A Frame Cabin
A Frame Cabin
(OP)
Hello everyone,
I'm assessing a friend of mine who is an architect who is working on a project of an A-Frame cabin as in the attached picture, he is using some drawings and references form some do it yourself books and asked me to check the designs. I don´t have much experience on wood design buy do have decent experience on steel and concrete design.
I'm concerned about the lateral resisting system to wind loading. Can´t see any brace on the plane of the rafters. Is the external plywood supposed to provide lateral restraint working as sheer wall? I've seen other drawings in which no braces are provided. Any source of information regarding this is welcomed!
Another doubt is concerning the unbraced length of the rafters, being attached continuously to sheeting provides sufficient restraint as to consider it continuously braced, no LTB?
Thanks!!
I'm assessing a friend of mine who is an architect who is working on a project of an A-Frame cabin as in the attached picture, he is using some drawings and references form some do it yourself books and asked me to check the designs. I don´t have much experience on wood design buy do have decent experience on steel and concrete design.
I'm concerned about the lateral resisting system to wind loading. Can´t see any brace on the plane of the rafters. Is the external plywood supposed to provide lateral restraint working as sheer wall? I've seen other drawings in which no braces are provided. Any source of information regarding this is welcomed!
Another doubt is concerning the unbraced length of the rafters, being attached continuously to sheeting provides sufficient restraint as to consider it continuously braced, no LTB?
Thanks!!






RE: A Frame Cabin
To your 2nd question: yes, the continuous nailing to the compression face does allow you to consider LTB as zero.
RE: A Frame Cabin
RE: A Frame Cabin
Dik
RE: A Frame Cabin
BA
RE: A Frame Cabin
BAretired, yes, inside face of rafters do get compressed by flexure but bending moments on this direction are lower allowing for larger unbraced lengths.
Anyone has recommendations on how to nail down and connect plywood sheeting to each other and to rafters to assure right performance as diaphragm?
Thanks!
RE: A Frame Cabin
Mike McCann, PE, SE (WA)
RE: A Frame Cabin
Not sure which building code you are using but if you can't find anything there, the following may be helpful:
http://www.ce.udel.edu/courses/CIEG407/APA%20Diaph...
BA
RE: A Frame Cabin
As far as the lateral forces on the endwalls. You could model the endwalls as trusses with diagonal braces and try to resist the lateral forces that way, although Im not sure how well it will work. If you want to use the plywood sheathing you could try to use the triangular segments at the end as shearwalls, you would just need to have hold downs from the studs to the bottom chord of the truss at the edge of the door and window openings. Either way the endwall needs to be able resist out of plane wind loads as well. You will need to have a built up column in between the door and the window that goes from the floor to the roof diaphram that will support a majority of the wind load.
RE: A Frame Cabin
dnlv, thanks for the tips I haven't finished reading APA's document yet, maybe i'll come back later on your suggestions! I was supposing all lateral forces (parallel to the plane of the A frames) go through the horizontal component of axial force on the rafters, as will be for A's on the middle of the walls, far away from end walls. In my structural scheme no lateral forces will be resisted by the endwalls. I'm not sure if you where referring to this situation though, maybe I'm missing something!
Thanks!
RE: A Frame Cabin
You're right, my dumb brain is stuck on conventional residential framing. If you can use each truss as a lateral resisting element, that is the way to go. You will need to tie the trusses down to the beam to resist overturning, if the forces or light enough you can probably just use typical hurricane ties or truss screws, also size the beam for those loads and tie the beam down to the pilasters for adequate uplift resistance. In the direction parallel to the end walls you will not have any diaphragm shear, since each frame is taking equal force. In the other direction you will have diaphragm shear, you will probably be fine with just typical 6/12 nailing pattern with no blocking, assuming you are not in a high wind area. You should however block at the panel edges of at the ridge and at the base so you have proper diaphragm boundary elements.
As far as the end walls, you will need to size the studs to transfer wind loads into the diaphragm, you should also block between the trusses in the first few exterior bays to help carry the horizontal reaction from the studs into the diapragm. I attached a few details that will hopefully help.
RE: A Frame Cabin
I'm sorry I'm replying so late, thank you very much for your word, really helpful. Also thanks for taking the time to do the sketches! All will be reflected on the final design!
Thanks again!
PLQQD.