pyramid roof

huangyhg

pyramid roof
i have a pyramid roof for a tower about 20'x20'.  the roof is 4:12.  the client wants to express the ceiling so i cant put trusses.  so i decided to use the double plates (2x6) as tension ring.  so i model it risa 3d and assume that each hip will be pinned on both sides.  i got about 8 kips of thrust on each direction.  so i decided to fix the top connections to try to reduce the thrust (create a moment connection).  this is the part i dont understand, as soon i fixed the top, my thrust went up to 11+ kips!!  can someone explain it to me why?  i dont undestand why.  i rechecked my model like 10x and i cant figure out why it doesnt go down.  am i missing something here?  it is suppose to go down right?
are you running a pdelta on it?  
i would think that with the fixity, it would reduce since the reliance on the tension ring.
did you just fix the straight bending moments going through to the opposite ridge   
no, im not running p-delta.  i fixed everything at the peak (no releases).  
jae, this is what i did, i only drew 4 members (each hip).  the bottom connection is all pinned,  i let the peak node free.  then at each   
try release the rotational restraint on the top node, so you have pin-pin condition, and see the difference in results.
kslee, the top node is free.  i am only changing the end release at the top to either fixed or free.  the fixed one yields higher thrust which i dont understand.
i ran it quickly in staad & modeled it with the bottom corners fixed, w/ beam releases so no moment transfer. i used a triangular load which started at 0 at the corner, and increased to 250# at the peak on each hip.
i got 3.5k of thrust in each direction (x & z)
looking at the forces, they cancel out with the adjacent   
if you model it in 2-d then your l would be 28'.  the height of the peak should be about 3.5 ft.
sorry, i should say release the   
well, there is internal moment at the peak on all 4   
ahhh, now i see what you did.  yes, it makes sense. you made an arch.  your thrust force is definitely going up