A Frame

[knowsenoughtogetintrouble]

To start off I have very little academic engineering knowledge but know just enough to get me in trouble. I am designing an Aframe cabin with an open loft. The frame is a 32' equilateral triangle. the dimensions of the cabin are 32' x28'. The rafters/ frame is 16" oc. I am using 2"x12" x16's sistered together with an 8'x12"x 3/4 inch ply on each side to make up the vertical sections with appropriate strength structural nails/screws(have not got that far). I have also though of using metal bracing, but saw a video of someone using the plywood and thought it might work. The top is joined with 3/4 ply as well. I will have a rafter approximately 48" inches from the peak. I guess my question if I was to leave the A open with no rafter lower down is that strong enough and is my method of sistering solid? I have attached a Fusion drawing for reference.

 

[phamENG]

Sounds like you need to engage the services of a structural engineer. There are likely several in your area that would be happy to assist you in the design of this structure - of an honest fee, of course.

 

[SWComposites]

OP -
are you designing this to build it yourself? are you going to need a building permit? where are you located?
or are you designing this to build to sell to someone?
or something else?

 

[knowsenoughtogetintrouble]

OP -
are you designing this to build it yourself? are you going to need a building permit? where are you located?
or are you designing this to build to sell to someone?
or something else?

I plan on building it myself as of right now. This just a preliminary plan I'm using for material cost and design. I do plan on handing this over to an engineer/architecture service, but I wanted to see if I was in the ball park first. I m currently located in central Missouri, I am unsure about the building permits. This

 

[JoshPlumSE]

Yeah, there are lots of issues with an "A Frame" type system that we see in residential construction. Even when done by good, experience carpenters.

a) One way is to use a true ridge beam That resists the vertical loading of the rafters and carries this load to supporting columns on either side.
b) Another way is to just have ridge board for framing. In this case there will be an outward thrust at the bottoms of the rafters and a vertical sag at the ridge line that must be investigated / calculated to make sure it will be acceptable both from a capacity calculation and a serviceability one.

 

[XR250]

a) One way is to use a true ridge beam That resists the vertical loading of the rafters and carries this load to supporting columns on either side.
b) Another way is to just have ridge board for framing. In this case there will be an outward thrust at the bottoms of the rafters and a vertical sag at the ridge line that must be investigated / calculated to make sure it will be acceptable both from a capacity calculation and a serviceability one.

I disagree with both these statements.
1) A-Frames are triangles - why would you use a structural ridge? The floor system is supposed to resist the thrust.
2) Ridge boards don't visibly sag unless the rafters are spreading - which should not happen if they are tied correctly

 

[ANE91]

I think OP wants a vaulted ceiling with no ceiling joists or rafters ties, in which case a ridge beam is the answer.

 

[Goatenstein]

I think OP wants a vaulted ceiling with no ceiling joists or rafters ties, in which case a ridge beam is the answer.

Picture C shows the rafters tied to the floor system. That should prevent any spreading. The ridge will act like a ridge board.

 

[JoshPlumSE]

XR250: 1) A-Frames are triangles - why would you use a structural ridge? The floor system is supposed to resist the thrust.

2) Ridge boards don't visibly sag unless the rafters are spreading - which should not happen if they are tied correctly

There is nothing wrong with this type of framing..... when it is done CORRECTLY.

Maybe it's just that I've seen a lot of track houses where I live that have this problem. Where you can see walls pushed out a little and the roof sagging noticeably. Since the OP is not an engineer at all, I figured it was wise to point out the problems that arise when the framing is not done correctly. I find the most egregious mistakes with residential wood construction are related to a poor understanding of load paths.

1) Honestly, I think we're saying the same thing here, sorta.....A ridge beam is required if you don't have any tension struts at the bottom to resist the thrust.
2) My main point was that you have to design the roof framing properly. To resist the thrust or else it's going to be problematic. It'll push the walls out and sag.

 

[LittleInch]

Is this a true complete A frame building?

i.e. the end of the very long joists just sit on the foundations close to the ground? / attached to the ground floor? As shown in your b picture.

I think most people would be looking to tie the two sides together at the base / floor level. i.e. in your b picture, run the floor joists the other way and use them to resist the horizontal forces coming into the floor beam / plate which is trying to push them out.

Not sure how they will work to have what I guess are horizontal beams holding up the mezzanine level or is that going to be a stand alone frame inside the main structure?

Those main spars though look very long. Can you get them in a single length?