Span & Load ???

[PlicketyCat]

Trying to make sure we hunt for the right sized timbers for our cabin and I'm having a bitch of a time finding load and span tables for Black or White Spruce unsawn timbers.  Our local co-op extension office says timber in our area are easily #2 grade, so can I just use the Pine/Fir/Spruce tables for unsawn timbers? Are all Spruce (Picea) created equal enough to do the calculations? At most, we'd only be lightly sawing the top surfaces of the beams and rafters (maybe 2-3" wide to achieve a flat plane) so we can install the decking easier.

Also, anyone have any experience with calculating spans and loads on radials/domes? We're planning to have a 13-rafter reciprocal roof which results in cone/dome shape with a central "occulus". This should eliminate the need for a central post as the roof load should be distributed across the dome and onto the purlin plate of the 13 mid-span posts and the 13 outer posts (all posts carry to the ground) . Trying to figure out if 13 rafters will be enough to support the weight of a 20' radius outer wall with a 2' overhang with just 1" decking, or whether we'll need additional rafters and/or some concentric purlins. Roof snow loads in this area are no less than 50psi; but we'll have a green roof so we must be able to handle an additional 22.5 lbs per square foot (3" of lightweight media and membrane) on top of normal dead load (live load for green roof shouldn't be more than snow load since we don't get a lot of rain and will have a water-shedding membrane just in case). You can't really pre-determine pitch well on recipro roofs since they flex when they lock together, but I'd say 3-4/12  is about right and that's the steepest recommended for green roofs.

NOTE FROM ADMIN. DIRECT LINKS TO OFFSITE PHOTOS ARE NOT ALLOWED. IMAGES MUST BE IN YOUR FORESTRY FORUM GALLERY

We may have to 1) beef up the size of the rafters (once we determine what size they should be), 2) add more rafters, 3) add purlins, or 4) add knee braces from the edge of the occulus to the mid-span post (creating, in essence, something similar to a birdcage or cruck).

ICK - the technical math is so not my favorite part of the design process   I'll have to post my model once I fight with SketUp some more.

[beenthere]

So you are looking for the size of spruce rafter that will support 1/13 of the total expected roof load?

The load point would be about 1/6 of the way from the top end,  or ?

Depending on the opening, the rafters might be close to 10' long plus overhang?

Have you any rough estimates as to what you calculate the load to be at the loading point ?

[PlicketyCat]

So you are looking for the size of spruce rafter that will support 1/13 of the total expected roof load?

Yes, plus a good working figure/estimate for the dead load given my design.

The load point would be about 1/6 of the way from the top end,  or ?

Assuming nothing is different about domes and load points, yes 1/6 sounds about right.

Depending on the opening, the rafters might be close to 10' long plus overhang?

Assuming opening is 2' radius, and perimeter wall is 20' radius, the rafter will more likely be closer to 20' plus overhang (compensating for increased length due to pitch)  -- we can say 24' total if that helps with the math

Have you any rough estimates as to what you calculate the load to be at the loading point ?

Not really - we know snow load  has to be 50psi, green roof average is 23.5 lb/ft, and then whatever the actual timber load is .

[beenthere]

Caught me on the 10' radius instead of 20'.  ;D

So roughly 1260 sq ft of roof area, at maybe close to 100 psf, or total of 126,000 lbs.

Supported by 13 rafters, for almost 10 k per rafter?

The load at 1/6 from the tip, and support points at the tip and on the wall, going to be quite a bending moment.
Will think on that some.  :

[PlicketyCat]

Of course, we could always add another set of rafters - so 26 with 5k lbs... but that's assuming that the additional rafters don't have to be supported to the ground on their own posts on the inner ring (mid-span). If we had to do that, then we'd only have about 2' of room between the posts on the inner ring and that's a mighty tight squeeze. Now, if it's ok to distribute the load on the purlin plate down those original 13 mid-span posts, we could add 13 additional posts to the outer ring to help handle the load if that would work... those would be 4-5' apart which is plenty big enough for doors and windows on the outer wall.

(working with pie shapes is so much fun isn't it?)

[PlicketyCat]

Hubby reminds - with a reciprocating roof the end is supported by virtue of the occulus, it's not just hanging in space. Bending moment may not be as extreme as anticipated, or even present.

(isn't working with domes so much fun?)

