# Best method for building balsa-wood bridge [closed]

I'm building a bridge out of balsa-wood strips for school, and wanted some advice. These are the specifications:

• Height: 2 to 6 in
• Length: 12 inches, plus 1-3 inches on each side resting on tables
• Width: 2 +- 1/16 in at the base
• Weight: <= 50 g

The objective is maximum efficiency (load it can hold divided by weight), not just load. It will be tested with a block placed in the middle of the base that has a bucket hanging from it. The bucket will be filled with sand slowly.

What type of truss do you recommend I use for this structure? Currently, I was planning on a warren truss-type configuration, but with an arch. What other trusses would you suggest? What height should I use? What length?

Any and all tips or resources are greatly appreciated. Also, I would love to know the reasoning behind any choices (just for personal interest).

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## closed as off topic by David Z♦Mar 9 '13 at 6:12

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I've built a few balsa wood bridges for school projects and have only one piece of advice to add to Josh's: Use super glue. It's strong, and dries quickly so you will save a lot of time, possibly enough time to build a test bridge and then improve the design. – TaylorR137 Feb 27 '13 at 2:14

Well I can give you the following pieces of advice from personal experience in doing precisely the same thing (once in 6th grade, and once as a senior in high school)

1. Truss + arching is a very good plan. One of the groups in my 6th grade class broke the all-time record with this very design.

2. The most common point of failure for such balsa bridges seemed always to be the center. In high school, I tried to replicate the success of the group in my 6th grade class by using warren + arch, but I failed miserably because there was not enough wood at the central points from which the weight was hanging. The wood actually failed structurally and broke even while the rest of the bridge remained it tact. To make sure this doesn't happen, reinforce the center!

3. I assume you're using glue. Although glue can significantly add to the weight, sometimes the glue itself can add significant strength to the bridge (especially joints), so don't skimp too much on the glue to cut weight!

4. In general, I would err on the side of making a heavier bridge that is more structurally sound than going for less weight. In both of the times we did this in class, most of the really high-performing bridges more than compensated for being heavier by being extremely strong.

Unfortunately, not being a structural engineer, I can't give you much of the reasoning behind all of this, but with engineering, trial and error is just as important, and often is significantly more important, than physics. I hope it helps!

Let us know how it goes!!! I'm really curious to see your final design and to hear about the results (I wish I could do that project again!)

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How do you arch a warren truss? – Anubian Noob May 16 '15 at 3:02
@AnubianNoob Changed to truss given that apparently, Warren truss is un-arched by definition. Is that what you were getting at? – joshphysics May 16 '15 at 3:23
Yeah. I'm still having trouble visualizing an arched truss though. – Anubian Noob May 16 '15 at 15:46

Use paint thinner combined with glue. It thins the glue out so that the glue can get into all of the crevices that people can and CANNOT see. This makes the bridge much more compact and all aspects of the bridge, especially joints, will be bound together with more strength. My bridge was 20 inches long, 160 grams and held 4,287 pounds!!! P.S - As well, use a gusset technique. If you don't know what that is, then search it up online... PS - my bridge had to cover a 15" span and be 5cm tall by 5 cm wide. Mine was 4" by 4".

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