I've stepped into the hammerbeam debate previously and would like to add a historical oddity from the sketchbook of Villard de Honnecourt circa 1230 with a hope it will clarify the basic hammerbeam form.
http://classes.bnf.fr/villard/grand/carnet/34.htm The drawings at the top left represent a possible early stage in the hammerbeam developement. The hammerbeam in this case acts as a cantilever beam supporting the roof system in a balanced fashion. The principle rafter ties to the beam over a corbelled wall, thus counter weighting and bedding the beam. The beam extends modestly into the room and receives the hammer post that extends to the rafter below the collar. This seems to be a conservative design low in drama, well suited to thick walls and small rooms.
The later English hammerbeams maintained this basic principle of the well designed cantilever but added lenght, struts, spandrels and Gothic motif. The current American style has lost the structural system of old but has kept the style and suggests the form, a rendering into a genuine ersatz hammerbeam.
My suggestion is to tie the rafter to hammerbeam and bed (not bracket) the hammerbeam to create a cantilever with a second post in the room and then complete struts. I'm sure a good form could be developed, but not what is expected.
