Jonny,

Across the design value tables Doug Fir is 50% or more stronger than Western Red Cedar. I wouldn't advise designing for tension, but that goes for Doug Fir too.

The Guild's "Timber Frame Joinery & Design Workbook" is available through the store on this website. It has all the design values and lots of other info that would help you with this problem. If you like good reference books, buy "Understanding Wood" by R. Bruce Hoadley. You will be glad you have both.

Good luck with your projects. Sounds like fun!

Rudy R. Christian