Engineers spend a lot of time running KPS tests, or stress tests, on various building materials. It’s important for the tests to be run so that they know if the materials they use will be able to withstand the stresses the materials will face after being put together. I have a budding engineer, so for this Engineering in Action we completed a KPS on a craft stick and glue suspension bridge.

This was a fantastic engineering activity. Not only did my children build a craft stick suspension bridge, we placed weights on it until it broke to pieces. My third grader loved participating in this experience. He loved designing and constructing the bridge. Everyone else really enjoyed smashing the bridge – so much so, that after we were finished, I was asked if we could build something else and smash it, too. I love it when my children really get into these topics.

This activity took two weeks to complete – but mostly because we had other things come up (like fishing trips), so we weren’t able to work on it very much each day.

Materials to Build the Bridge:
One box of craft sticks.
Craft glue
Scratch paper
Fishing line – the heavier the weight the better
Scissors
Cardboard

Materials Needed to Smash the Bridge:
A length of rope at least 5 feet long
A wooden platform with holes in the corners for the rope to go through (This creates a platform that can be hung from the bridge so that heavy things can be suspended on it until the bridge smashes.)
Heavy things (like books) to place on the platform

Method:
Start by explaining that the children are going to build a bridge, and then they are going to perform a KPS test on the bridge. Explain that a KPS test checks to see how strong a building material is and when the material will break.

Now, explain the specifications that the bridge will need to meet. I told my children that the bridge had to span at least 12 inches, and that a small toy car needed to be able to cross from one side to the other.

Next, open the box of craft sticks and let the children engineer.

As they are building, talk to them about what they are doing. We talked about how triangles are the strongest geometric shape. We also talked about the type of bridge they were making, the elements that bridge needed, etc. We also discussed if the bridge was meeting the specifications of the project, and how we were going to connect the cardboard to the two sides of the bridge.

It was fun to work alongside my many engineers and watch how they were working on this problem.

Be sure to let the bridge dry for at least 24 hours before adding any structurally strengthening items. We wrapped fishing line (albeit only 4 lb. fishing line) through the sides of our bridge and around the deck of our bridge. We also used the fishing line for the cables of the suspension bridge.

After all the final additions for the bridge are competed, then it’s time to get ready to smash it!

First, set the bridge up so that it is spanning the distance it’s supposed to span. Remember this is a bridge, so not very much of the bridge should be supported by something. I recommend taking a picture at this point – just for the memories.

Next, loop some of the rope over the middle of the bridge. Secure the wooden platform to the ends of the rope, under the bridge.

Now, it’s time to  SMASH A BRIDGE!
Place heavy objects, one at a time, on the platform until the bridge collapses.

I placed the objects (we used heavy books) because I didn’t want any parts of the bridge coming off and hitting one of my children.

After the bridge is smashed, weigh the heavy items on a scale. This gives the weight limit, or breaking point of the bridge. Our bridge supported 27 lbs.

To complete the KPS for the bridge, divide the weight by 1,000. Divide that number by the length of the span – in our case 16 in. Then, divide all that by 12. Our KPS for our bridge – a fantastic 0.00014, and our moment (or how much torque each pier of the suspension bridge could tolerate before breaking) was 18 ft. lbs.  (All numbers were computed by my father who is a P.E. and civil engineer.)

Not too bad for craft sticks, glue, cardboard, and fishing line. . . but maybe we’ll be out done. Can your family, or class, build a bridge from craft sticks, glue, cardboard, and fishing line that has a higher KPS rating? I’d love to feature photos and stories of any family, or class, that tries.