Wednesday, September 22, 2010

This One Rings a Bell (14)

I wonder how many readers out there, in the segment of the woodworking population who tend to not spend much time in the design phase of a project, who may well be getting restless with this thread? After all, I haven't cut a stick of wood yet. I'm starting to get to that maxed-out point with the drawing myself, after another 20 hours or so of work since the last post. It's getting really close now though, with 95% of the structural parts now in place. A lot of the drawing time at this point is more perspiration than inspiration, I'll say that. SketchUp doesn't always deliver on some of its functions so well when the surfaces which intersect are of complex curvature. I'm doing the best I can.

I thought that this time I would show the structural details in a manner similar to that of the parts being added to the roof. Whaddya say?

Starting with the blank slate, so to speak, of the lower exposed eave, the first move is a set of interior beams, along with a ring of hanegi osae (which serve as fulcrums for the various cantilevers):


Since the previous post, those interior beams have been re-sized and reconfigured a fair bit. They bear more or less directly down upon the principal structural timbers carrying the bell and will be through-bolted to those support timbers.

On now with the ha-ne-gi (桔木), the cantilevers, and a few stub posts sprinkled in here and there:


Configuring the cantilevers is challenging in a small building with a square plan, as the cantilevers all are essentially needing to meet more or less in the middle of the roof and be as long as possible. The solution in this case meant that some of the hanegi had to arrive in the middle at different heights.

Here's a closer view showing how the hanegi come together:


The result of the positioning and re-postioning required to get them into a good arrangement is that four of the pieces are of curved form and will need to come from bowed logs. I've got a source for some excellent material in that regard.

Next layer is the purlins:


With the field hips being irregular, some creative solutions were required to support those hips properly in the roof.

Next, the field rafters:


Here's a look at the asymmetrical positioning of a cluster of purlins around one of those hips- some of the purlins serve partially as cantilevers themselves:


And a view from the other side of the hip:


The purlins I have shaped so as to curve up with the field rafters, and then for the last 50~100 cm they straighten out to run under the hip and lap over one another. That simplifies the connections between the hip and the purlins. Additionally, I strengthened the area generally with the addition of corbels on one side at purlin level 2, and the purlin entering from the opposite side of the hip (which is the 3rd one up on the high pitched side) is at that location sandwiched in between the corbel and the purlin it carries.

Another view of the same corner:


Those stub posts in the second picture of this post support the decorative purlins which carry the verge board assembly:


In front of the central stub post is roof for the assembly which will infill the gable and conceal the posts. As this gable infill is primarily a decorative element and the client has not had an opportunity to view some of the options in that regard, I have left off designing it for the time being.

Another view of the gable:


Last one for today - the planking is in place and the roof form is shown:


All that remains is the minoko, along with the decorative ridge, and a few more interior posts and beams. Those posts and beams will serve to support those purlins, in case you might have been thinking that I was intending to run them across the roof without support. Uh-uh. it will hopefully all make sense when all the parts are in place, so please stay tuned.

All the structural components should be complete and presented in the following post in this thread. Thanks for coming by today. Oh yes, we're nearly there...

--> on to post 15

2 comments:

  1. Chris

    Now I see why the decorative hips are so large: they carry the interior beams! I'm going to have to study this post for a while (there is a lot going on here), but I wanted you to know it has been really exciting to see the bell tower grow bit by bit on line. Thanks.

    Tom

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  2. Hi Tom,

    the decorative hips do not carry the interior beams actually - I'm not sure what you are seeing but that is not how it is arranged. The decorative hips do help carry the eave build up, but that load is equally carried by the fanning cantilevers of which there are four on each side of the building. Those cantilevers are fastened at their upper ends using threaded rod, which is not illustrated, so perhaps that is the source of the mis-apprehension?

    The bulk of the heavy lifting, so to speak, in the roof is done by the cantilevers, with supporting roles taken by the fan rafters, decorative hips and to a small extent by the field hips.

    Glad you are enjoying watching the bell tower come together in the design phase - me too!!


    ~Chris

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