structural question from a mechanical guy

huangyhg

structural question from a mechanical guy
hi all-
this is probably a simple question for you structural guys, but i want to make sure i'm correct in my thinking.
i have a house, and i'd like to vault the ceilings. it's a one-story, with only attic space on the second floor. attached is a rough schematic of the layout. it's an older house, wood-frame construction (pine). all the joists are 2x8's, 16" on center. based on the layout attached, i basically just want to take out the ceiling. foundation is good. no cracks in any of the joists that i can see. based on the static load calcs i did, it looks like it should be fine, but i wasn't sure if there was something i was missing.
i'm going to hire a structural engineer to help me, but i want to make sure it's feasible (without much extra work) in the first place.
if i'm missing any important info, please let me know.
thanks for all the help, in advance.
v
if you tske out the ceiling joists, what's going to complete the roof rafters ?
how's the "bearing wall" in the middle of the room going to be connected to any sort of load ??
there are ways to resolve these questions, i'm sure; just as i'm sure it's not a matter of just cutting out the ceiling joists (not that i think that's what you're planning on doing).  
i'm not sure i'm buying that the wall is actually a load bearing wall?  if it were, i would expect it to go all the way across the house.  as it stands, some joists have to span the entire house while others don't.  that doesn't quite make sense.  the only option i see is if one of the bearing walls is "extended", so to speak, by a beam that heads off the joists.
what i would be more concerned about is the performance of the roof when you remove the joists.  they are likely acting as ties to keep horizontal loads off of the exterior walls.  once you remove them, all of the horizontal thrust is going to go to the tops of the exterior walls.  if you have a true ridge beam (one that is adequately designed and has proper supports on each end), then this will help to minimize the thrust, but won't eliminate it.
the ceiling joists are not only supporting the weight of the ceiling but are also keeping the exterior walls from spreading at the top.
it does not look feasible unless you can come up with a way of resisting the horizontal thrust at the top of the exterior walls. note that the weight of the roof joists causes an outward horizontal thrust. also, wind load may cause an inward thrust (direct pressure) or outward thrust (wind suction) depending upon which way it is blowing.  
i shouldn't say the the house is one floor. to get to bedroom 1, 2, and the master, you need to go up a half flight of stairs--so it's really more of a split.
there's a horizontal beam across the center of the house between the living room, and the dining room/kitchen. that "bearing" wall supports that beam, so i assumed it was load bearing.
my concern was, as a couple of people said, the horizontal loads on the top of the walls.
i guess my question should be, what kind of supports are used in this situation? obviously there are many houses with vaulted ceilings... so how do they achieve this?
v
if you would like to remove the ceiling joists above the living room, dining room and kitchen ceiling, you will need to install a structural ridge beam. each end of the structural ridge beam will have to be supported by a column. the location of these columns will need to be reviewed to ensure the load from the roof is properly transferred to the foundation walls/footings. a structural ridge beam will significantly reduce the potential for horizontal thrust at the top of the exterior walls due to gravity loads.
the existing beam and interior load bearing walls (likely only support the ceiling joist load) will have to be investigated onsite to determine their purpose and design for their removal accordingly.
depending on your location (i.e. climate)and local building codes, you will need to carefully consider insulation, vapour retarder and roof ventilation details for the new cathedral ceiling.
there is a ridge beam already installed (sorry for not detailing it). i was hoping to use big enough collar ties to handle the horizontal loads.
i'm on long island in ny, usa so i don't believe climate is too much of an issue.
there's no hvac that goes through this section of the roof. no wiring, or anything else like that.
would it be possible (generally) to use big enough collar ties to react the horizontal loads?
thanks for all the opinions so far.
v
make sure it is a ridge beam and not a ridge board! these function completely different!
the lower the collar tie is p[laced, the lower the tension forces and bolting requirements, and the less the horizontal spreading.  it is possible to do what you mention but the structural will also have to check out the bending of the existing rafters with the collar ties installed.  you may need to have double collar ties at each rafter, but that will get you where you want to go.
mike mccann
mmc engineering
the slope is definitely greater than 4 in 12.
i'm pretty sure it's a ridge beam as it's a pretty thick board, but i'll double check. thanks.
mike-
i was going to double up the collar ties anyway, but you make a good point about the bending of the rafters themselves.
  
v
the ridge beam is currently supported by collar ties at looks to be 8 foot intervals. i was under the impression that they had to be supported at 4' spans, or is that just a local code thing?
man, i think i need to buy myself a aisc manual.
v