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Best truss bridge design for this school project?

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Summary:: In my course, we have to design a truss bridge (made out of balsa wood sticks) that can withstand the most weight compared to its mass. It’s our final, so very important haha. One of the rules is we can’t stack the wood right on top of each other to support it (just preventing us from gluing 20 sticks together for the final lol)

One of the rules is we can’t stack the wood right on top of each other to support it (just preventing us from gluing 20 sticks together for the final lol). I do understand that triangles are the best way to go, but I was thinking about having an arched top with Xs on the top and layering Xs on the bottom so it doesn’t break easily. Yet, that’s adding more mass and I’m wondering if it’s necessary, as our teacher is going to compare the weight and mass rather than just weight.
 

Answers and Replies

berkeman
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Welcome to the PF. :smile:

Please post all of the rules for this competition. There is a trivial answer if it is not covered by the rules.

Also, what are your thoughts so far? We cannot provide tutorial help unless we see a lot of effort on your part. Thanks.
 
Welcome to the PF. :smile:

Please post all of the rules for this competition. There is a trivial answer if it is not covered by the rules.

Also, what are your thoughts so far? We cannot provide tutorial help unless we see a lot of effort on your part. Thanks.
The rules include the height being 6cm to 10cm, length 25-30cm, and width 6-7.5 cm. I have been coming up with personal designs and now I have a life size blueprint of the side so far. My idea for the bottom was to have a double layer of Xs to reinforce the bottom. My teacher said she’ll have a piece of wood on top with a hook hanging from it. She’ll place it on our bridge and hang a bucket on it. We’ll be putting sand in increments until our bridge breaks, then she’ll measure how much our bridge could withstand compared to the mass of our bridge. Attached is the side blueprint
 

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berkeman
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Nice design to start with! Do you need a flat spot on top to accommodate the teacher's "piece of wood" loading structure?
 
berkeman
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56,085
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BTW, how are you going to make the joins between the pieces of wood? Are you going to be making small notches, or bolting things, or just gluing flat surfaces? Can you use little glued pins in the members at those joins to increase strength?

https://www.thesprucecrafts.com/thmb/n14sec2a1aubO9Kcw6jozB3qNPI=/960x0/filters:no_upscale():max_bytes(150000):strip_icc()/wood-joinery-types-3536631-v3-5b9827b84cedfd002536486c.png

View attachment 253624
I’ll be doing mitered butt, and no I won’t be using a flat piece to accommodate her structure. We can only use sticks and can’t layer them up to reinforce. It’s a long piece of wood she’s using to place in the middle of our bridge. And our bridge should have a small area for the hook to fall through in the middle respectively.
 

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