# Cable Tie Truss Bridges

Students design and build truss bridges, then test the strength of the bridge by attaching a scale.

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## Step 1: Materials and tools

Materials and Tools

## Step 2: Advanced Ideas: Truss Armor / House Frame

Truss armor is made of a simple craft stick and cube construction, but includes a handle near one end. This allows the user to grasp the structure from within and wear it like armor.

Another alternative is the house frame. Although it does not rely heavily upon trusses for it's sturdiness, it does show the basic idea of what timber looks like within the frame of real houses.

burns987 months ago

thanks!!! I built this for a socials project as a Canadian Pacific Railway truss bridge!

yapoyo2 years ago
This thing is EPIC. I spent about an hour and it could actually support my weight, and I weigh 135 pounds!
2 years ago
Now i extended this so its like six feet long!!
WYE_Lance (author)  yapoyo2 years ago
Whoa, awesome! Thanks for sharing this - I'm glad you had huge success with the bridge!
kelseymh2 years ago
Engineering awesomeness!

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.
WYE_Lance (author)  kelseymh2 years ago
These are some superb ideas for high school students. Truly a great way to make math more engaging - thanks!
2 years ago
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.
WYE_Lance (author)  80\$man2 years ago
Thank you for your thoughtful reply. I hope the words of you and kelseymh inspire high school teachers to apply this project to their own classroom
2 years ago
I'm in high school, and do you know a site that shows the mathematics behind it?
wiredcav2 years ago
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.
Biodynamic2 years ago
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.
wilgubeast2 years ago
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.

Good luck in the Teacher Contest.