|MadSci Network: Engineering|
Hi Carmen! Sorry it took so long for me to get back to you. I'm going back to school as well. I hope you get this in time for your project... You've asked a very good question, and I can only answer a small part of it. The short answer to your question is that Basswood, scientific name Tilia americana (Capital "T", lower case "a") weighs 417 kg per cubic meter. This is the same as 6.83 g per cubic inch. If you're using 3/32 square, and you're limited to 25 grams, then you can use 416 inches worth of wood. (Less if you're going to use glue.) So if you guesstimate about 300 to 350 inches worth of rod, then sketch the bridge with that in mind. Next step: Bridge design. I'm not even close to being an expert on bridges, but here's a quick summary. If you're planning on using wood and glue, the strongest design would be what is called a Warren truss. This is the typical design you see on railroad bridges. Basically, you have the flat bottom, two arched or trapezoidal walls of triangles (aka trusses), and truss the top section. Here's a good picture. Good luck building the bridge, and have fun! Jeff Yap Mad Scientist Items of note: Your note is pretty specific about using Basswood. That's an interesting choice. Balsa wood is about a third the weight but about a tenth the strength. That's a good trade. However, cost and availability factors into the project, so if you've got a cheap source of wood, you'll have a superior bridge. As long as 25 grams of Basswood is enough for your bridge. Let us know how it turns out. For style points, you might want to consider making a suspension bridge or a cantilevered bridge. They're a lot prettier, and depending on how they're made, they can often hold just as much weight. Links: American Hardwood Export Council Association for Bridge Construction and Design (Possibly the coolest bridge site I've seen.) Bridge Designs Tacoma Narrows Bridge Beware of Resonance!!!
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