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Dunceath Observatory - an Explora-Dome build

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#26 HunterofPhotons

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Posted 01 January 2014 - 12:36 PM

....The Explora-Dome instructions call for 2X10s for the main cross members, doubled in the case of the full length ones (for snow loading), and side braces cut from 2X10s. First, we don't know what snow is down here (not quite true, but close). Second, I think 2X10s are over kill for the support of a 180# dome. The roof of my home has 2X6 rafters on 16" centers, and the spans are well over 10'. It has survived multiple hurricanes with NO damage. I did use 2X10s for the two full length cross members, but undercut them in the middle portion to essentially 2X6 dimensions....


The two full length 'cross members' are essentially taking the full weight of the roof. Cutting into the 2 X 10s reduces the load carrying capacity to that of a 2 X 6, not a 2 X 10. For all practical purposes your roof load is being taken by two single 2 X 6s, quite a reduction from two doubled 2 X 10s.

dan k.

P.S.
Plywood is traditionally laid perpendicular to supporting joists/rafters to maximize stiffness.

#27 Jack Morris

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Posted 01 January 2014 - 01:34 PM

dan k.

Not true by a long shot! The 2X6 side braces every 14-1/2", assembled with 4" deck screws plus steel angle brackets, are capable of carrying the full load directly to the stud walls! That coupled with the diagonal 2X6s which form the octagon make this a VERY stiff structure in all directions.

#28 Midnight Dan

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Posted 01 January 2014 - 07:08 PM

The 2X6 side braces every 14-1/2", assembled with 4" deck screws plus steel angle brackets, are capable of carrying the full load directly to the stud walls!


Well, that's not completely accurate. The side brace are only supporting one side of the main beams, and are at a shallow angle, so they'll only transfer a portion of the load. The rest goes downward on the beam which tends to push it away from the side braces, towards the middle of the dome.

On the other hand, there are a large number of side braces which, along with the angle brackets, will tend to contain the side forces to a large degree. The secondary cross members also capture the primary ones to keep them from moving sideways, at least near the ends. And the fact that you have essentially no snow load helps a lot.

I think you're ok, but I don't think I would have done it that way up here in the snowy north. Just my 2 cents. :grin:

-Dan

#29 Jack Morris

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Posted 02 January 2014 - 10:24 AM

Hi Dan,
I'm sorry I didn't take the time to explain myself better. It is correct that when the main cross members are first placed on the walls, they have nothing for support but themselves, and adding the shallow angle brackets would have essentially no effect. Adding the orthogonal (shorter) cross braces helps some, but not nearly enough. However, when you complete the octagon you now have a very strong three dimensional structure. It certainly would help if there were more angle on the side braces, and I considered that (shorten the walls and steepen the roof), but then the Explora-Dome roof panels wouldn't work. The principal here is the same that has been used for centuries to build the magnificent domed structures so common in the eastern Mediterranean and Middle East regions. They achieve enormous open spaces with relatively light roof structure. This is due to what I call the "egg shell" effect. Any sector of the dome, taken alone, would collapse under its own weight (just as half an arched stone bridge would) if to couldn't "lean" on the other part(s). I even considered a round 10' diameter building with a, perhaps 30 degree, conical roof at one point. I think that would be cute, and functional, but in the end elected to go with the simplicity of the Explora-Dome panels. I hope this helps put this one to bed.

This will probably be my last post for a couple of days. The weather prognosticators are promising me a couple of clear nights, for a change. We'll see.

#30 Midnight Dan

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Posted 02 January 2014 - 11:26 AM

However, when you complete the octagon you now have a very strong three dimensional structure.


Ah, good point! I didn't notice the corner braces that complete the octagon and transfer inward load from any one side into outward load on the two adjoining sides.

The reduced beam thickness will result in less of the load going vertically straight down from the beam on to the wall, and more of it being transferred into an overall outward load on the walls. But the outer ring of side braces and beams will act a bit like an I-beam shape to resist those forces. So yeah, it should be pretty solid. :waytogo:

-Dan

#31 lunar

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Posted 02 January 2014 - 03:46 PM

Nice observatory!

#32 Jack Morris

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Posted 07 January 2014 - 08:56 PM

The door is a standard 36" X 6' 8" steel clad, foam core, utility exterior door from Home Depot cut down to fit the 49-1/2" opening available. I can provide details if anyone is interested. Incidentally, the work table in "The Office" (first post) is half of the top of the door, painted to match the pier extension.

