Girder Orientation

[thedresden40]

I'm having some last minute jitters before starting the floor structure on my small cabin:

She will be just 12' x 16', set on six 12" sonotubes. The span between piers is 10' on the short side - with plans to cantilever 1'. The 16' span is bisected by two middle piers, forming spans of 8'. Each sonotube is anchored and epoxied into bedrock at around 2.5' below grade and vary above grade between 1.5' and 3'.

I've been waffling for weeks about the best way to orient the girders that will sit atop the sonotubes; two 16'ers or three 12'ers. The roof will be a steep 16/12 pitch with metal roofing, but with an 8' shed dormer with a 4/12 in the mix. Gable end obviously on the 12' ends. Snow country up here in Maine, although I'm guessing minimal load given the metal and steep pitch.

This will be stick framed with 2x6s (original plans for timber framing this one had to be ditched last month, regrettably - excuse my interloping in the wrong subforum if I have) and I would like to utilize a structural ridge beam to keep my ceiling spans open. I had originally thought to span my girders along the 16' ends, but I worry now that the ridge would not be appropriately supported that way, with the half the roof weight landing midway on the largest unsupported span.

I'd love any insight that you all may have. This is my first cabin build and although I'm incredibly excited to finally get this going after months of clearing, I'm more than a little daunted as well ...

[Don P]

The building is 20-25' tall, the "foundation" is 10' wide and unbraced, as are the girders that rest atop it. The ridgebeam support posts land at midspan on a beam that is also supporting the floor. There is ample reason to have the jitters but the problems run deeper than the question.

[thedresden40]

I appreciate the reply. And the skepticism. I would love to hear more.

I consulted a a very reputable engineer for my original timber frame design (same dimensions only without the 1' cantilevers) and was given a solid green light from them. The Sonotubes were arranged, anchored, and fitted with J-bolts exactly as recommended by him. A timber frame is an entirely different beast, I do understand, but I wonder what problems with the foundation would be created by the switch from a timber frame to a stick build?

The plan was to tie the girders to the Sonotubes with 6x6 Simpson post bases, and the joist structure then tied to the girders by way of hurricane ties and angle brackets. The girder plan came from a very successful carpenter friend of mine, who's expertise I trust, though I will admit that my understanding of that approach hasn't quite clicked versus a continuous sill of some kind.

Perhaps those extra details change things, perhaps not. I am not tied at all to this particular design if there are better to be had - I am all ears for hearing what other approaches could be taken. The tubes are in and poured though, so no need to spend time on recommending a slab or otherwise, which would not have been possible on this site.

I've gleaned so much knowledge from the sidelines on this site and I'm anxious to hear thoughts ...

[Don P]

I would run the joists the 12' direction to support the cantilevered walls and lower ends of the roof. The gable end joists would then need to be sized to carry the ridge as well as any other loads accumulated along the way.