OK, customer wants a traditional crafted frame with no modern metal fasteners, sounds good. You have 8x10 posts, with 8X12 tie beams and 6X10 girts coming in for a three way connection at the post. You want to use a through half dovetail joint on the tie beam, nice.

So, what to do about the girt to post joinery?

Will the 6X10 girts be flush with the outside of the posts (where will the girt be located on the 10" wide post)? Will you have braces for the girts? How do you plan to house the girts? How long are the girts, how many 6X6 joists will there be on the girt, what are the loads on the girt? Do the tops of the 6X6 joists need to be flush with the top of the girt, what joinery is planned here? How about the summer beam joinery to the tie beam? What are the details of the tie beam through half dovetail joint, how much material will be removed from the post?

I would be very reluctant to proceed with a three way joint as suggested, without further information.

Your options are to consider locating the girt to post joinery either above or below the tie beam joinery, or to take advantage of the sheathing as suggested by Mark. However, it sounds like the customer would object to using the sheathing.

So, one option is to locate the girt to post joinery above the tie beam joinery. Probably not a practical approach, it would require several changes, but it might be possible.

Second option is to keep the girt/post/tie/summer/joist design as is but indirectly connect the girt to the post with support framing below the girt. I've seen this accomplished in two different ways.

One method is with a horizontal member below the girt that connects to the post (spline joinery or pegged mortise/tenon) and supports the girt. The horizontal support member can be connected to the girt in a variety of ways, I saw a nice design where the member was keyed to the girt using nicely contrasting wood, similar to what one might see in a scarf joint. This member can be a full length beam or kept short and braced.

The other method is to use a vertical member that attaches to the side of the post, the girt is supported by and attached to the vertical member. The bottom of the vertical member is supported by a sloped shoulder housing in the side of the post, and the vertical member is kept attached to the post using a through free tenon/spline that is wedged/pegged.

--Chip