I've seen a lot of variation in what's been written about and practiced with regard to dovetail layout. Before cutting the pockets in 7x7 tie beams to connect with 4x6 floor joists (2' on center), I thought I'd put it out there to the forum members for input on what I've laid up (see image below). The ties and joists are eastern hemlock, spanning 10'x10' bays, forming the floor of a loft in an equipment shed (loft size of 10'x30').

Some questions that I've been wrestling with:
1)How do you optimize the balance between wood removed from the tie beam (vertical dimension of dovetail pocket) and thickness of the joist at the tenon?
2) What is the minimum acceptable length of dovetail tenon (horizontal dimension - I've set mine at 2.5", leaving 2" between opposing joists.)

Thanks for your thoughts.

Kevin
If you house the entire end of the joist into the beam to 1" deep This will carry the load. Then use a smaller dovetail at the top... say 2" deep to prevent pull-out.
Another way is to have a tenon near the center point(neutral zone) of the beam then peg it.

Thane,

I debated about whether to go with a housed dovetail, but given the smaller size of the ties (7x7), I wasn't sure if I had enough room to include the housing while still having adequate length tenon. I ended up sticking with the un-housed dovetail for simplicity's sake but I still wonder/worry about any weakening of the tie beam. I also debated as to whether or not I should alternate the joists so as not to have them opposing one another, but, after seeing a minimum spec of 2 inches between pockets in a Benson book, I thought I'd be okay. In the end, I drew up 2 1/2 inch long tenons (with a 1 in five angle) that are 3 1/2 inches tall. (If I'd been working with 8x8 tie beams I'd probably have drawn up the pockets at 4 inches deep.)

Perhaps I'm fretting over small differences?

Kevin - am I to understand you are taking ((3.5 inches x ~2.5 inches) x 2)) @ 2' OC out of your tie?

Mike,

That's my current thinking. Any thoughts?

Kevin

What are you basing your timber sizing on? What floor load are you working with?

I know 10' isn't much of a span for the tie... but that is alot of meat being removed.

It will be the floor for a loft that will be used for storage. Don't know for sure what the total load will be - assorted boxed-up household-type items that now sit in a storage unit. The loft is 10'x30' with a 2' knee wall (dropped tie) and a 6:12 roof. The peak of the roof is centered over the loft, making the maximum height in the loft a bit over 5 feet. Loaded up, I'm not sure if that space falls within a typical residential live load floor spec.

Timber sizing was based on formula given in Benson's "Building the Timber Frame House." The tables in the appendix do not list values for eastern hemlock, so I ran the calculations for pine.

The part I'm unsure of is how much the numbers change with the removal of wood for the joist pockets. (I'm assuming that we're primarily dealing with a loss of compressive strength on the top of the beam.) Does the remaining 2" between joist pockets provide ample resistance to compression?

Additionally, the braces are 3.5' from the shoulder of the tie, therefore supporting the tie at approximately equal intervals along its 10' length.

Thanks for any input you can give me on this.

Essentially, the exercise seems to come down to finding the balance between the loss of compressive strength in the tie beams (larger joist pockets) and the loss of shear strength on the joist tenons (smaller tenons).

Here's what I've come up with on the shear strength of the tenons: Assuming I'm doing the math right, a 100 sq ft space with a load of 50 psf (5000 lbs) distributed across 5 joists will result in a load of 500 lbs on each joist tenon. Calculating for horizontal shear (H), I'd divide the cross-section of the joist tenon where it enters the tie (3.5" x 3" = 10.5 sq in) into the reactive force (Rf) of 500 lbs, then multiply times 3/2. This gives a result of 71.43. Referring to a table published by Northeastern Lumber Manufacturing Association (http://www.nelma.org/files/File/Eastern_Hemlock-Balsam_Fir_Table.pdf), the maximum allowable horizontal shear for eastern hemlock is 140. Unless my math is flawed, I'm well within that spec, right?

Kevin-
I just completed a frame with a similar loft-- an 8x8 spanning 10' between queenposts holding 5 4x8x12' joists 28" o.c.
I used very similar proportions to what you describe--
3" long tenons with 1" x4" housed portion and a 2" x3" stub.

Above the neutral plane, everything is in compression, so assuming the 7x7 is capable of bearing the loads with a good margin of safety, I don't see a problem. If this were main tie, I could see getting more picky.

-Devin Smith
Rockingham, VT

Hi everyone

One thing that I would do is after the dovetails are all in place I would wedge around the dovetail ends and sides to ensure that the upper surface cannot sag due to voids that might give in the compression mode.

NH