I posted this on the Engineering page but maybe I should also post it here.

I am building a 40 x 48 queenpost shop/barn and have been trying to figure out if I should use a dovetail or tusk tenon for the floor joists. I am using doug fir #2 btr heart center for all timbers except the girts which are #1 btr free of heart center (10x10). The floor joists I have are 6x10's. There are four bays and five bents in this frame.

I have read all of the tf books I have (and I think I have all of them) and I am leaning towards the tusk tenon for the floor joists for added strength for the second floor.

The questions I have are:
Since the wood is doug fir vs pine would cutting two dovetails (instead of tusk tenons)in the top of each girt compromise the girt too much.
I realize the tusk tenon adds complexity to the raising which I am ok with but I just read the post on using Timberlinx connectors for connecting the posts to the slab and was planning on using them myself.

My concern is that if I used the tusk tenon for the floor joists as well as the Timberlinx in the slab it might be too difficult (if not impossible) to raise the frame.

My raising expierence is very limited so it might be fine to use the tusk tennon and the Timberlinx slab connectors but I would like some professional opinions if I could get them.

Thanks,
Tyson

I have no experience with Fir, (and very little experience with anything else!), but I'll tell you what I did...

I used dovetails, and kept them short... 2.0". I began using dovetails that were 2.75" long, but then I went to a TimberPeg frame raising in Cinci, OH and saw the dovetails that the CNC hundeggar machine produces - they can't be over 1.5" long. (can anyone else confirm this?) I realized that my girt housings were only 1.25" deep, so why wouldn't 2" dovetail support a floor joist? Sure it will. Everywhere I had girts or rafters that were less than 10" wide, and had joist or purlins coming into them from both sides, I used the 2" dovetail. Elsewhere I used 2.75" dovetails. (I had already made most of my floor joists and purlins at the beginning of the project, so I eventually had to lop off .75" from many of the joists and purlins.)

As they describe in the books, I undercut the male dovetails slightly, and at the raising (my raising is still going on!) I drove dried hardwood wedges on both sides of the dovetail. You would not believe how tight this will pull the joints together... tighter than a 2 ton come-along will pull them... and they stay!!! If you use wedges, be careful when you drive the wedges, as it will pull so tight that it can rack your frame out of square if you haven't pegged everything yet and your girts are a little bowed.

BTW, there is another benefit to keeping your dovetails short - you can use a right hand skil saw and a left hand (worm drive) skill saw to cut the dovetails so long as you have the end of your beam square.

FWIW, I did not use housed dovetails (except for one place in my house), because they add 2x the work for only a small benefit. In fact, for my 1st floor, I omitted the dovetail (simple lap joint) on every 2nd or 3rd joist to save a little more time. I used lap joints in very few places upstairs, since the dovetails really help hold the structure together.

To summarize, my joists and purlins are hardwood 6x8s (mostly oak), spanning 10' to 13', terminated with the dovetails 6" tall and 2.0" (or 2.75") long. I used 6x10 purlins where the span was 15'. The girts, summer beams, and principal rafters are 8x10, 8x12, 10x10, and 10x12 depending on the span and whether joists join from both sides or one side. I would highly recommend using dovetails over tusk tenons - mainly to simplify (or make possible!) the raising.

-Thomas

I will post some dovetail pictures on my blog in the next few hours. (hundeggar dovetails as well as the more traditional ones that I used)

my web page with soon to be added dovetail pictures

Thomas,
Thanks for your reply. I looked at your Blog and you are putting together a very impressive frame. I will continue to watch as it is going to be a beautiful house.

Tyson

I just assembled a bunch of trusses with timber-linx's. I was impressed with the product. The architect originally spec'd steel knife plates and we revised it to a timber linx. With the timber linx we were able to create a very tight joint (much tighter than with the steel plate). Although I would have prefered to stick with mortise and tenon joinery, the engineering was already completed on the project when I got involved.

That said, the timber linx's require very accurate alignment of the bore and cross-bore holes. I would recommend using the timber linx for anchoring an interior post that is exposed on 4 sides (ie: hidden anchorage). For any posts that are against a wall, I would recommend using a simpson style anchor strap on the hidden side. I think you will find the strap cheaper, more forgiving for alignment and easier to deal with during the raising.

Regarding the dovetail joint, bear in mind that if your timbers are green, the dovetail will loosen as the timbers shrink. I don't think a 'hundegger style' dovetail would work with green timber. Besides, I don't think you can hand cut a 'hundegger dovetail'. If you choose not to house your dovetails, you have to make the dovetail very deep (ie: the depth needs to be almost as deep as the joist). Otherwise, the joist may fail in shear at the dovetail. The downside of this is you end up greatly weakening the supporting beam when you cut a dovetail that is almost the same height as the joist especially if you have dovetails on both sides of the supporting beam (girt, summerbeam).