Spline joinery at building corner

[Griffon]

This is a development of an un-responded inquiry I posted on the TFG forum.

I'm researching tension in corner joints for douglas posts and reviewing proposed solutions. Concern centres over:

• relish remaining in tenon after peg (NDS 7 x diameter )
• poor tensile quality of douglas perpendicular to the grain
• beam shrinks and left suspended above seating, held by peg(s)

For the case of same-level beams crossing a post, a long spline solves these problems elegantly, with ample space and relish available for 3 pegs in line for each beam.

For a corner joint, the answer is not so evident, especially where exterior wall depth (thickness) restrains relish. A spline with keys outside the post offers the solution if sufficient wall depth (or room space) is available (7"). Otherwise, a peg in the post is the solution; the limit here is in the number of pegs usable. The beam connection may still take three pegs, but of what use when an 8" post may only take one at center, in consideration of perpendicular tension on post wood and spline relish to exterior (with eg. 3" spline protrusion for a 8" post).

Now, I'm aware that modern building reglamentation has pulled many away from traditional solutions, so I feel no shame in proposing another: 4x6 glued blocks on the spline tail. Though not as compact as a Timlock this would beat the expense and otherwise rely on wood rather than metal. It offers more strength than a single peg in post, and no problem with beam un-seating.




I do not have great experience with glue-ing wood, but others assure me that a correctly made glued joint, when put under stress, would first break wood fibres rather than at the glued joint interface. The same item could be solid machined from one lump of wood, but glue-ing offers economy and plywood-like cross-grain lamination. The stopper would be hidden in available wall space. Does anyone know of strength ratings for glued joints (proportional to surface area) ?

Is all this worthy of consideration? Apart from NDS rules, has anyone witnessed beam un-seating or joint failure in a real timber frame structure?

[Raphael]

  How about a hardwood wedge through the spline rather than the glued on blocks, like the wedges in the through tennons of a classic Dutch barn.  That way when the post shrinks you can tighten the joinery.

  I know of several people who recomend the wedged half dovetail in the situation you are describing to alleviate the worries of relish failure in a pegged tennon and there is no question of the tennon coming up off it's seat.  This would work well with Doug fir.

  In my Sobon frame the posts are through morticed so the tennons off the beam have plenty of relish for the recomended 13/16" pegs and just enough for the 7/8" alternative pegs.  I suppose a hint of down pressure in the draw bore wouldn't hurt.

[Griffon]

I had noted the wedge (keyed) methods, and I do like the half dovetail, but neither are valid unless mallet access remains. Likely by the time our slow progress has completed, all the shrinking will have done anyway  ;D

Yes, you do have a point in respect of wood shrinkage; even so, using a wedge method, supplementation with glued blocks could allow for a shorter, stronger protrusion. Lee

[Don P]

Just as an aside, the 7Diameters or any of the diameter requirements for edge and end distance are the number of diameters that a steel fastener carrying the same load would require, not 7 hardwood peg diameters.