sag. This is called a truss bridge. A truss consists of straight members connected at joints. Truss Bridge

Stronger than beam bridge. It has extra support from above

Truss Bridge

A truss can also support from below

You Try  Position two stacks of books 12 inches apart.  Try building a truss bridge that connects one stack of books to the other by using the toothpicks and small marshmallows given to you.

Try supporting it on a book at each end so that the middle has nothing but air underneath it. Put some moderately heavy object on the middle of your bridge, or gently push down on the middle with your hand. What do you notice? You Try

Using Triangles Build the following two objects. Each is a four-sided box, but the one on the right has a cross piece which really makes it two triangles. Gently push the upper-left and the lower-right corners towards each other. Whoa! That's enough. Not too hard now. Which object is better at holding its shape? Why do you think this is true?

The two triangles are better at holding their shape. The cross member adds a lot of strength. In engineering terms, the box with the cross member is more rigid. The reason is that 3 lengths uniquely define a triangle. So, a truss bridge is really just a beam bridge that has been made way more rigid by the addition of more layers and possibly strengthened even more with triangles. Add triangles to your truss bridge and see how much stiffer it is: Using Triangles

Suspension Bridge

Suppose we want to build a bridge across a wide space, but don't want to use as much steel as all those triangles would require. After all, steel is heavy and expensive. What else could we do to support our bridge and keep it from crashing down to the water and rocks below? I know, how about we hire a fleet of helicopters to suspend the bridge with strong ropes as shown below: Suspension Bridge What's that you say? "You can't be serious!"? You are right; I'm not. If you thought steel was expensive, try a fleet of helicopters! The idea of suspending the bridge is good, but this particular implementation is goofy.

OK, let's put a cross piece across two tall towers and suspend the bridge from that: Suspension Bridge This looks much better, except the crosspiece is essentially a beam bridge by itself and will sag enormously. Well, as long as we know the thing doing the suspending will sag anyway, why not make it cheaper and lighter by making it into a sagging cable and suspending the bridge from that?

This approach is called a suspension bridge. But, if it were to be built exactly like drawn above, the two towers would try to fold in on each other: Suspension Bridge So, suspension bridges also have an extra set of cables on the outside to help keep the towers from collapsing inwards as shown here:

Suspension Bridge One of the tricks to building suspension bridges is how large you can make the distance between the two towers. Because of advances in engineering, bridge designers have been able to make that distance larger and larger.

First suspension bridge in America Newburyport, Mass. Built around 1900

Golden Gate Bridge Perhaps the most famous bridge in the world Why is it called the Golden Gate?

Golden Gate Bridge Perhaps the most famous bridge in the world It spans the Golden Gate Strait

Arch Bridge

Suppose we want to build a bridge across a wide space, but don't want to string cables for a suspension bridge. Are there any other approaches? Yes, in fact, there are. One of these is an arch bridge:

Arch Bridge Very strong Can last a very long time

Arch Bridge This is Pont du Gard a 2000 year old Roman aqueduct This is a bridge built so that a stream of water could cross a stream of water

Cable Stayed Bridge

Cable Stayed bridge Elegant structure Long spans

Sunshine Skyway Bridge St. Petersburg, FL Requires less material Requires Less labor

Lift Bridge

Lifting Bridge It is over 100 years old Tower Bridge

Lifting Bridge Blocks trafic

Lifting Bridge Allows the passage of ships

Lifting Bridge Rather expensive Subject to mechanical breakdown

Many different bridges for many different jobs

See these websites for more information on bridge design