I am working with 4.5 feet to 2.5 feet of 6"x6" post hight to brace . Posts 5' apart.
Long "X" braces top of one post to bottom of the next seem best. Bu, if I do that the angle is not 45 degrees. How much does that matter..especially on shorter posts, the angle will be far from that.
If that does work, is bracing to post or the beam on top best? And can I cross the two braces in the "X"..one will be cut to let the other one pass without compromising the strength of the cut brace?
Or, can I get by just as well with Y braces...2'4" feet down post on the 4.5 foot posts? That sure is easier.
Please disregard the end representation...they will be cut into post on lower and upper connection and butt against post if on the beam on top. Held in compression.
(https://i1185.photobucket.com/albums/z354/alextrent/House%20Uploads1/Screenshot2011-12-23at13719PM.png)
Don_P recently posted one of the better methods of bracing piers. Something like this does not depend on mechanical fasteners for its strength. Some variation on this theme would be advised.
(https://i180.photobucket.com/albums/x109/windyhilll/postbrace.jpg)
An alternative would be to construct what would be called a shear wall or braced wall between the piers. Framed much like a normal wall it would be sheathed with PT plywood, well nailed to the piers, upper beam and the 2x members that make up the wall framing. Sorry, no images of that idea to post. I have some but they're not complete; been placed on a "back burner" for time being.
A bit in here about braces, and just for general knowledge these are really good articles;
http://www.timberframeengineeringcouncil.org/images/pdf/TFG%20Article%20December%202007.pdf
http://www.timberframeengineeringcouncil.org/images/pdf/TFG%20Article%20March%202008.pdf