[PlicketyCat]

Did a little more digging and found a general rule of thumb for rafter sizing in reciprocal framing on Simon Dale's site:

"...rafter diameter should be no less than 1" for every 5' of perimeter diameter unless span between rafters at the butt end is less than 4'... "  This assumes hardwood rafters with no support mid-span.

In our case, that would equate to 8"d rafters for a 40'd home, but would require 26 rafters to keep them within a 4' span at the butt end.  Or, more likely, 13 full-length rafters and 13 half-length rafters (spanning from the outer wall to the mid-span purlin plate) since the 4' diameter of the center occulus results in a circumference that can't accomodate 26 rafters 8" in diameter butting into it. The span between the full-length rafters should have tapered to within 4' at the mid-span purlin plate, so extending the rafters beyond this point shouldn't be necessary (I think?)

So, if we assume that we need to beef up the rafters a little because spruce is softwood, we can safely estimate that our principal rafters should be no less than 9" diameter... and our secondary rafters, if half-length, should be the same diameter... or could be lesser diameter if they are full-length (??).  Did I wrap my brain around that right?

So then, the question remains, can we rest the secondary rafters on the purlin plate if the butt end rests on a post on the outer wall, or do they need their own post? And does this result in roof loads that are too high and/or would require huge posts to support?

[Gary_C]

A very important question you should ask yourself at this point is this. Is this reciprocating roof a good choice for high snow load country? From what I know of roof design, low pitched roofs of 3-4/12 are not the best idea in snow country.

I've never seen that style of roof before, but my first impression is that is a low pitched indian tepee and it will be a difficult, perhaps risky, and time consuming design. After reading some of your excellent blog, I wonder if sooner and safer would be preferred over wishes and desires. If you are set on the round shape of construction, would a geodesic dome be a better choice? Or perhaps a geodesic dome for a quicker and more permanent structure while you build your dream home? Later the dome could be converted into a garage or warm work space.

Don't want to be negative at all, but now seems to be a good time to ask those questions. 

[PlicketyCat]

All excellent questions Gary! I've been batting these things around myself... here are some alternatives:

1) If we simply framed the occulus and butted the rafters straight to it, we could increase the pitch for better snow load.
2) We can sacrifice the occulus and build a tridecagon roof in a cathedral or gazebo style. We'd then have to add dormers or several small portal skylights to acheive the daylighting we lose when we omit the occulus.
3) We give up all pretense of organic form and plant a square hip roof onto a cylindrical house and do the same dormer or skylight thing as #2
4) We design a geodesic dome roof to plop on our cylindrical house... but I've never seen a round-timber geodesic design, so I don't know how that would work

Any increase in pitch and we lose the green roof without additional battening and/or terracing. But, it would be excellent for thatching if I can source the proper materials and fly someone over from Europe to teach me how   Or I can get froggy and do it all in spruce shakes. Of course, I could capitulate on organic roofing in total and go with metal (it's relatively cheap and fire retardant).

We absolutely do not want a geodesic dome house... we want a round permieter, but like plumb walls. Dome roof good, dome walls bad. In any case, our roof would likely be more conical than dome anyway since we don't want to mess with bending members.

[moonhill]

How about a cable around the outside of the wall, like the tension band on a barrel and on Yurts?

Tim

[pineywoods]

I'd pretty much agree with Gary_C. I'd take a hard look at geodesic domes or some variation thereof.
They look complex, but really aren't. Only 2 sizes of timbers in the whole thing.  Round timbers would be a problem. You have a chainsaw, chainsaw mills are cheap and portable, and they make 2X6's real easy.   a pile of 2X6's and a compound miter saw, and you could frame up a 30 ft dome in just a few days. Make one heck of a workshop/garage. During the cold dark winter, get yourself a pile of sticks and a bottle of super-glue and build some models. Many years ago, I built a 3 ft diameter dome out of popcicle sticks. It supported a 100 pound load and nothing broke.

[PlicketyCat]

We have considered a ring beam and/or tensioning cables, those might still work. We've got a guy who does custom metal fabrication, so that might be an option. I'll need to make some scale models to see if it really works ;)

Geodesic isn't entirely complicated, and we might be able to just do the roof. Again, time for more models. Once I get SketchUp behaving properly I can fiddle in the ether and then settle on a few that I want to make with toothpicks and popsicle sticks. Not a lot of crafting space here in our little wall tent ;)

[PlicketyCat]

Rooster mentioned trying a cupola instead so I can increase the pitch without losing my daylight. So I drew up a few plans & views in Illustrator (I'll do a 3D in SketchUp at some point).

