Hi Ian.
Steve Chappel's book is the only one I'm aware of that discusses Hammer Beams in any detail. It contains some rules of thumb about their design, including the goal of having the main brace meet the post approximately 2/3 of the way down. My understanding is that this design was originally used in buildings with stone buttresses to resist the rafter thrust. Since this is rarely done today, pay special attention to how well your design will handle that thrust. CB.
