Toothpicks bridge

[chickentown]

I have been assigned a new project in physics. After surfing throught the web, I kind of get the idea but I'm still not sure which design should I use to make a best bridge. So I think that I'll try different designs to see which works the best. But my rules are a little diffenrent since my teacher said that toothpicks must be glued with no more than a 1 cm overlap between any tow toothpicks??? What does that mean??? How am I suppost to build a strong base without sticking 2 or more toothpicks together??? Follow are the rules, if anybody can help me by giving some ideas or suggestions ; I appreciate it so much


Objective : to build a bridge of standard toothpicks and wood clue that will span a 50 cm space and hold as much weight as possible

Rules
1) the only two materials alowed are tow ended, round toothpicks ( origianl length nor more than 7 cm ) and wood glue
2) Maximum allowed mass of the bridge is 100g
3) Toothpick must be glued with nor more than a 1cm overlap
4) The test load will be hung on a 90cm long chain placed around the center of the bride . The chain will rest on a block of wood
5) A bridge is considered broken when the block of wood drops 5 cm below its original rest position
6) All bridges will be tested unitl they break

The score for a bridge will be determined by dividing the maximum load it supports by the mass of the bridge itself
Quynh Le

[chickentown]

yeah furthermore , I plan to build diffrent types of Truss bridges which I think would be easier to construct .So anybody have any ideas for me to start ...

[Louise]

yeah furthermore , I plan to build diffrent types of Truss bridges which I think would be easier to construct .So anybody have any ideas for me to start ...

I'm not exactly sure what you are asking for but, here are some thoughts.

I think this is the right thing to do (the truss design). I guess the reason why your teacher says only 1 cm of overlap is so people think about the physics and engineering of the bridge... if you use geometry, you can spread the forces out. Some people may just think of gluing many overlapping toothpicks together with out any design.

There are many different types of truss bridges. I found a nice discussion here:
http://homepage2.nifty.com/SUBAL/carrie/CarrieProE.htm

See also:
http://www.pisymphony.com/tp_index.html

though obviously they (the original) violate your rules, since the basic unit is two completely overlapped toothpicks. However, you could probably adapt it to your rules. There are several bridges on that page that would work.

One thing that seems to be important is good glue!


Louise

[deleted-71588]

Check with your teacher to see if I'm interpreting the maximum 1 cm glue joint between any two toothpicks properly.

My interpretation of the 1cm glue rule is:
1) You can have as much overlap between tooth picks as you wish, they just can't have more than 1 cm of that overlap glued together.

2) If you split that 1cm into 4 equal 1/4 cm sections of glue joint, you can make a strong bond with 100% overlap of the toothpicks and still be within the rules.

Truss Design:
Truss designs are good engineering solutions to making strong beams out of smaller dimensional beams and saving weight. Since the judging criterion in you case defines "best" as the design that holds the most weight per structure weight, you are on the right track.

Joint Design
The tapered round toothpicks that you must work with aren't the easist to make strong traditional joints with. I'm thinking this is a case where a class in "underwater basket weaving" might actually be appropriate

Check with your teacher and verify that you can soak your toothpicks in water and make center slits in them with an "exacto" knife and weave them together.

The winning design might not actually require glue!

The 90cm long chain constraint:
I'm not sure if this contains a hidden requirement for "ground clearance" or height of the span. Check with your teacher on this as well. Any materials that are required to meet any center span height requirement will work against some designs by adding weight that is not as useful.

In other words, can you build a bridge that requires something like cement blocks placed 50 cm apart to get the ground clearance required.

Unspecified issues:
What will be used to "hold" the bridge up in the air for testing. How much surface area and in what shape is it that the bridge will rest on.

What size and shape block of wood do you have to accomodate for load testing. Is it something you can provide or is everybody tested with the same one.

Again ask you teacher.

The test setup in this project is incompletely specified which means you really can't start and know you aren't coming up with something that will perform poorly because of the test setup.

[chickentown]

hi Craige , thanks for your suggestions ,your interpretation about the overlapping thingy is right....you can overlap as much as you want but
you said " If you split that 1cm into 4 equal 1/4 cm sections of glue joint, you can make a strong bond with 100% overlap of the toothpicks and still be within the rules. " what does it mean, can you explain it to me a little clearer

[deleted-71588]

Rules clarification on multiple glue sections:
If you put a very small drop of glue at four places along a toothpick (each small enough so that when you press two toothpicks together, each drop spreads to less than 1/4 cm in length), do you stay within the rules.