Specs: 40' diameter outer wall, 20' diameter inner ring, 10' diameter cupola.  New pitch closer to 8/12. Min snow load 50psi. Spruce unsawn timber. Elevated pier foundation. Walls - 2' straw bale & plaster. Main floor wall height +/- 8', Loft floor wall wall height +/- 5'. Central atrium is open floor to roof.

Omitted to increase pitch: reciprocal roof rafters & green roof system.









Principals drawn only, I'll probably need additional rafters & beams, and (of course) joists and purlins.

So, it's sort of a hammer beam, except we have inner posts. And it's sort of an arch brace/sling brace hybrid, but with the cupola ring replacing the collar since we're cutting off the peak.

Questions? Comments? Helpful tips? Will this work or will something collapse, cave in or blow out?

Edited to add: Ok - just noticed that the scale on my my cross- section is off...  it's drawn 10'-10-'10' when it's really 10'-20'-10'... so the cupola would be bigger too and not look so dorky. (it's late/early)

[moonhill]

http://greenbuildingelements.com/2008/10/01/the-reciprocal-roof-beauty-strength-and-simplicity-in-a-roof-frame/

Is this what your roof may look like or was going to look like?  With the internal support system I could see a lower pitch and a live roof, I would peel the the logs unlike the last photo in the link.  There will be a lot of bugs dropping sawdust in his morning tea.

A recent article in the Timber Framers Guild publication Timber Framing had a very similar example of a round barn.  I have loaned out my copy, perhaps someone else will insert more detail. 

I am curious if you will be curving the brace stock as depicted in the sketch?  The brace length looks short, how long are the braces?  The longer the better.   

What is the size of the post stock? 

Building in the round is more challenging than typical, but it seem as if the two of you are up for that.

You may have answered this already, with straw bale construction is it applied to the outside of the frame and what does the foundation look like to support the bale wall?  You have mentioned permafrost, so an insulated floor with plenty of air movement underneath?

Have you though about hewing some of the stock, no mill needed?
   
Tim

[PlicketyCat]

That's exactly the roof system I was talking about. If you look at the example on the bottom, you can see that he has the outer ring of posts, and an inner ring of posts with a plate... exactly what we're trying to do.  If you look at Simon Dale's site, he's used whole trees in the center and built a loft inside those... as opposed to a ring loft on the outside like we're trying to do.  Both Simon and Tony Wrench built the ultra-rustic way, they didn't peel their logs and nothing is of a truly consistent measure.  That's a little extreme for me.  If you check out The Pavillion, they have all the math-y technical stuff explaining how the roof supports itself and how it's stronger than it looks.

The reciprocating roof isn't entirely off the table. If we can support the ring with braces to the inner posts, and we frame the inner ring and cut the rafters to butt into the ring (rather than rely on their overlapping) we should be able to increase the pitch. Eliminating the green roof halves the dead load, so it wouldn't have to be as beefy.  Sacrificing the green roof would allow us to put a water catchment system in, so that's not a horrible sacrifice.

Rooster sent me to an off-site gallery where he and his crew restore old barns. He had a lovely example of a large round barn with a dome/cone roof and cupola. I've checked out lots of round TF barns since then and think we might be able to get what we're looking for with a little more fiddling.

We've got stock running from 6" up 24" on our property. Once we settle on the design, we can do the math and then go fell the right timbers. The trees here are really straight and the big ones have minimal taper for at least 20'.  Not opposed to beefing up the braces as long as we dont end up blocking light, air, or look like we live in a bird cage.  Hubby has mentioned pre-stressing members.... but he's a techy guy, don't know if I want to attempt that. An alternative could be to build a sort-of cruck truss for the second story rather than taking the posts straight up.

Building round will be a challenge, but more organic. Just trying for that "minimally processed" look. We do plan to flatten the decking facies and may also square at the notches rather than completely full-scribe if that doesn't weaken the structure. It's pretty easy to square the timber if we get too frustrated.

There will be additional posts directly under the outer edge of the bales to support their weight and provide a ledger for the deck when we add it. Yes, thick floor, lots of insulation and up at least 24" to keep from thawing the permafrost.