MadSci Network: Engineering

Re: Do you know how much a 3/32 inch square cross-section basswood weighs?

Date: Sat Feb 1 20:16:15 2003
Posted By: Jeff Yap, Materials Engineer
Area of science: Engineering
ID: 1041554407.Eg

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.

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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