Joinery question - dovetail or soffit tenon - girder to sill

[TFnovice]

I am partway through my first TF shed following Will Beemer's book and referencing both Jack Sobon and Steve Chappell books. I have enlarged the frame to 16x20 and therefore have inserted a girder as per Dave Sheppard's drawing below from a different post. I have been searching the net, FF and re-reading the books to answer the following question:  What type of joinery should I use to connect the 8"x10" girder to the 20' sills? 

Initially, I was thinking of a housed lapped dovetail (pg. 49 Sobon) because it will help tie the two long sills together and the housing on this joint removes minimal wood to preserve the strength of the sill and give lots of support for the girder. I have also read that a soffit tenon joint could work, however, my concern was only the peg would be acting to stop the long sills from potentially bowing outwards. I am using ponderosa pine for the timbers. Any thoughts on the joinery would be much appreciated and any suggestions on how to cut the joinery would also be a welcome bonus. Thank you very much.

[Dave Shepard]

Just a mortise and tenon joint. Here's my five second sketch: 



 

[TFnovice]

Wow, thank you very much Dave for the quick response. This is going to save me a lot of work! 

[everythingwood]

I agree with Dave on the mortice and tenon but I would chamfer the bottom of the girder and house the joint... otherwise all of the weight will be concentrated on the tenon alone.

[TFnovice]

Thank you for the added advice and suggestion about the weight "Everythingwood". The weight  was a concern of mine as well, however, me being a novice I am looking for a few other opinions. Thanks again to all.

[Don P]

I would support the sill and girder. I suspect it would be in distress without both supported. I remembered some notching stuff from the NDS in my gallery;


 

[Jim_Rogers]

I am as well concerned about the tenon in Dave's picture holding all the weight.
What we usually do with a "tying" joint or joist is to lower the tenon so that it is stronger and won't split the timber that the tenon is on the end of.
And by cutting a scoop on the bottom surface, eliminate the point of fracture.
Like this:


 

Jim Rogers

PS. if you house the girt, then you have to take into consideration that the timber is going to shrink vertically and that would again put pressure on the tenon.
In that case you compensate for the estimated shrinkage by making the tenon sloppy in the mortise. not much but the same amount as the shrinkage.
So, when the timber dries and shrinks it will settle down and if it does it will not split the timber at the bottom edge of the tenon.

[Dave Shepard]

It depends on whether the center sill is supported or not. 

Both sills will shrink, so if it's housed, both the housing and the end of the center sill will shrink equally.

[TFnovice]

Thank you to everyone who has put themselves out there and offered advice and pictures/diagrams. I should have said in my original question yes both the girder and the Sills will be supported. You have all given me confidence to move forward. Thank you very much this forum is wonderful!