Hi there,
I've done a load of blog posts about platforms I've made, which you can look at here:
https://thedevolutionary.wordpress.com/?s=platform
Answers will depend on the size of the yurt and what you're going to put in it. I'd recommend at least 15cm joists, about 60cm apart (sorry it's not in inches, guys) as shown in the more recent blog posts. The yurt I'm sitting in right now is on 8cm joists resting on breeze blocks, with no
, and has happily gone through several winters, and a spell of -20C. I can put an 18' platform together for about 1,000 euros and support the joists on acacia posts pounded into the ground.
Check out the
platforms for ideas - they're excellent.
Good luck with it all.