"The galvinized steel plate is there to hold the post in placeand to seperate wood from concrete."
the bottom plate is made of aluminum not steel which by my understanding is a soft metal. It is called an "Aluminum Plinth". I found a similar one here at homedepot to show what it looks like -
http://www.homedepot.com/catalog/pdfImages/0d/0d77a15d-4080-4e57-964b-45244fe21d77.pdf
the fact that the columns are standing on aluminum plates makes me think they can't be holding a whole lotta weight but that is just a guess.
"You said the rafters are cantilevered over the wall, perhaps the post are just decritive."
I think they are not decorative since they are holding up a beam made of 2 - 2x10 nailed together (the rafters are resting on this beam and the columns are holding up this beam). So that looks structural to me.
"Is the beam under the roof that the posts are holding up? " yes
"Did you open an end one or one of the middle ones?" Both. Over the years I have replace many individual boards on these columns because of rot. I am done replacing individual boards and want the whole darn column replaced with something that will not keep rotting out ever couple years.
"I have installed hundreds of posts, never a box, " I agree Its the damnedest thing I have ever seen also (I am in south carolina after all, they do crazy *** construction down here). My 3 car carport was also supported by these stupid box columns and those columns on that structure were holding up some serious weight (until they rotted). I replaced those columns with treated 6x6s and it hasn't fallen down yet! (crossing my fingers).