I don't think the I-joists will like supporting the loft floor system. I would be designing this with dimensional lumber.

If you want to go this route, I don't recommend i-joists. No capacity in axial orientation, connection detailing is difficult, bracing is difficult to justify, no shear wall capacities (maybe treat as 2 pieces of a 3-sided diaphragm?), axial buckling and slenderness, etc.

Not sure I agree that you can ignore collar ties at the top. Internal pressure that is normal to the roof surface is going to push those planes apart, is sheathing boundary nailing enough to hold the system in place without strapping?

On a similar topic, how are people approaching lateral design on A_frames? I haven't spent a lot of time on it, but I haven't yet figured out a good way to justify resolving lateral loads on these structures, particularly because the front wall is always glass.

I haven't done one, but I thought a good bit about it when an RFP for one crossed my desk last year.

I had planned to approach it as a series of frames taking the lateral load and ignoring any potential diaphragm contribution. The diaphragm resists loads in the long direction, parallel to the ridge, but the rafters take the transverse as frames.

For wind/seismic perpendicular to the ridge line I would think a triangle is pretty stable and requires no other bracing other than itself...checking stresses of course.

Need the floor diaphragm of the loft to be stable via shearwalls because:

1) The rafters will likely splice there.
2) Even if they did not, they would likely be overspanned for wind.

Yikes. That looks like a nightmare to design, just because it is so different from the box diaphragm building model we are all so used to dealing with. Regarding lateral loads in the transverse direction, I agree with phamENG and JAE; it will be resisted by frame action (the triangles); no diaphragm is necessary. I am speaking only of the A-frame portion. Of course diaphragm action will be required at the base of the A-frame since the "basement" level appears to be above grade. Lateral loads parallel to the ridge will require diaphragm action. I am used to seeing the A-frames based at ground level, not like this one where the A-frames sit on a lower story nevermind, I see now that apparently it is a basement.

My family has an A-frame cabin in northern MI built in 1970. Lots of other similar A-frame cabins in the midwest. Ours has IIRC 2x10 rafters, doubled with 2x spacers between, on 4' spacing, with 2x12 second floor joists between each rafter pair; no ridge board. Roof sheathing is from inside to outside - 1x8 tongue and groove boards, 2-3" foam insulation, plywood sheets, shingles. Has been rock solid and hasn't moved a bit in 50+ years. Snow mostly slides right off.

All the documentation I've seen for several brands of I-joists have all required a ridge beam.

I doubt the manufacturers would go for bolting floor joists to I-joist rafters. But I have never been presented with a situation like this one before, so I haven't asked.