Because my students are third graders, I would like to keep things simple, so I decided to talk about three main bridge types: beam, arch, and suspension. I will also add on movable bridges as a fourth category.
The beam bridge is the simplest bridge to make and usually the cheapest. It is a simple structure that carries usually one road or pedestrian path over it. The beam bridge is the extension of that bridge I alluded to earlier, which first sparked the idea of a bridge in humans. At first it might have been a simple log going across a small stream but soon the builders realized it could be made longer if some kind of pier were out into the water. At first piers were made of piled rocks and later they were made of wood, iron, and steel. As with most things the advancement to other bridge types was born from a necessity to cross spaces the beam bridge could not handle. These bridges are rarely longer than 250 feet. The beam bridge is the most common bridge seen on highways in the United States.
The students will have already built a beam type bridge in building the truss bridge. They can take the truss bridge design and turn it into a covered bridge like those found throughout the United States by adding a fence of popsicle sticks on either side of the bridge and then adding a tag board or cardboard roof. The bridge can then be painted. Many of the covered bridges are red in color. It must be noted that a system of trusses is often incorporated into an arch or suspension bridge design. In fact many bridges use a combination of design techniques.
Most modern arch bridges can be up to 1800 feet. The arch is used most successfully where the two legs or sides of the arch can be stood on either side of a riverbank or area to be crossed. Sometimes the bridge can be a series of arches across a river if necessary. When there is a need to span a longer length a suspension bridge is used. Suspension bridges cover the longest distances from 2,000 to 8,000 feet in length. The uniqueness of a suspension bridge is that it eliminates the need for numerous piers. In a suspension bridge, cables tied to a few widely placed towers hold up the deck. Often digging piers is time consuming and expensive. Over long spans the suspension bridge is a better use of money and materials, and the results are visually spectacular.
Demonstration: Making a Suspension Bridge
Students can work in pairs to make these suspension bridges. First they can put their desk chairs back to back about three feet away from each other. The backs of the chairs will be the towers of the bridge. Then place a large book on the seat of both chairs. Tie a string to one side of one of the books and then loop it around and across to the other chair where you should tie it to the other book. Put another string around the other side of the book and attach it onto the other book so that there are now two strings going from one chair to the other. Make sure that the strings are tight. Then make a cardboard deck a bit wider than the distance to the cables and able to reach from one chair to the other. Put the deck down and cover it with a book on either chair. Then tie a series of strings from the main cables. Punch holes in the sides of the deck where the pieces of string are hanging. Tie the strings to the deck making sure they are tight and that the deck is even.3 Place a load on the bridge to see what it can handle. If it sags, how can you fix it by using more cable? Try seeing how your bridge handles wind by having a fan blow across the bridge. What can you do with the cables to help stabilize the deck?
The last category of bridge is that of the movable bridges. In Medieval times the drawbridge was used as part of the castle to help protect the town and its people. The castle was usually built surrounded by a moat or body of water. When the drawbridge was down the people could come and go. In times of trouble the drawbridge would be raised, making it difficult for invaders to successfully attack the town.
Demonstration: Making a Movable Bridge
An easy way for students to make a drawbridge model its by using a cereal box. The box top is cut off. The front of the box is cut and allowed to fall like the drawbridge. The students then punch a hole in the top of the front panel and in the top of the back of the box. A string is drawn between each side of the drawbridge and the top of the deck and then it can be drawn up or let down by the student pulling the cables (see illustration in Appendix B).4
There are also bridges in which the deck of the bridge moves up and down to allow trains to pass and then retreat so that water traffic can get by. The process for making a model of this bridge is similar to the first. This time you need two cereal boxes. Cut the tops off and put a whole in each corner of the boxes both back and front. Then cut a square piece of stiff cardboard to be the deck. Put a whole in each corner. You will need about 5 feet of string run it from the box to the deck (see illustration in Appendix B).5
Finally there is the swing bridge. To make a model of this bridge you will need a rectangular baking pan, the lid of a margarine container or coffee tin, a paper fastener and some cardboard. Cut two ramps for the side of the bridge and lean them over the edge of the baking dish. Cut another piece of cardboard long enough to cover the span and fasten it to the plastic lid. Put water in the pan, making sure that the cardboard does not get wet. Then you can float boats in the pan and the bridge can open to let them by (see illustration in Appendix B).6