Adapted from “Bridge Game” by Robert Gregson

Put on your engineering hat and build a bridge!

Two-thousand years ago, the Romans used arches to build bridges out of stone. In the early 19th century the iron truss bridge was invented. Truss bridges carried the railroad across rivers and streams from coast to coast. In 1886 the Brooklyn Bridge in New York became the model for the suspension bridge. It used long cables and metal suspenders to hold up the bridge deck.

For this activity, divide into two teams. One team will build a truss bridge and the other will build a suspension bridge.

Materials for Suspension Bridge
* 2 straight-backed chairs
* string
* scissors
* cardboard or foamboard 5 feet long
* tape

Materials for Truss Bridge
* Plastic straws or make paper straws by rolling sheets of 8 1/2 x 11 paper from corner to corner and taping them so they won’t unroll. Make the paper straws so that a paper clip can be inserted in each end (not too loose or too tight)
* Paper clips
* tape
* cardboard or foamboard 5 feet long
* scissors
* two tables
* heavy books

Instructions
1. Divide into two teams: Team Truss and Team Suspension
2. Make an area for each team to work
3. Team Truss should place its two tables about 4 feet apart. Team Suspension should place its two chairs about 4 feet apart.
4. Start building!

Team Suspension
1. Your two chairs are your bridge towers (see the picture). Place them 4 feet apart with the backs facing each other.
2.  Create two suspension cables. First tie an end of two very long strings to the front legs of one chair. Looping each one up over the chair backs, allowing the string to sag between the two chairs. Loop them over the other chair back and tie the ends to the front legs of the second chair.
3. On each side along the suspension cables, tie pieces of string every six inches. Let it hang down and tape it to the bottom of the bridge deck. Try to tape them at the length that keeps your bridge deck level.

Team Truss
1. Make a series of trusses (triangular supports) from the plastic or paper straws and paper clips. Twist the paper clips open and slightly widen the narrow end. Bend them at an angle to make three straws into a triangle. Wedge one end of the paper clip into each end of the straw. Add a third straw and two more paper clips to create a triangle. Add another paper clip to connect two triangles to make the truss.
2. Use paper clips or tape to attached the truss framework to the bridge deck.
3. Suspend the truss bridge across the two tables. If the truss bridge sags in the middle, you might need a support pier. Stack some books or blocks underneath to support it.

Which bridge is stronger?

Test the bridge strength by rolling a toy car or truck. Then, add some weight to the truck. Add some more until your bridge starts to fail. Observe where each bridge is weakest. What part did not work? Was it the tape? Did the string come untied? Were the holes in your straws too large and the paper clips slipped out? Problem solve to see which team can get their bridge to hold the most weight. You’ve just been a bridge engineer!