Wooden Trestle beam size? Warning: a bit of a ramble ahead

[Jake1210]

Hello all. Friday in my tech-ed class during the "first period of x class where you don't do any work, but the teacher just explains what you will be doing during the year" we were talking about bridges. At one point he held up a Pratt truss bridge made from 1/8 inch balsa wood (which we were supposed to do last year, but my teacher [different one from my current one-yay!!!] kept babbling on about how bad the kids at my school were so we never go to do it) At the end of class I asked him about where he got the wood & how much it cost. (I'll post those details if you want me to) And after having a quick talk with him I left for my algebra class (9th grade curriculum in 8th grade... being smart is fuuuunnn.) thinking "Would the 1/8 inch balsa be too thin? After 5 secs with a calculator I found that it would be about 10 7/8" thick if it were full scale. Does any one think this is too thin in the prototype? And no, I'm not modelling X bridge in the prototype. I'm just building my own to see if it would be cool to do when I start the real construction, even then, I probably wont be building x bridge in the prototype.

[selector]

I guess it depends on what is available close by.  The engineer(s) could always specify that the trusses are tighter and more of them to share the same load...I think.  On any of the trestles that I see up here in Canada, the timbers are at least 12", but not much bigger.  So, the size you mention is not all that small.

The posts in the trestle below are 1/8", and look about right, but not compared to the obviously wider sway braces at the sides.  They were all I had.

[twcenterprises]

No, 10" is actually fairly thin according to RR standards when bridge building.  I've seen 12" and 14" square, as well as 12" by 16" beams, up to 24 or more feet in length.  I'd venture to guess that using 1/8" stock would be just fine.

Brad 

[UP2CSX]

12" beams for Pratt truss bridges is just about right on as the most common size. The Pratt truss bridge used smaller members than previous designs like the Howe truss bridge but was stonger due its design, where the members were in both compression and tension. There's a great site that will tell you everything you ever wanted to know about a bridge at http://pghbridges.com/basics.htm .