a-frame structural members

[scoutineric]

I'm looking at the a-frame plans from the ndsu ag extension office and they call for double 2x6's 4' on center or double 2x10's 8' on center. The plans are from the 60's. Is this really sufficient for the rafters?

http://www.ag.ndsu.edu/aben-plans/5964.pdf
http://www.ag.ndsu.edu/aben-plans/6003.pdf

[John Raabe]

Have a local plan checker look at the plan. There are your local conditions for loads as well as the changes in the rated stress values of currently available lumber. Almost all lumber was from dense old growth timber when those plans were designed. As the old man says, "they don't make 'em that way anymore".

[Squirl]

From what I know, the rafter span is usually measured from the center line of the roof to the load bearing edge.  From link this it appears that it is 12 ft.

Things are partially dependant on your ground snow load.

http://publicecodes.cyberregs.com/st/ny/st/b400v10/st_ny_st_b400v10_8_sec002.htm
Let us use as an example a 30 pound snow load. Since the IRC gets its design numbers from the National Wood Council I would use chart:
TABLE R802.5.1(5) RAFTER SPANS FOR COMMON LUMBER SPECIES (Ground snow load=30 psf, ceiling attached to rafters, L/Δ = 240)
2-2x6's 48" O/C, is the same amount of lumber as 2x6's 24" O/C, although Don_P has pointed out how they don't always perform the exactly the same, many people use this as an equivalent.  Table R802.5.1(5) gives a span of Spruce-Pine-Fir No. 2 (one of the most common offered) at 9 feet 9 inches.  This is a little shy of what the design appears to be.  Also this is without the load bearing second floor attached to the middle of the rafters.  Calculating how that effects the bending of the rafter is beyond my engineering capability.

I do know that as you move a ceiling joist up the rafter, it reduces the allowable span of the lumber.  So much so, that if you move the ceiling joist up 1/3 up from the bottom of the rafter, that it decreases the allowable span of the lumber by 1/3.  I would assume that the load bearing second floor would have a negative effect of span of the rafters.

[Don_P]

This is really a design that should be checked by an engineer. The span of the rafter on the lower level of the 20' wide A frame is actually about 4', the horizontal distance between the 2nd floor joist and the 1st floor joist. But the loading is a combination of the wind or snow from the entire roof plus the load of the second floor on those lower rafter sections. The upper rafter section has a span of about 6' and is loaded just by the roof. Each floor provides the appropriate tie for it's section, which at this slope is minor, steep roofs don't thrust much. I don't doubt that the framing is adequate for many areas. The connections... I'd fill the middle ply of the build up solid under the floor joists so that they are not hanging on bolts but are resting on wood, as per code. This would beef the rafter strength as well.

They lack insulation, I'd go over the roof with foam boards and then strapping and a layer of ply or osb to act as a nail base for the finish roofing.

The piers need an engineer's ok as well, I'm not thrilled there. A crawlspace would be better and would help with making it easier to plumb as well as stronger.

I'd start with a conversation with an engineer or two and see if it seems workable and ask what it would cost to develop these plans. I don't think it would be too bad and I think these are close

[scoutineric]

Thanks for the feedback.

The connections... I'd fill the middle ply of the build up solid under the floor joists so that they are not hanging on bolts but are resting on wood, as per code.

I'm not sure I follow you on this. Do you mean adding lumber underneath the joist in the center of the doubled up beam all the way down to the base?

[Don_P]

Yes, beneath the 1'st floor joists, solid fill between the foundation and underside of the floor. That 2x6 would be treated. Then between the top of the 1st floor joists and the bottom of the loft joists fill that solid again. Now the joists are bearing on wood posts rather than suspened on bolts and subject to splitting.

I just checked the joists at current design loading for wood of the bending strength they specified on the 20' wide frame. It took me 2x12 joists to get it to work after lowering the loft floor load (12' span @ 4' on center) to 30 psf live load... which is code minimum for sleeping, non public type, rooms).

One way to help visualize the lower part of the rafters, between the main and loft floors, is as walls, they simply happen to be angled. Take a section slice thru a typical vertical wall to help visualize further, it becomes a column. A column handles load axially, along its' length in compression. A horizontal member like a joist resists load by bending. A member that has both types of loads is sometimes referred to as a beam-column or as a member with combined bending and axial loading.

Think about a thin flexible stick between your thumb and index finger, a column, and as you squeeze your fingers together that delivers an axial load which will at some point cause the column to buckle, think about the force that required. Notice we don't care about the orientation of the column, you can do the same work if it is upright or on its' side. Now release that force and with the index finger of the other hand push sideways on the middle of the stick that is between those fingers, a bending load. Again note the force required and notice that orientation does not matter. Now squeeze axially and push the column from the side in bending. I hope you can visualize that the buckling effect happens at a lower force in bending and axial directions because the forces are combined. That is what we are dealing with here. It actually happens in a vertical wall when the wind blows. There is an axial gravity load and a bending load from the wind... gravity, orientation now matters! As the wall tips the ratio of the bending and axial loads changes and that buckling point needs careful consideration. when the wall tips more we call it a rafter and size it as a beam in bending but it also has a combined load. This is really a simplification as well, many of these members are really resisting combined forces to a greater or lesser degree. When I filled that middle ply I became more comfortable... but hopefully this helps explain why I would sit down with someone who really knows their stuff.