In this example, you have four very small glue joints between two toothpicks instead of one longer glue joint.

[Louise]

Rules clarification on multiple glue sections:
If you put a very small drop of glue at four places along a toothpick (each small enough so that when you press two toothpicks together, each drop spreads to less than 1/4 cm in length), do you stay within the rules.

In this example, you have four very small glue joints between two toothpicks instead of one longer glue joint.

I'm impressed you figured out that is what the rules meant... I read it as the toothpicks couldn't overlap (so that you couldn't make a "super-toothpick" of mutliple toothpicks bonded together to use as your base unit)

I'm still not entirely sure this interpretation is correct though, since policing this rule requires looking at every glued portion and messuring it, while my interpretation is much easier to see.

Louise

[deleted-71588]

Louise, Careful, one of the Webster dictionary definitions of an engineer is "one who schemes".

In this case figuring out what the rules really mean is part of the process of determining what is and isn't allowed. In contests like this, the best schemers usually cause a rules change for the next contest!

[Louise]

Louise, Careful, one of the Webster dictionary definitions of an engineer is "one who schemes".

In this case figuring out what the rules really mean is part of the process of determining what is and isn't allowed. In contests like this, the best schemers usually cause a rules change for the next contest!

Scheme away then! I just wanted to throw my cavaet of a different interpretation in since we are getting the rules second-hand. If there is one thing that I have learned on these forums, it is that much of the time we are playing the game "telephone"... I don't want this kid to get disqualified because he/she misunderstood what the teacher said, or miscommunicated your question/clarification to the teacher, or whatever.

So, chickentown, make sure you absolutely understand what is and is not meant by these rules. If you have any questions, please double check with your teacher!

I would look again at:
http://www.pisymphony.com/tp_index.html

One thing the author pointed out is that while he made his bridges very strong, they were much lighter than the other bridges entered in the competetion, and so got beaten. The lightness of the bridge wasn't sufficient to increase the max load/mass bridge ratio.

Louise

[deleted-71447]

3) Toothpick must be glued with no more than a 1cm overlap

I gather that the issue at hand is whether "1cm overlap" refers to the toothpick or the glue joint. My initial interpretation was the same as Louise's. I also agree with Craig that it is ambiguously worded, and that the interpretation of this rule could easily determine the winner. This is definitely worth asking about, as well as the size and shape of the mysterious "block of wood" that will be used to suspend the chain.

Good luck!

[chickentown]

this is what it really means --- toothpicks must be glued with no more than a 1 cm overlap between any two toothpicks.. [/url][/code]

[chickentown]

I get the overlapping lap already and now I just wonder which kind of truss should I use - Warren, Warren (vertical support) , Warren (subdivided), Howe Truss, Baltimore Truss. Which one should be the best??

[chickentown]

http://www.toothpickdesign.com/trusstoo ... ridges.htm
here the link if you wonder

[Louise]

http://www.toothpickdesign.com/trusstoo ... ridges.htm
here the link if you wonder

This site is trying to sell kits to build bridges. I think you would be much better off looking at other websites and designing your own. Look at the website I gave you.


Louise

[deleted-71588]

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 web site you pointed to implies there are some "internal clearance" requirements (aka can a "train" go through it?) AND some "ground clearance" requirements. Ask you teacher about these!

The strength of plain overlap glue joints between two rounded pieces is poor at best and fairly independent of the orientation of the toothpicks and the forces on the toothpicks because it comes down to shear and torsion of the glue. You need to modify the glue surfaces so you have more contact surface area if you want to increase the strength of glue joints.

Straight grained toothpicks can withstand a LOT of tension! The joints will pull apart first! So if you can shave and soak a toothpick, you can wrap it around a joint in tension and reinforce it. Is this allowed?

Full length tapered round toothpicks don't do too well in compression because their ends will bend and break easily. If you can cut the ends off without damaging the fibers and shape the end to match what it will butt against, you can make a strong joint and the compression limits will be a function of how well you control any bending and the shear angle of any unopposed glue joint. Shorter is better. The alternative is to use some wet techinques to put a slit in the middle of a toothpick and make use of the thin end to fit into the slit. You can then split the protruding piece to make it harder to pull out. This kind of wedge like technique is what is used to hold hammer heads onto wooden handles. Again, are these wet techniques allowed?

[chickentown]

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???

[chickentown]

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

[deleted-71588]

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.