Engineering help needed for a scarf joint

[surrealrealred]

Hello, everyone! This is my very first post here and I wanted to start by saying that your work is inspiring!

I am on a quest to understand how scarf joints work to use them properly. I got confused when I read (a Google translation of) a German blog proposing a counterintuitive orientation of the scarf joint. Here is the post itself:



 

 

 

Now I understand that the behavior of the joint depends on many factors including its configuration, the presence of wedges, pegs, or bolts, and many others. It would be great if there are engineers in the forum that could explain how these types of joints behave. Since the recommendations for the size of the scarf joint and the wedges vary, it would be helpful to understand what is considered when choosing these dimensions. Also, is there software that could run verification calculations on carpentry joints that anyone here is using and is happy with it?

Thanks

PS the German site is baubeaver.com

[Den-Den]

This is not exactly what you asked for but with what others add, perhaps it will be enough.
All scarf joints are far less strong than a continuous beam even with good design and perfect workmanship.  The effects of less than perfect workmanship make the engineering more of an educated guess than a precise value. 
With a specific design, it would be possible to calculate resistance to bending forces based on perfect fit-up and allowable stresses for the wood.  Throw in some tolerances for the joinery and it becomes complicated, choosing what those tolerances should be is educated guessing.

In summary, I doubt that an engineered design for a scarf joint would be an accurate prediction of its strength.

[beenthere]

Placement of the scarf joint is where the moment in bending is near zero.. i.e. not over the top of a post. 

You want it to hold together in tension, albeit not (as mentioned) the same strength in tension as solid wood continuous beam.

[canopy]

That is a simple, yet popular scarf joint. There are more elaborate types. An engineer may suggest a scarf be undersquinted and with V nose so the two members lock securely together in all planes.

[Don P]

Which reintroduces those red split lines.

Why do you feel the need to use one?

Edit: this came in today it is from the AITC, Timber Construction Manual... which is under the APA, the host of this document;
APA Publication Search - APA – The Engineered Wood Association (apawood.org)
See pages 10-12 in this doc, and the manual has more.

[Don P]

Another edit, Hewett's chapter on the evolution of scarfs;
hewett_scarfs_in_evolutionary_sequence_1969.pdf (woodworkinghistory.com)

Why do timber frames fail? (dowsingarchaeology.org.uk)

[Don P]

@surrealrealred this post came to mind... I hope you two are talking.

Continuous Beams - A Structural Analysis in Timber Framing/Log construction (forestryforum.com)

@DangleSnipe267