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Advice on cloud lift arbor beam

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Forum topic by Vate posted 10-18-2012 11:38 PM 1287 views 1 time favorited 9 replies Add to Favorites Watch
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Vate

7 posts in 1420 days


10-18-2012 11:38 PM

Topic tags/keywords: greene arts and crafts arbor question

I’m building tapered columns and an arbor as part of an exterior Arts and Crafts style remodel of our 70’s ranch. The architect’s rendering of the arbor had a straight 6X8 beam going across the columns (picture 1). For both aesthetic and practical reasons, I’ve redesigned the beam to incorporate a Greene-and-Greene style cloud lift (picture 2). This will both raise the arbor higher, and will help me with clearance on the gutters and roof. I’m building the beam out of sapele to match the front door.

Well needless to say it might be hard to find a 24/4 thick slab of sapele so I’m going to build up the beam using thinner pieces. Also, the length of the beam is approximately 15 feet. My thought is that gluing and bolting together these pieces will also make the cutting/shaping easier, and should result in a much more strong/stable final result (full sun and rain exposure).

The third and fourth pictures show my planned beam design and layout. The center beam has the pieces cut a little different so that the “break lines” don’t line up. My plan is to glue the pieces, and to bolt them together as well, plugging the holes with rectangular ebony (or ebony-like) plugs, again Greene and Green style. My thought is that this should provide sufficient strength for the weakest part of this design, where the upper and lower beams come together.

Any feedback, ideas or insight would be appreciated. Thanks.


9 replies so far

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Granddaddy1

181 posts in 860 days


#1 posted 10-19-2012 02:18 AM

Unless your roof structure is cantilever trusses bearing in the wall of the house, you have rafters bearing on this beam. You need an engineer’s design here. In 36 years of design/supply in this field I’ve never seen a structural component like your proposed design. Also, design properties of exotic woods like sapele will not be commonly known to your engineer or public building officials. You might want to consider wrapping an engineered beam (such as lvl) with sapele. Good luck with it!

-- Ron Wilson - maker of fine firewood!

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mbs

1439 posts in 1598 days


#2 posted 10-19-2012 02:23 AM

I like your idea and your plan to do it. Does the entire beam need to be sapele or can you put a less expensive wood in the center?

You may have a hard time getting the beams flat enough for regular glue joint. Consider using tinted epoxy which is strong, has a longer set time and will fill any gaps.

I normally see the lower mating pc to the upper cloud lift. Have you considered incorporating the mating pc below your current pc of sapele? It would basically make the base a little thicker at the post and have a air gap between the two cloud lifts.

I believe the cloud lift form is made with big radii rather that straight intersecting angles.

I noticed some beams (dark grey) that are on the inside of the house. Can you make those cloud lift too?

-- Sorry the reply is so long. I didn't have time to write a short reply.

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Vate

7 posts in 1420 days


#3 posted 10-19-2012 02:59 AM

>>>Unless your roof structure is cantilever trusses bearing in the wall of the house, you have rafters bearing on this beam.<<<
It may not be clear from the Sketchup renderings, but the beam is not supporting any part of the house. It is only supporting itself, and some cross-beams that will also be part of the arbor. The total weight supported by the beam will be the beam itself plus 8 other smaller beams – 200 pounds at most. however, I do like the idea of incorporating an LVL beam both for strength and cost – thanks.

>>>Consider using tinted epoxy <<<
That’s a good idea, especially considering the exposure.

>>>I believe the cloud lift form is made with big radii rather that straight intersecting angles.<<<
Yes, I’m pondering that, and how to best do that. I don’t want big round corners, but I think straight angles will look too harsh.

>>>I normally see the lower mating pc to the upper cloud lift…<<<
I’m sorry, you lost me with that paragraph.

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Vate

7 posts in 1420 days


#4 posted 10-19-2012 03:04 AM

Here’s another view, from above, that may help clarify the design. The lower part of the beam will fit under the soffit of the house, but will not support the house in any way.

View JJohnston's profile

JJohnston

1578 posts in 1949 days


#5 posted 10-19-2012 03:20 AM

My engineering sense (yes, I am) tells me you should take the upper section of the beam all the way to the end, that you shouldn’t notch the end section like that right near the support. This is a “high shear” area, and the end piece seems likely to crack at the corner of the notch. Note that the section of end piece beyond the notch (on which the center section is sitting) is carrying all the weight of the center section (half per end); structurally, the portion of beam above the corner of the notch is doing very little.

Don’t do it like in the center of the 3 laminations, either. It’s the same situation as the outer ones, just upsidedown, and the crack will go toward the center of the span rather than toward the support.

In the last picture (first set), take the blue line to the end of the beam and delete the vertical joint intersecting it. The main section of beam goes all the way to the end, and the narrow lower section will act more like a cosmetic corbel.

-- "Sorry I'm late. Somebody tampered with my brakes." "You should have been early, then."

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MrRon

2835 posts in 1901 days


#6 posted 10-19-2012 10:43 PM

This is how I would make the joint. I would make the sloping surface at least 12”. With a good epoxy, the bolts wouldn’t even be necessary. Have you thought of laminating plywood, at least for the center plies? It’s the tapered edge coming to a feathered point that I don’t like. (no strength). (sketch not to scale)

View Charlie's profile

Charlie

1017 posts in 944 days


#7 posted 10-19-2012 11:39 PM

Cloud lifts were created using arcs and I have never seen one made of pieces like this. If I were doing this I’d either use a larger dimensioned beam and cut away the lower center and upper ends to form the cloud lift, OR… I’d use a straight flitch beam, make the bottom (ends) of the cloud lift design and set those on the columns, the flitch beam on those ends, and then apply the top (center) piece.

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Vate

7 posts in 1420 days


#8 posted 10-21-2012 03:11 PM

Thanks to all for your help – great ideas that are improving this design.

Below is the current design. I’ve radiused the edges to soften the look toward the more traditional cloud lift. I’ve incorporated MrRon’s joint design, along with a couple of floating tenons to help with both strength and positioning.

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mbs

1439 posts in 1598 days


#9 posted 10-24-2012 02:10 AM

Vate, check the gamble house out for some ideas. Here is the link to the living room. You can pan around the room. I like the archway on the left of the picture. Can you do something like that?

-- Sorry the reply is so long. I didn't have time to write a short reply.

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