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Popsicle Stick Bridges

Competitive Crafting - How to Build a Stronger Popsicle Stick Bridge

Could someone help me solve a Popsicle stick glue joint problem? I am in a contest in which I need to glue Popsicle sticks together for strength. I am having a problem achieving a strong joint because of some kind of coating that comes on the surface of the sticks.

I have purchased sticks from many sources, even hobby shops, and all have the same slightly shiny appearance. I think there is some kind of substance applied during manufacturing to give them a long shelf life. This material goes into the grain of the wood because, after sanding, enough is left behind to inhibit a strong joint.

I have tried many solvents including paint thinners, lacquer thinner and even acetone. This has helped a little but the joint still fails before the sticks break. I am using good clamping techniques.

For this project, I am restricted to using only carpenter’s glue which is very strong under the right conditions, such as using it on a clean smooth surface. These sticks, as the come from the store, work well enough for simple craft projects but not for strong joints.

Any help would be greatly appreciated.

Don

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You’ve piqued my interest. I’m curious as to what type of contest you’ve entered. Are you recreating a trebuchet (a medieval siege engine used to batter castle walls or to throw projectiles over them); or perhaps a Popsicle stick bridge that will support 50 pounds? That was a rhetorical question. Don’t answer and give the construction secrets away to the competition!

I assume you’re using pure wood craft sticks, not one of the new composites made of pressed thermoplastic and sawdust. Traditional Popsicle or craft sticks are carved from wood, sanded and buffed to a smooth finish to avoid splinters. Since these are items that go in people’s mouths and are used by small children, there are no additives or coatings allowed. The smoothness you detect is from the buffing and from any natural oils remaining in the wood.

This is more of an engineering question than a crafting one but I’ll give it a whirl. I also invite other readers to weigh in with their ideas. Click on the "Contact" button on the left is you can help Don.

I don’t think you can get a weight-bearing joint with just one layer of Popsicle sticks. Have you tried layering and staggering? Think of the way bricks are laid. Two bricks cemented end to end could not support much weight; but bricks layered and staggered can support a roof!

I don’t know how strong a joint you need, but the minimum number of layers for the brick design is three. For example, if you want a strong span that is three 6-inch sticks long, take nine 6-inch Popsicle sticks.

Cut one stick into two 3-inch lengths to use in the middle layer. Lay three sticks end to end against a straightedge. Draw a line of glue down the center. Lay the middle layer thus: 3-inch stick, 6-inch stick, 6-inch stick, and 3-inch stick. Draw a glue line as before. The top layer is identical to the bottom: three whole sticks end to end. Do you see the brick pattern?

This design creates a “beam” which will support quite a bit of weight. When you need to form angles, gluing beam to beam (rather than one stick to one stick) also gives you more surface area to form a bond and therefore a stronger joint. Finally, when your “whatever” is completed, you can surround the critical joints with more carpenter’s or white glue to reinforce the joint. One of the nice things about this glue is that it dried clear.

Good luck in the competition! I’d be interested to hear how you solve the problem. After the contest, feel free to send me a photo to put on-line with your letter.

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Thanks for your interest and for responding to my questions regarding popsicle stick glue joints.

I first encountered the problem last fall when I entered my first popsicle bridge building contest. Although I won the contest with my bridge carrying 183 lbs. before it failed, this coming fall I will be entering another bridge contest and want to solve the joint failure problem before hand.

When the bridge failed, it did so by the joints pulling apart with out leaving much wood behind. I found this puzzling because I took great care in selecting good quality, all wood, craft sticks and by sanding the face of each stick, using new glue, making and using a clamping jig, allowing each gluing to remain clamped over night and using a three stick joint design in most of the joints for maximum strength.

I even was careful to avoid using sticks with cross grain, knowing that these sticks would probably fail first by splitting apart.

I know that every bridge has it's limits but if I could find a way to get the sticks to glue properly, my next bridge could easily carry over 200 lbs.

Attached is a picture of last years bridge and the contest rules. If, for some reason, they do not make it through in one piece please let me know.

Any insight you can give me would be greatly appreciated. If you are interested I can also send the pictures of the other bridges I competed against, each with a short list of stats including builder, age category, number of sticks used and weight capacity before failure - 11 in all.

Don

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I’m really impressed that your first effort supported 183 lbs.! Your gluing technique sounds flawless, but you might find something in this page on gluing wood from Popular Mechanics that sparks an idea.

I’m wondering what kind of a “post mortem” you performed on last year’s design. Do you think the failure was solely due to individual differences in the wood surfaces and glue thickness? That seems to be the tack you’ve taken because you’re selecting your sticks as carefully as possible and looking for ways to make the wood accept the glue better.

You mentioned that the joints perform a little better after you soak the wood in some kind of solvent. I suspect this is because the solvent swells the wood a bit, giving it some “tooth”, like a piece of Velcro. Can you enhance the effect by roughening the surface after soaking? What I mean is to use sandpaper to make the surface a bit rougher rather than smoother.

When you apply the thin layer of glue to each piece, apply it in the direction to make the “teeth” stand up. On a microscopic level, there will be more surface area for the glue to grab onto.

Did you note where last year’s joint failure began and how it spread? If you think it began at a weak point in your design, do you have any sticks left over to reinforce that area? Since you’re limited to 200 sticks, if you’ve already used your quota, taking some from one area to reinforce another is kind of a Catch-22.

I noticed in the rules of competition to “keep in mind that the majority of the bridges in competition collapse due to joint failure, not member failure.” I guess that says it all, doesn’t it?

Good luck, Don! I’ll pass on any tips our readers submit. Please let us know how you do in this year’s competition.

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Thank you for your feedback and suggestions. I am coming to understand that my last bridge failed for two reasons. The surface of my sticks was too smooth and I used too much clamping pressure.

In September I get another chance.

Thanks again.

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I just came across your popsicle stick bridge article.

The third week in February is National Engineers Week, nationwide.

Many of the Engineering Societies do various projects to create a desire to learn more about the field of engineering. We've had a popsicle stick bridge contest every year. The kids really get a kick out of it, plus we award cash prizes.

Our span has a 24", 500 gm limit; Elmers glue, no paint and no bumps in roadway. A "car" 2"x2"x4" must pass through it. There are some other rules.

Two years ago we had a sixth grade girl's bridge take 350 lbs to break. It might have taken more but the pull chain shackle popped open.

Here is more information on National Engineers Week.

I belong to North Carolina Chapter 082 of the Society of Manufacturing Engineers.

John

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My son wants to try to build a popsicle stick bridge for his school project, do you have plans or instructions step by step to build it.

We really need help. It has to be done by 4th of Feb.

Toby

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Go to Frank Petersohn's Rainbow Bridge site. As long as you are not using them for profit, the plans are free for your use.

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