The Lesson Plan The total lesson time from start of lecture to weight testing and wrap-up is between 1.5 and 2 hours. I usually break up this project...

Prep: Bring more craft sticks than you think you'll need Setup: Set up glue guns

Start-of-class lecture

Introduce the project name and design goals

Discuss what defines a truss and the significance of trusses. If possible, point out examples of trusses in the room (sometimes used for tables and chairs).

Show step-by-step how to build the Warren pattern (do not make an entire bridge during the lecture, just part of one)

Explain how the bridge pieces are connected

Pass out copies of the bridge patterns document and give your students two minutes to look it over before building

Something I'd appreciate (as a physicist) would be some of the mathematics to let students actually calculate the forces, and make a guess as to how much weight their bridge "should" hold (and then test that hypothesis with their butts on the line :-).

Since forces decompose linearly, in principle the students could do the math by hand, with only arithmetic (give them values of sin/cos 30, 60, 45 to use, to avoid the scariness of "trig").

I could imagine an introductory session where they measure

how much force/weight is needed to snap one stick in half, when pushing the ends together (for the vertical members);

how much force is needed to snap a stick in half when bending (for the horizontal members);

how much force is needed to break a glue joint using a scissors movement (both kinds of glue are very weak against peeling, but strong in shear).

Then, once they have a truss built, take a uniform load and distribute it across the top (or worse, a point load at the center!). Draw force arrows to show how the load (a) pushes on the horizontal top members, and (b) runs down the diagonals to the bottom. The latter is where those sines and cosines come in, which you could "just give" to a younger class.

Don't forget the upward force arrows at the endpoints where the bridge is supported. On some members, you'll have arrows going in both directions, and the students get to learn about tension vs. compression, as well as bending.

Where the arrows meet at the bottom, you've got net forces on the glue joints, and possibly (depending on the design) unbalanced net bending forces on the horizontal members.

Can the joints hold up, based on the earlier measurements? As you add more weight, will the bridge fail at joints, or fail within members?

Then go and test the predictions. I suspect that it might even be possible to get within a factor of two or so between calculation and reality (allowing for measurement error, variations in glue joints, etc.), which is not bad.

This is a great way for high school students to engage in maths and physics and be expose to material science and strength of materials. The concepts of stress, stiffness, strain and strength should be something a young engineer should grasp at an early age. As a post-grad student in mechanical engineering, I appreciate the efforts of lakiyama and the Workshop for Young Engineers (cool site, such a great initiative! respect!). From previous experience in the high-school bridge building competition University of Cape Town, (2003), the problem with these bridge designs are failure due to Euler buckling in members with glued connections, which involves complicated formula to predict failure loading (rigid connections, beta=4, effectively four times weaker Euler buckling mode failure, with combined loading the math becomes very ugly and unpredictable), still: yes, it can be approximated with constants but rather:

With regards to kelseymh comment, I would rather have young engineers be exposed to and grasp the concepts of stress, strain, stiffness and strength in addition to the different loading conditions and their interaction/coupling. The development of a students ability to interpret and draw graphs (stress/strain) and diagrams (free body diagrams) to explain and understand property relations and loading conditions are of much greater value than simplified calculations that 'can' predict failure. Doing basic experiments to explain these properties and the governing mathematical relations would be of great value. Whilst the student gains knowledge of these concepts, their ability to intuitively build upon and improve previous designs will grow. Learning from failure, especially during experimentation, is crucial in the development of an engineer's problem solving abilities.

Finally, I have a great appreciation for the efforts taken in education of young people in the fields of mathematics, science and engineering, with commend your initiative and endeavour.

Fantastic! One of my fondest memories was competing in a toothpick bridge contest in high school. I still feel I got as much out of hands on engineering projects in high school (R/C planes, skateboards, etc.) as I got out of the my engineering course work in college and grad school. Kids building stuff = *much* better engineers.

Great job! I do a similar project with my students using toothpicks and wood glue. It takes them a little longer to build, but the results are just as cool. I like the fact that the kids who finish early still have something they can work on.

This is a spectacular Instructable. Way to take a rather pedestrian engineering project (most kids try this at some point), but you took it to a whole 'nother level. Great work, and well-deserving of having been featured.

Bio:Hi! I create project-based engineering lessons for my after school program, The Workshop for Young Engineers. I also develop hands-on science curriculum for a summer camp company, Galileo Learning. I'...read more »

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Something I'd appreciate (as a physicist) would be some of the mathematics to let students actually calculate the forces, and make a guess as to how much weight their bridge "should" hold (and then test that hypothesis with their butts on the line :-).

Since forces decompose linearly, in principle the students could do the math by hand, with only arithmetic (give them values of sin/cos 30, 60, 45 to use, to avoid the scariness of "trig").

I could imagine an introductory session where they measure

Then, once they have a truss built, take a uniform load and distribute it across the top (or worse, a point load at the center!). Draw force arrows to show how the load (a) pushes on the horizontal top members, and (b) runs down the diagonals to the bottom. The latter is where those sines and cosines come in, which you could "just give" to a younger class.

Don't forget the upward force arrows at the endpoints where the bridge is supported. On some members, you'll have arrows going in both directions, and the students get to learn about tension vs. compression, as well as bending.

Where the arrows meet at the bottom, you've got net forces on the glue joints, and possibly (depending on the design) unbalanced net bending forces on the horizontal members.

Can the joints hold up, based on the earlier measurements? As you add more weight, will the bridge fail at joints, or fail within members?

Then go and test the predictions. I suspect that it might even be possible to get within a factor of two or so between calculation and reality (allowing for measurement error, variations in glue joints, etc.), which is not bad.

With regards to kelseymh comment, I would rather have young engineers be exposed to and grasp the concepts of stress, strain, stiffness and strength in addition to the different loading conditions and their interaction/coupling. The development of a students ability to interpret and draw graphs (stress/strain) and diagrams (free body diagrams) to explain and understand property relations and loading conditions are of much greater value than simplified calculations that 'can' predict failure. Doing basic experiments to explain these properties and the governing mathematical relations would be of great value. Whilst the student gains knowledge of these concepts, their ability to intuitively build upon and improve previous designs will grow. Learning from failure, especially during experimentation, is crucial in the development of an engineer's problem solving abilities.

Finally, I have a great appreciation for the efforts taken in education of young people in the fields of mathematics, science and engineering, with commend your initiative and endeavour.

Good luck in the Teacher Contest.