hip roof with cathedral ceiling

huangyhg

hip roof with cathedral ceiling
i am reviewing a set of plans in which a hip roof with a cathederal ceing is specified.  the roof system consists of a ridge beam and 4 hip beams.  the interior is pure cathederal with no columns supporting  the ridge or hips and no collar ties.  the roof is rectangular 48' x 28', roof pitch 6:12, snow load 25 psf.
what if anything can be used to oppose the thrust forces generated at the outer walls?.  can the roof sheating act as a diaphram to oppose the trust?  if so how can this problem be modeled?
should i send this back to the architect?  it seems to me this is a unstable design.  
thanks in advance.  

if you drop the ceiling slightly, this allows for a small, flat area around the perimeter of the ceiling between the wall and the point where the ceiling begins to slope upward.
this flat area can be utilized as a horizontal truss spanning from wall to wall and taking the horizontal thrust to the sidewalls where it can be properly resisted.
the flat area "truss" can be an actual truss designed for this purpose, and hung from the rafters, or it can be some form of plywood structural panel with built-up 2x chords and webs of plywood.
dave 169:
another option would be to design a "scissors truss" (inside your building section profile)  which would take care of the horizontal trust from the roof loads. obviously, the slope on your cathedral ceiling would vary slightly (less steep)  from your roof framing rafters but would still provide the architect with the cathedral ceiling look on the inside.
hopefully it helps
:0)
rarebug
if you have a beam support, you can likley put in a strap tie at the ridge to hold the opposite pieces together and then only deal with vertical loads at the walls...
dik
see if the architect will accept tension ties at the bottom of the rafters.  i once used 3x6s at 8 ft on center in each direction.  the architect got the open effect he was looking for.
dik,
i don't think that works because the ridge beam is supported by the hip beams (not a gable end condition).
although i've never designed one, i've seen a tension ring type of design used. it's done by connecting cables to the tails of the hip rafters which resist the horizontal thrust of the roof.
thanks for the help.  i think i'll try to talk the architect into using scissor trusses or something to support the ridge beam such as columns or trusses.
a tension ring or flat ceiling around the perimiter that acts as a tension ring is a good idea but would interfere with the other portions of the home in this case
thanks again.
dave169,
at each of the four corners create two plywood shear walls acting as vertical diaphragms ( total 8 walls, each at least 6 feet long) with tie down anchors at each end to take the horizontal hip thrust as an uplift force to the foundation.  also create a continuous top chord   
we recently did a 16' x 24' accessory building with a ridge beam supported by hip beams restrained by connecting to the top plate of the wall with a welded plate assembly. thru-bolts  in the hip beams (6 x 10) and lag bolts to the top plate. the plate assembly is "l"-shaped as you look at it in plan, so the wall corner could take the outward thrust (tension in the wall double top plate).
the roof slope was only 3:12, so the thrust was large. the roof is concrete tile. roof dead load=24 psf, and the live load = 20 psf construction duration.