Tension or Compression? To the Editor: I have just seen Harris Hyman's article, "Removing Collar Ties" (4/92), for the second time. I did not write after the first publication because I felt sure that someone else would express the same surprise that I felt. Since no one has, here I am. In conventional "stick" construction, it is the job only of the attic floor joists to act as ties to prevent the spread of the sidewalls. Clearly, if the sidewalls are firmly tied by the joists, then any sag in the rafters will put a collar in compression. This is the true function of the collars; to prevent a swayback ridge. The "real effect" of removing collars in a structure