Structural Limitations of 8x8 Post

[Rougespear]

I am trying to better understand the upper structural limitations of an 8x8 post: primarily when and why one must transition to an 8x10 post.  Far as I can see, 8x8 posts seem widely used in frames up to 16-18' wide supporting a common rafter roof system.  However, when surpassing 18' wide bents, posts seem to be more commonly sized as 8x10.  Is there a rule of thumb about when to "up" the post size?  Does the distance between the rafter plate and the tie beam set the limit due to outward thrust of the roof?  At what rafter plate to tie beam spacing is a "rule of thumb" denoting when to upsize the post?  If increasing the post size, does convention also say up the size of the rafter plate?

I ask these questions because I am considering "upping" my posts from 8x8s to 8x10s.  I have 12' posts, with 3' of that above the tie beam.  I am wondering if the outward spreading force of the common rafters is too great for an 8x8 post and rafter plate with 18' wide bents?

[Ljohnsaw]

Others please chime in if I get this wrong...

A post's role is to support vertical loads.  As the designer, it is your responsibility to handle the side thrust from rafters with appropriate tie beams or a supported ridge beam (to eliminate outward thrust).  if your ridge is unsupported, 3' seems like a lot of leverage on your joints.  A lot will depend on you loading (snow load).

The size of the post is determined by the load it has to carry.  A piece of wood can carry so much weight per square inch of footprint.  However, that "footprint" is reduced by the mortises and housings that you cut into it.  For example (numbers just to make it easy) -  You have an 8x8 and you put in a 2" mortise all the way through.  You have effectively removed 1/4 of the "footprint".  The mortises are not additive.  That is, a 2" mortise near the top and 2" one near the middle don't remove 1/2" the footprint (1/4 each).  It only is "somewhat" additive if they are near each other (don't know the specifics).  That being said, a post can hold a tremendous amount of load!  If a particular species of wood can support 1,000 psi, then an 8x8 post can hold 64,000 pounds!  The limiting factor (reason to up-size) will likely be aesthetics and/or the number or type of joints needed.  My cabin's central posts have to support 20,000 pounds.  For the look I wanted, I decided to make them 8x10 since the were housing a 10x16 beam - both of which were oversized because I wanted the look. 

IIRC, I read all this in a Jack Soubon (sp) book a couple of years ago.