When the dome and other components arrived, the first order of business was to install the aluminum "support ring" and the support and guide rollers (commercially available inline skate or roller blade wheels) in the pre-drilled holes. Putting the support ring in place is an easy two man operation. The support ring is supported by eight "L" brackets which rest on the eight sides of the wood support octagon. These were bolted to the support ring, but were not fastened to the support structure until after the roof panels were installed to allow the roof panels to "position" the support ring.

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#33 Jack Morris

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Posted 08 January 2014 - 04:54 AM

I made a last minute decision to purchase the gear, gear track and motor mount for future automation of dome rotation. It turns out that this was a good decision, it would be difficult to install the track later. As a result of the late decision, the parts were shipped loose so we had to install the track before the "wheel ring" could be installed. The track is mounted on the top of the inside edge of the wheel ring with 10-32X2" SS screws. This required drilling and tapping MANY holes around the top of the aluminum wheel ring, not difficult, but TEDIOUS. Order early and Polydome will do it for you!

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#34 Jack Morris

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Posted 08 January 2014 - 05:00 AM

Here is a shot of the wheel ring (and dome) installed, showing the location of the gear track and motor mount installed. The motor mount is on the north axis. I don't know if that is the best place, but it can be moved if necessary.

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#35 Jack Morris

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Posted 08 January 2014 - 05:24 AM

I got ahead of myself :o! Of course, the roof panels were installed earlier. This went quickly (too quickly as you will see later :foreheadslap:), so I didn't get any pictures. Because of the ridges on the R panel siding, the bottom edge of the roof panels (the part that turns back to the siding) had to be trimmed back almost flush with the inside of the vertical portion, about 1-1/4". The Explora-Dome instructions call for a 1/4" spacer to be attached to the outside 2X3 before installing the panels. I just used the cut off pieces for the spacers. Also, the outside seal tape was placed at the top of the siding. This makes a tight seal around the siding, under the lip of the roof panels. As noted above the roof panels went on too fast. If you go back and look at the support plan, there is a note to relieve the top of the outside 2X6, 2X3 and center support to accommodate the overlap in the panels. I forgot to do that :tonofbricks:! I will come back to that later.

The next step was to lift the dome into place. The tractor and loader turned a 3-4 man STRETCH into an easy two man job. :cool:

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#36 MHamburg

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Posted 08 January 2014 - 11:28 AM

That's really coming along nicely, Jack.
Michael

#37 csa/montana

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Posted 08 January 2014 - 12:13 PM

Fantastic build, Jack! Looks like awesome skies there for you to really enjoy your new observatory!

#38 Jack Morris

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Posted 08 January 2014 - 01:20 PM

"Looks like awesome skies there for you to really enjoy your new observatory!"

Funny you should say that, Carol.

First light was Thanksgiving eve (it wasn't finished, but I couldn't wait), and that and the next two nights were glorious. Since then it has been, "Curse you southern branch of the jet stream!!". There has been a steady flow of relatively warm, moist air flowing across northern Mexico, the NW Gulf and right over us on it's way to the NE. We have averaged about one relatively clear night every ten days to two weeks, and the transparency or seeing, or both, have been very poor. This last blast of frigid air from the far nauwth has finally cleared it out, temporarily at least, but it has been FRIGID (for us). Late fall and winter are usually my favorite observing (summer is often not good due to the humidity), but this year it practically ended the day after Thanksgiving.

Wishing you the clearest of skies and the darkest of nights,
Jack

#39 Jack Morris

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Posted 16 July 2014 - 05:13 AM

Sorry folks! Personal tragedy interrupted my life in January, and I stopped posting and, for the most part, observing. But I will try to finish the story now.

As I mentioned earlier, I got ahead of myself and installed the roof panels before notching the roof supports at the center of the walls to accommodate the overlapping roof panels. If you look back at some of the building pictures you will be able to see the hump in the roof edge at the center of each side. Here is a closeup of one side.

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#40 Jack Morris

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Posted 16 July 2014 - 05:24 AM

At first, I just let it go. However, my perfectionist nature ultimately got to me, and I decided to go back fix the problem. This was done by removing the roof screws along one side at a time, and using wedges to lift the center part of the roof enough to work on the support structure.

