Ken:
To stop a post from moving at the foot, sometimes is as easy as having a person "foot" the post. That is place his hands on the post, near the base, and hold it from sliding.
In larger frames, there are some blocks attached to the "deck" whatever that maybe, to hold the feet of the posts from sliding.
I've often heard them called "kickers", but never understood why they would be called that. You don't kick them away after they are done, and the aren't kicked into place, but they do stop the post from kicking out..... so maybe it is a good name....
Jim Rogers
PS. I may have some pictures of these if you needed to see them....
