Beam bridges are everywhere. Today, we're going to build a beam bridge and learn about the two different forces that act on a bridge.The compressive force is a force that compresses or shortens the thing it is acting on. The tensile force is a force that expands or lengthens the section it is acting on.

The beam bridges are the simplest and least expensive bridges to build. A beam bridge consists of a horizontal beam that is supported at each end by land, columns, or piers, just like the logs that sit across a stream.

When a load is placed on the beam bridge, the top of the beam bridge is pushed down by a compressive force as a tensile force stretches the lower portion of the beam bridge. The farther apart the supports structures are, the weaker a beam bridge becomes.

Step 1: Gather Your Supplies

Supplies:

A bag/box of straws
Tape
Scissors
Small cups (or wedding favor mini buckets from the Dollar Tree stores)
Paper clips
Weights – pennies, paper clips, etc. Anything is OK as long as they weigh the same. You want to compare how much different bridge designs can support

Step 2: Building the Side Support Structure

1. Take 6 straws and cut off all the bendy parts, if you have bendy straws.

2. On one end, pull a straw 1 1/2 inch up between two straws and tape them together.

3. On the other end, tape together the 2 straws without anything in between them. This should result in a triangular shaped support structure for your beam bridge.

4. Repeat steps 3 and 4 and make the second support structure.

Step 3: Taping the Side Support Structure

1. Tape the wide ends of the support structure, with a single straw pointing down to the bottom, to tables, chairs, desks, etc. that are 12 inches apart.

2. Place a straw between the support towers.

Step 4: Ready for a Load Test

1. Hang a small cup in the middle of the bridge (use hole puncher and paper clips) and load it with pennies, paper clips, etc. to weigh it down and test its strength.

Bottom row of pictures:

1st picture - an empty bucket

2nd picture - some coins added

Note how the beam bends toward the bottom. The load (a bucket of coins) is shortening the top of the bridge in compression, and lengthening the bottom of the bridge in tension.

Step 5: My Students' Improvements

I've done this project with my students, and I was blown away by how much a bridge like this could hold.

Top picture - This bridges is made of 3 reinforced beams, but without any other special features.

Bottom left - This is a variation on the beam bridge with side supports and 3 reinforced beams.

Bottom right - This bridge has 3 reinforced beams, but the beams themselves are neither taped to the table nor to the support structure.

My students' beam bridges held up to 500 pennies.

Do you think you can beat that? I bet you can. I thought this type of bridge could hold up to ~200 pennies, at the most. Boy, was I wrong!.

If you'd like to get more information on beam bridge project, I have more details on my blog, http://kto6science.blogspot.com/. I also have a simpler project, which helps younger kids understand the differences between compressive and tensile forces, on my blog.

I hope you have a lot of fun with this project.