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Scarf joint advice #32797 02/15/15 03:38 AM
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RunningWaters Offline OP
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Hi folks,

This is my first time posting. I am helping a friend with a building project in Sonora Mexico and need advice about a scarf joint in a ridge beam. My friend has two 4.5"x10" Arizona black walnut beams. They are split from the same 9"x10" timber. My friend wants to splice these two beams together to span 19 feet between adobe end walls. The beam will hold 18 pole rafters and a thatch roof. The pole rafters are between 4" and 6" diameter salt cedars.

Is it possible to do a traditional scarf joint to join the two beams at the center of the span? If so, what type of joint would be best? Generally, what would you recommend for this scenario? Any advice would be greatly appreciated.

Thanks in advance,
David

Re: Scarf joint advice [Re: RunningWaters] #32798 02/15/15 05:00 PM
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Jay White Cloud Offline
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Hi R.W.,

Quote:
Is it possible to do a traditional scarf joint to join the two beams at the center of the span?


Depends...???

There are a number of scarf joints and/or corbeling scenarios that may be possible either in a format from the Iberian Peninsula which has roots in the Spanish and Pueblo areas of this country, or some other vernacular form...like Japanese, which can deal with such a "thin" beam. It is going to be really challenging to suggest any valued assistance without seeing a model of the structure intended and photos of the beams.

4.5" is extremely narrow (too narrow?) for a major ridge beam...

Quote:
If so, what type of joint would be best?


Really can't without photos and more design info like...there are a dozen or so I can think of just off the top of my head but without more information (i.e. photos, blueprints, etc) I can't even begin to make a good offering...

Quote:
Generally, what would you recommend for this scenario?


"Generally," I would suggest a corbel supported scarf, which fits into the design parameters of many Pueblo architectural forms of the region...or something Asian.

So...this would include a larger corbel under the scarf, which in turn is supported by an appropriate sized post. It would also include a corbel generating from the adobe walls as well, supporting the end of you black walnut ridge beam...

Please note! This is a rough guide of basic needs, and not an approved nor engineered perspective. I could probably facilitate something with the material to fit within your described parameters but as I routinely do, I suggest getting help from a PE if you are unable to do the engineering yourself, or have enough experience as a Timberwright to understand your margins of safety in sizing beams and joint design.

Regards,

j

Re: Scarf joint advice [Re: RunningWaters] #32800 02/16/15 02:04 PM
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Gabel Offline
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Hi David

The traditional solution for when you don't have a long enough ridge is to place a post or some other support at the joint. You could use a truss to hold the two ridge pieces up, but it is a lot more work to make a truss than a post. Of course, posts in the middle of a room aren't always an acceptable solution.

If you do use a post to support the joint, a simple and strong approach is a corbeled butt joint, which is a variant of what Jay was mentioning in his post. Y-- place a samson block or corbel about 4' long on top of the post and underneath the ridge. Simply butt the ridge pieces to each other and secure with adequate fasteners.

Generally speaking, I would not splice a structural ridge in my own house, so I can't really recommend someone else to do that.

Also, like Jay, I suspect the ridge is quite undersized as a 4.5x10.

Another approach might be to skip the ridge altogether and tie the feet of each rafter pair together with a third pole. If done properly that would eliminate thrust on the walls. Now if the roof pitch is below 25 degrees or so, I would stick to the ridge idea since the thrust increases as the roof pitch gets shallower.

(insert the usual disclaimer about incomplete information and how I am not an engineer, and you shouldn't trust structural advice from internet strangers, etc)

Best of luck, sounds like an interesting project.

Re: Scarf joint advice [Re: RunningWaters] #32807 02/20/15 03:59 AM
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RunningWaters Offline OP
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Thank you both for your responses. I had the feeling that scarf joint would not be able to take the bending stresses in the ridge beam.

It is good to know that the traditional solution is a ppost supported corbel. However, we do not want a post in the middle of the room (it's a small one room structure) so the corbel is out.

We are intrigued by the idea of a truss. What might that look like?

We are also thinking about the potential of using metal fasteners to support the joint/splice. Any advice in that direction?

Rafters ties won't work because the side walls are about 5' high. 53 degree roof pitch. What about collar ties?

As for beam sizing, the roof will be quite light. 5" round salt cedar rafters supporting a thatch roof and no snow load.

I am on limited internet so can't upload photos or drawings but will try to soon.

Re: Scarf joint advice [Re: RunningWaters] #32808 02/20/15 04:34 AM
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Jay White Cloud Offline
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Hello David,

There are a number of "truss types" that could be applicable to your scenario...yet without a rough plan or even basic plan view/elevation view concept it is difficult to impossible to suggest one.

Metal fasteners changes nothing...and has no effect, for the most part...in strengthening a splice to the point of not still requiring other support. Now, with the correct adhesives and scarf joint you may improve strength considerably yet that does not change the "thinness" of the beam which is concerning without proper design. I can think of a few Asian systems that have such thinness. "Tidewater Capes" and few other timber frame styles also have beams in this size range..

If this is a 53° pitch that suggest a "3,4,5" classic roof style pitch or what is called 合掌 (Gassho) or "praying hands" pitch. If the walls are not already built (or even if they are add the height needed) I would suggest rafter ties anyway so there is better utilization of space even if only a half loft. "Collar Ties" from a structural perspective do little for a frames strength, and in most high tectonic or wind events...can weaken the roof by causing a "hinge effect."

Hmmm....I am not sure how a conclusion of 'roof weight' was reached without some actual engineering done? You have live and dead load to consider, as well as, dynamic wind loads. Thatch is not "lightweight" when done well in most traditions, unless this one of the 'tropical' modalities in hanging the thatch.

There is much to consider on this project, even if it is diminutive in size. From proper rafter spacing to actual design overall. This further suggest guidance and submitting an actual drawing here for feedback. If you can't load drawings here, post a link to them...

Regards,

j

Re: Scarf joint advice [Re: Jay White Cloud] #32824 02/28/15 10:52 PM
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SBE Builders Offline
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Jay, that's neat information about the 53° roof pitch slope.
Got any more information on the other roof slope angles?

I had to open this html tag up in an HTML editor to see what it looked like.

合掌

Sloping roof with praying hands symbol?

Sim

Last edited by SBE Builders; 02/28/15 10:55 PM.
Re: Scarf joint advice [Re: RunningWaters] #32825 03/01/15 04:04 AM
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Jay White Cloud Offline
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Hi SBE,

Ya...I am sorry about that...our forum here at the TFG currently is a tad "wanting" in ease of use and properly displaying "html" text like Japanese and related Kanji. If you take the number expression and the word (Gassho) together and do a google search, you will get the kanji and a lot of image examples. Send me an email at:

tosatomo@gmail.com and we can go into all kinds of details.

Regards,

j


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