danf26
03-04-2011, 07:01 AM
Hi all,
So I’m looking for any advice/info that may be out there regarding a stick-framed deck that Im building for a small cabin I’m currently building in western Massachusetts. The deck will be on 10” sonotubes, reinforced with rebar. The deck has a number of qualities that make set it aside from a more standard deck on piers. For one, the cabin is a traditional timber frame. This means each of the 6 posts need to have pier directly below them to successfully transfer all the loads. See rough drawing of Cabin interior dimensions.1927
The second quality is that the structurally timber frame will be ‘wrapped’ with straw bales, acting as both the walls and insulation. This means there is a considerable difference between the intior footprint of the house (14’x20) and the exterior (depends on the orientation of the bales). See rough drawing of Cabin exterior dimensions. 1928
One side of the house (North) will transition into a screened in porch. Another side of the house (West) may be a narrow (~3’) walkway. The other two sides (or three if I don’t use the walkway) will stop where the exterior walls end. This is important as it affects pier placement and the cantilevers I’ll be needing. See picture here, and pardon its roughness: 1929
Its important to note if I did use a walkway as shown on the West side, more piers would be placed to carry the extra load. This would mean no more cantilever on the West side.
So, with piers needing to support the interior posts, I am looking to cantilever the girders and joists out over the piers to catch and support the straw bales. The straw bales, with both interior and exterior coats of sand based plaster, have a dead load of 22 to 25 lbs/square foot per course of bales (a course is one ‘level’ of stacked straw bales – this is easy to imagine with bricks). My cabin, from floor to eave, measures 11’6”. That’s around 9.8 courses of bales laid on their flat edge (36” long x 18” wide x 14” high). But, with the roof at a 12:12 pitch, we actually won’t be able to fit exactly that many courses up to the top plate, because of the rafters. But, to play it safe, 9.8 X 22 = 215.6 lbs./square foot dead load.
Using 4x12 girders, with 2x12 joists running on top of them, and with a 19.5” r past the pier and with this calculated dead load, how can I make sure my cantilevered joists will be strong enough?
I feel like this should be fine, but I want to come as close to qualifying it as I can. In the book “Serious Straw Bale” the authors write: “Working with a design load of 22 to cantileve5lbs/square foot per course of flat-laid bales and plaster, the load imposed by a standard 6- to 8- course bale wall should be well within the tolerances for a cantilevered 2-by-10 or 2-by-12 floor joist system.” My cabin will have closer to 9 courses, but that passage, along with other reassurances I’ve gotten from people, make me feel like my 19.5” cantilever will be sound no problem, but I want to come as close to qualifying this as I can.
I understand a building inspector will run these numbers themselves, but I don’t want to bring my design to them without knowing what I’ve drawn up will work.
Any thoughts would be appreciated.
Dan
PS – After figuring this out, I would need to design for proper insulation and air sealing for this cantilever. In the same book, it is written that “the overhang portion of the deck should be detailed pretty much like an outside wall, with proper attention to air tightness and insulation.” What details are they referring to? IE – How do people detail a conventional outside wall?
So I’m looking for any advice/info that may be out there regarding a stick-framed deck that Im building for a small cabin I’m currently building in western Massachusetts. The deck will be on 10” sonotubes, reinforced with rebar. The deck has a number of qualities that make set it aside from a more standard deck on piers. For one, the cabin is a traditional timber frame. This means each of the 6 posts need to have pier directly below them to successfully transfer all the loads. See rough drawing of Cabin interior dimensions.1927
The second quality is that the structurally timber frame will be ‘wrapped’ with straw bales, acting as both the walls and insulation. This means there is a considerable difference between the intior footprint of the house (14’x20) and the exterior (depends on the orientation of the bales). See rough drawing of Cabin exterior dimensions. 1928
One side of the house (North) will transition into a screened in porch. Another side of the house (West) may be a narrow (~3’) walkway. The other two sides (or three if I don’t use the walkway) will stop where the exterior walls end. This is important as it affects pier placement and the cantilevers I’ll be needing. See picture here, and pardon its roughness: 1929
Its important to note if I did use a walkway as shown on the West side, more piers would be placed to carry the extra load. This would mean no more cantilever on the West side.
So, with piers needing to support the interior posts, I am looking to cantilever the girders and joists out over the piers to catch and support the straw bales. The straw bales, with both interior and exterior coats of sand based plaster, have a dead load of 22 to 25 lbs/square foot per course of bales (a course is one ‘level’ of stacked straw bales – this is easy to imagine with bricks). My cabin, from floor to eave, measures 11’6”. That’s around 9.8 courses of bales laid on their flat edge (36” long x 18” wide x 14” high). But, with the roof at a 12:12 pitch, we actually won’t be able to fit exactly that many courses up to the top plate, because of the rafters. But, to play it safe, 9.8 X 22 = 215.6 lbs./square foot dead load.
Using 4x12 girders, with 2x12 joists running on top of them, and with a 19.5” r past the pier and with this calculated dead load, how can I make sure my cantilevered joists will be strong enough?
I feel like this should be fine, but I want to come as close to qualifying it as I can. In the book “Serious Straw Bale” the authors write: “Working with a design load of 22 to cantileve5lbs/square foot per course of flat-laid bales and plaster, the load imposed by a standard 6- to 8- course bale wall should be well within the tolerances for a cantilevered 2-by-10 or 2-by-12 floor joist system.” My cabin will have closer to 9 courses, but that passage, along with other reassurances I’ve gotten from people, make me feel like my 19.5” cantilever will be sound no problem, but I want to come as close to qualifying this as I can.
I understand a building inspector will run these numbers themselves, but I don’t want to bring my design to them without knowing what I’ve drawn up will work.
Any thoughts would be appreciated.
Dan
PS – After figuring this out, I would need to design for proper insulation and air sealing for this cantilever. In the same book, it is written that “the overhang portion of the deck should be detailed pretty much like an outside wall, with proper attention to air tightness and insulation.” What details are they referring to? IE – How do people detail a conventional outside wall?