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#41 Jack Morris

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Posted 16 July 2014 - 05:32 AM

The center roof support was removed and 1/4" of its top was trimmed off. A reciprocating saw was used to notch the outer support and spacer in place, and the center support was then replaced. The wedges were removed and the roof screws were replaced. Here is the finished product:

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#42 Jack Morris

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Posted 16 July 2014 - 05:43 AM

And here is what it looks like underneath (note: there is a turned down lip on the lower roof panel that was trimmed as well):

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#43 Jack Morris

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Posted 18 July 2014 - 06:57 AM

The next problem I encountered was that the dome was seriously out of round (>1 1/2"). This would mean that the brush seal, which I had purchased from Explora-Dome, would not work if installed as instructed - mounted on the edge of the retaining ring facing inward so that the brush contacts the "collar" of the roof panels. I discussed this with Dan at E-D and he suggested two possible solutions: 1) force the skirt of the dome round and bolt it to the wheel ring, but he warned me to bolt only where the dome was in natural contact with the ring or the dome would warp the ring, OR 2) measure the gap between the dome skirt and the collar of the roof all the way around and vary the mounting extension of the brush seal accordingly. I decided to go with the first option. After further examining the gap between the dome and the wheel ring I found that they were only touching in three locations (no surprise there) and two of the locations were very close together with the third on the opposite side. I did not think that it would be possible to force the dome round using just those three locations. Therefore, I carefully measured the gap between dome and wheel ring at four orthogonal points: front (center of shutter opening), back and both sides, and fashioned, by trial and error, wood shims to fit in the gaps at those points. I then marked the location for each shim on the dome and the wheel ring, and we removed the dome and ring from the building. With the dome up-side-down we placed the ring in the dome, used orthogonal ratchet straps to force the dome round, and centered the wheel ring. We then drilled 3/8" holes through dome and ring, front, back and both sides, and then placed the shims and bolted the wheel ring and dome together.

It had also been noted that the collar itself was not round. The collar is in loose contact with the support ring opposite the corners, but there is a very significant gap where the roof panels overlap. While the dome was off, we used Great Stuff foam sealant to seal the gap between the support ring and the roof collar.

After replacing the dome and wheel ring on the support ring we found it necessary to loosen the bolts and adjust the shims slightly to get the wheel ring to ride evenly on all rollers. We then used Great Stuff to fill the gaps between the dome and the wheel ring all the way around.

After all this we found that the dome skirt was not perfectly round but much closer (<1/2").

In the meantime, I had thought of another way to mount the brush seal which would avoid the problem of the out-of-round roof collar (and also make all the a-fore mentioned hassle with the dome roundness unnecessary javascript:void(0)! This entails mounting the brush on the inner edge of the retaining ring with the brush pointing up so that it rubs against the underside of the support ring:

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#44 Jack Morris

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Posted 18 July 2014 - 02:42 PM

My Bad! The preceding image doesn't accurately depict how high the roof "collar" extends up the support ring. Furthermore, after reexamination it appears that it "might" have been possible to get the retainer ring high enough for the brush seal to rub against the outside of the support ring above the collar, however, in my case the Great Stuff that was used to seal the gap between the collar and support ring bulges enough to be a problem.

In any case what we did works very well and I have found only an occasional dead wasp or lady bug inside the dome. I think those have come in around the shutter, not the skirt. Wasps have tried to nest under the skirt of the dome, but an occasional inspection with Wasp and Hornet Spray in hand has taken care of them.

Now for the mounting of the brush seal. As received, the retaining ring had a rounded edge on the inner surface. To provide a flat surface for mounting the brush, we fashioned a router jig for trimming about 1/8" off the inner edge of the retainer:

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#45 Jack Morris

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Posted 18 July 2014 - 02:54 PM

Three holes were drilled and tapped 10-32 vertically through the level portion of each retainer segment, one at each end and one in the middle. The brush was then mounted with sheet metal screws. Three 10-32 X 2" screws were threaded upward through the threaded holes to 1/4" short of the end of the brush. These were used as gauges when mounting the retainer segments. They were pressed against the bottom of the support ring and held in place while the retainer was screwed to the dome skirt:

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#46 Jack Morris

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Posted 18 July 2014 - 03:00 PM

The gauge screws were removed after the retainer had been attached!






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