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  #1  
Old 05-20-2003, 02:15 AM
Alan Mackintosh Alan Mackintosh is offline
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Anyone built a Reciprocal roof??

IS there anybody out there who has built a reciprocal roof. The span /diameter of the roof would be about 5m/16'. Any pointers from the professionals would be welcome.
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Old 05-20-2003, 11:30 AM
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Google turned up this link for a octagon pavilion design which uses reciprical rafters.
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Old 05-21-2003, 08:53 AM
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Thumbs up Square Reciprocal.........

Many years ago, we built a log home where the 4 ridge logs were offset by 6' and valleys came out of the 90 degree angle...sort of a pin wheel effect. The idea was to create a 6' square at the top in which to put a nice skylight. It worked and we got a wonderful "light well".
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Old 05-21-2003, 04:53 PM
Alan Mackintosh Alan Mackintosh is offline
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Thanks Blue devil and Greg.

Blue, the link you attached is what I was looking for. A way of figuring out the depth of the beams and the required notches. I like the idea of this type of roof which holds itself up. An eminently sensible thing for a roof to do, wouldn't you say?
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Old 05-21-2003, 07:41 PM
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An elegant roof indeed..

Yes, that octagon pavilion is neat, but look carefully at the construction of the posts and perimeter connections....all steel. Plus the posts are bolted to a concrete foundation. For a large habitable structure completely out of logs, I would want to see some serious engineering on those connectors. Just my 2 cents.
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Old 05-22-2003, 02:58 AM
Alan Mackintosh Alan Mackintosh is offline
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I agree about the need for robust construction. All the roof thrust is being pushed out to the walls. I imagine that a log wall is well suited to resist this thrust as long as the top round has deep square notches all the way round, and the beam intersections are well jointed so that the beams do not want to unwind. I suppose a good rule is to make it twice as strong as it needs to be.
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Old 05-22-2003, 07:17 AM
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Alan and Greg, I just may try that type of roof.........Not so sure that there is any thrust to the walls..........The
interlocking connections would act as collars???? Going to work to discuss with crew as we have a tower roof to build.........
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Old 05-22-2003, 09:01 AM
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Thrust

Quote:
Not so sure that there is any thrust to the walls.
Seems to me , Tim, that the larger the 'aperture" (the center opening) the less thrust. Look at the 2 extremes. If the aperture was zero, it would be a regular rafter system from the center with no center support. Therefore the thrust would be maximum. If the members were clear out at the perimeter, the 'aperture' would be at maximum and the thrust would be almost zero. (the rafters are just stacked on the posts, one after another. So, the project as presented would be somewhere in the middle, probably proportional to the distance from the center or perimeter.

Now I agree if the connections were strong enough, it could be fashioned with very low side thrust, but I think we are looking at steel and bolts to get that kind of shear. Just my 3 cents.
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Old 05-22-2003, 12:17 PM
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Greg
I have a pdf. picture of that pavilion ceiling.No metal connections are visible.It's possible they could have pinned them but there's no mention of that on the site.The other reciprocal roofs I looked at,were very primitive,useing pole rafters lashed together .If you saw the walls in these structures,you would laugh, actually scared me! So I don't think lateral thrust is an issue!

Alan.
Thats about all the constuction info I could find (most of the links are toast) but here's an interesting site to surf.
Ceiling details Also check out the sandstone pavers he used for floors.Cool!
The pavilion people say you can contact them if you have any questions.It is the same size as the structure you want to build so you may be able to use thier design.These do make beautiful ceilings!
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Last edited by blue eyed devil : 05-22-2003 at 12:22 PM.
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Old 05-22-2003, 04:30 PM
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Greg, We are going to put this baby to rest by building a scale model and jumping on it........LOL
Actually, We will load it and see what happens.........Will let you know!!!!!!!
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Old 05-23-2003, 03:34 AM
Fergus Fergus is offline
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Hi - I built the roof shown in the link - we had the beams (11" X 3" laminated Australian hardwoods which are very dense) up for 3 days with no fixings top or bottom. The weight of the members kept them secure which is remarkable in that all the bearing surfaces were parallel to the pitch of the beams so they could slide along their main axis.

My conclusion is there is absolutely zero lateral thrust - although the council insisted on (IMHO useless) ties at the apex.

The roof is like a falling structure suspended in midair - the harder it tries to fall the more 'locked-together' the beams become.

If you are building one I found the main trick is to get the first beam positioned precisely - higher is much, much better than lower.

Cheers
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Old 05-23-2003, 05:32 AM
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Fergus,

I really like the look of that roof! If I were in the room I would be laying on the floor looking up saying - "now, how does this work?"

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Old 05-23-2003, 06:40 PM
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Quote:
I would be laying on the floor looking up saying - "now, how does this work?"
Just to lie underneath the structure requires faith that it does work (-;

My understanding - no doubt simplistic - is:-

The outer ends of the beams can't fall as they are supported by the wall.

Therefore all the weight is acting to pull the inner end down.

Load on any individual member is transferred to the next by virtue of the fact that its inner end is resting on its neighbour.....and so on round the roof until the argument arrives back at the first beam - which is still supported by its neighbour - - which is still supported by its neighbour - which is still supported by its neighbour .... you get the picture I'm sure.

The only option for a beam to get closer to earth is to compress and the more it compresses the harder it resists further compression and the harder they "lock together".

Its a challenge to describe - possibly it is the ultimate demonstration of team-work or true community in action: all members are equally important and contribute equally to the success of the structure.

Unlike a conventional rectilinear rafter/ purlin arrangement it has no movement whatsoever and requires no bracing.

It does demand much more labour to sheet the roof as no two timbers meet at right angles, (throw away the set square and learn to love your sliding bevel). It does have the advantage that once you have figured out how to do one section all the others are the same.

I find it has been worth the extra effort as the ceiling has intrinsic 'movement' for the eyes and many visitors do indeed do just that - lie on the floor and gaze up for lengthy periods without appearing to get bored.
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Old 05-24-2003, 02:31 AM
Alan Mackintosh Alan Mackintosh is offline
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I have found someone over here who specialises in these rooves and was picking his brain a wee bit. Apparently there is no thrust to the walls as once all the roof beams are notched together the roof acts as a single unit, rather than rafters etc all trying to fall away from each other. He said that the biggest one the had done so far was a diameter of 20m. He also mentioned that they had to put in steel fixings to satisfy the lack of faith from the engineer and building control, but that it did not actually require any steel fixings.
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