My kiddos have been fascinated by bridges for a long time. Every 5-6 weeks we travel to the Oregon Coast, crossing numerous bridges. The most memorable one is the McCullough Memorial Bridge that spans Coos Bay. A recent trip to San Francisco really brought bridge design and construction to life (the Bay Bridge was under construction). Upon our return home, we read numerous books about bridges and the kids were asking lots of questions. When an opportunity to participate in a homeschool science fair presented itself, we knew immediately what project we wanted to undertake.
Join us - as we walk through the steps of designing and ultimately testing the strength of our toothpick bridges.
Bridges Are to Cross
Bridges Are to Cross by Philemon Sturges is a great introduction to bridges. Everyone knows bridges are to cross -- to get to the other side. But did you know that some bridges carry llamas loaded with firewood, some let people dance over lazy rivers, some were forts for defending castles, and some were crossed by emperors and popes? From a simple log to woven webs of steel, bridges reflect our values, our lifestyles. Feast your eyes on these bridges from around the world and you will come to realize that crossing is only one reason for having a bridge.
Planning & Design
Before we began construction, we read Bridges Are to Cross by Philemon Sturges. We then used the internet to make observations of many modern bridge designs, particularly those we were familiar with here in Oregon as well as those we'd seen first hand in San Francisco. I set up a little demonstration to show what structure was strongest ... squares or triangles. I then gave the kids graph paper with which I instructed them how to begin designing bridges of their own. They came up with several designs each - some of which weren't feasible for toothpicks. They then selected a design that would most easily be reproduced with toothpicks and we proceeded with construction.
Building Toothpick Bridges
This is the book that started it all. I had a copy of this book when I was teaching in the classroom. It is a great interdisciplinary tool to initiate bridge building projects. However, the bridges in the book are simple, composed of two sides and a few connecting toothpicks. It is a great introduction to bridge building but serious builders will want to go farther.
Read About Bridges
What Materials Do I Need?
In addition to the materials listed here, you will also need graph paper to design your bridge and a plastic jug or bucket in which to add weights (we used a gallon sized milk carton).
- School glue
- Wax paper
- Graph Paper
- Plastic bucket
The Great Bridge Builder
A great book with lot's of fascinating details about the technical challenges and the determination to see it through. I'll never cross another suspension bridge without thinking of this story.
In the classroom setting, I used to provide each team with a predetermined amount of money ($1,550,000) with which they would need to purchase their material ... land (cardboard - $500,000), lumber (toothpicks - $10,000 per piece), cable (string - $500 per cm), welding material (school glu - $850 per day's supply), building-plans paper (4 sheets graph paper; wax paper - $40,000), fines ($1,000 - $10,000), vandalism ($50,000). They were required to keep a balance sheet and to write checks to make purchases. This added a fun element to the project and enabled the teams to come up with company names as well as to assign specific tasks to members (i.e. the student who was good at math was the accountant, etc.). However, here at home since my kiddos are young and not yet skilled at math with such large numbers, their imagination and thereby their design were the only limitations. [ Checks and balance sheets are available in the Pollard book, "Building Toothpick Bridges" but you could certainly create your own. ]
To begin, a piece of wax paper is laid atop the design and taped down. Toothpicks are then laid down upon the paper and glue is applied to the intersecting toothpicks. Upon completion of the first side, it is left to dry overnight. The wax paper is then removed from the design paper and this process is repeated for the second side. When both sides are dry, the completed sides should be able to be removed from the wax paper easily.
The two sides are then brought together upright on the land (35cm x 15cm piece of cardboard). To do this, a tiny hole is poked into the cardboard where each of the eight toothpicks can be inserted. Cross-connecting toothpicks are then laid across and glue is added to adhere the two sides together. The river measures 15cm across (essentially a 15cm square in the center of the cardboard). Two 5cm squares should be drawn on each side of the river for the footing. The bridge should be built up from these two footing areas and continue across the river.
Upon the roadway, there should be space available for a small toy car to pass without obstruction. A small toy boat can pass freely under the span of the bridge.
One of the few real "How to" learning books about bridges. It explains how & why different types of bridges don't fall down and offers fun instructions on each major kind: arches, beams, trusses, cantilevers, and suspension bridges. It also shows why some bridges fail, like the Tacoma Narrows Bridge (you've seen this one go on disaster TV shows). A great resource book with many fun activities to get kids building.
Testing Bridge Strength
To test the strength of the bridges, a pencil is laid across the road way. A plastic bucket or container of some sort is then suspended from the pencil. Weights (marbles, metal washers, etc.) are added slowly to the bridge. Each new weight must be held for one minute before more weight is added. Continue in this way until the bridge collapses or breaks under the strain of the load.
An AP Physics Class Design Competition
Presenting at the Science Fair
Will post results & pictures soon.
Have you designed and tested toothpick bridges? Do you have questions on how to implement this project in your classroom? Take a moment to leave a comment and/or ask questions!