Toothpicks bridge
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chickentown
- Posts: 29
- Joined: Sun Jan 21, 2007 2:42 am
I thnk I am gonna build a truss bridge ( probably a vertical support truss ) , while searching some online resources, some people said it's better if you cut off the 2 ends for easy surface gluing.. I think it's true but as my project does not allow overlapping more than 1 cm , then do I still need to cut off the two ends.. I am wondering because it's ovbivious that i am gonna use a lot of toothpicks and cutting off the two ends of the whole 500 or more toothpicks is a long long process... what should I do???
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chickentown
- Posts: 29
- Joined: Sun Jan 21, 2007 2:42 am
Ok, now that the rules have been clarified with respect to overlap and glue, you still haven't answered my questions on:
1) "ground clearance" requirements
2) "size of test block"
3) Can you soak toothpicks in water and split them to make "rope" like bindings? A pair of 1/2 cm long shaving spirally wrapped in opposite directions and glued around two two toothpicks in tension will produce a stronger in tension joint that you can come up with by any other means using toothpicks. The question again, is this allowed?
The bridge is gonna place between a space of 2 wooden classroom desk
2) the test block has the size of 10 cm times 8.5 cm together with a 90cm long chain
3) I don't think I can split the toothpicks like bindings..
Once again, thank you for your help though
1) "ground clearance" requirements
2) "size of test block"
3) Can you soak toothpicks in water and split them to make "rope" like bindings? A pair of 1/2 cm long shaving spirally wrapped in opposite directions and glued around two two toothpicks in tension will produce a stronger in tension joint that you can come up with by any other means using toothpicks. The question again, is this allowed?
The bridge is gonna place between a space of 2 wooden classroom desk
2) the test block has the size of 10 cm times 8.5 cm together with a 90cm long chain
3) I don't think I can split the toothpicks like bindings..
Once again, thank you for your help though
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Craig_Bridge
- Former Expert
- Posts: 1297
- Joined: Mon Oct 16, 2006 11:47 am
Ok, now that you have ruled out "wet" techniques, you have simplified what you can do and what you need to consider for joints.
The desks for ground clearance and the 8.5x10cm (presumably flat) block of wood as a load bearing plate almost completes the knowledge of your requirements.
Because the block is NOT square, the orientation of the block matters. If the block is aligned with the long side parallel to the span, then it spreads the load over a longer portion of the span which is good, but it limits the width to 1 toothpick for chain clearance. If the block is oriented with the long side perpendicular to the span, you can use a wider design, but the load will be concentrated in a shorter portion of the span.
You probably want to design the load bearing surfaces where the block is to be placed to be flat. You probably don’t want the block resting on the ends of toothpicks because the failure of a single glue joint will cause the load to shift and that would likely be the end of the span.
Your bridge design needs to consider the possibilities that the block may not be perfectly centered and the chain will act like a pendulum, combined, these will generate both a static unbalanced side to side loading and dynamic unbalanced side loading on your span.
In other words, not only do you need to consider the downward forces, you need to provide for some side to side stability, and you need to think about ways of transferring unbalanced downward forces to the opposite side.
Back to glue joints and cutting ends of toothpicks. Strong glue joints require “fitted� surface area. The more surface area, the stronger the joint can be. Coming up with ways of quickly fitting joints is part of the challenge. Round tapered toothpicks complicate the challenge. The people who figure out these techniques will have an advantage if they figure out how to apply it!
Many truss designs have two or three vertical members meeting one horizontal member at the same place, like \/ or \|/ (sorry, I can’t show the horizontal, draw your own underline) Think about how two people of similar size could put their the toes of both shoes together and gasp wrists to make a “seat� and lean back and balance. Could they hold up somebody sitting on their hands? Does it matter if the shoes are glued to the floor? What might happen if the toes of the shoes aren’t touching and the floor is slippery? The aspects of physics that apply to these acrobats are why truss members are typically layed out like this.
Have you looked up solid beam design yet? You should! The formulas for the "inelastic" failure (the point a where the load bends the beam to the point where it breaks) of a solid beam in terms of beam height and width and clear span apply in a gross sense to uniform cross section truss designs as long as the loads on the individual members and joints aren't exceeded. This will give you a clue to the trade offs between wider and thicker.
Take a look at steel "I" beam design and LBL engineered wooden beams for a clue as to how one might use lines of horizontal toothpicks on either side of a group of verticals to make a strong light weight beam and then figure out how to couple mutiple beams together.
The desks for ground clearance and the 8.5x10cm (presumably flat) block of wood as a load bearing plate almost completes the knowledge of your requirements.
Because the block is NOT square, the orientation of the block matters. If the block is aligned with the long side parallel to the span, then it spreads the load over a longer portion of the span which is good, but it limits the width to 1 toothpick for chain clearance. If the block is oriented with the long side perpendicular to the span, you can use a wider design, but the load will be concentrated in a shorter portion of the span.
You probably want to design the load bearing surfaces where the block is to be placed to be flat. You probably don’t want the block resting on the ends of toothpicks because the failure of a single glue joint will cause the load to shift and that would likely be the end of the span.
Your bridge design needs to consider the possibilities that the block may not be perfectly centered and the chain will act like a pendulum, combined, these will generate both a static unbalanced side to side loading and dynamic unbalanced side loading on your span.
In other words, not only do you need to consider the downward forces, you need to provide for some side to side stability, and you need to think about ways of transferring unbalanced downward forces to the opposite side.
Back to glue joints and cutting ends of toothpicks. Strong glue joints require “fitted� surface area. The more surface area, the stronger the joint can be. Coming up with ways of quickly fitting joints is part of the challenge. Round tapered toothpicks complicate the challenge. The people who figure out these techniques will have an advantage if they figure out how to apply it!
Many truss designs have two or three vertical members meeting one horizontal member at the same place, like \/ or \|/ (sorry, I can’t show the horizontal, draw your own underline) Think about how two people of similar size could put their the toes of both shoes together and gasp wrists to make a “seat� and lean back and balance. Could they hold up somebody sitting on their hands? Does it matter if the shoes are glued to the floor? What might happen if the toes of the shoes aren’t touching and the floor is slippery? The aspects of physics that apply to these acrobats are why truss members are typically layed out like this.
Have you looked up solid beam design yet? You should! The formulas for the "inelastic" failure (the point a where the load bends the beam to the point where it breaks) of a solid beam in terms of beam height and width and clear span apply in a gross sense to uniform cross section truss designs as long as the loads on the individual members and joints aren't exceeded. This will give you a clue to the trade offs between wider and thicker.
Take a look at steel "I" beam design and LBL engineered wooden beams for a clue as to how one might use lines of horizontal toothpicks on either side of a group of verticals to make a strong light weight beam and then figure out how to couple mutiple beams together.
-Craig