More of joinery and joints, and the simplest of our intentional intertwinings of wood in the joining of the timber constructions which hold our roofs and roadways up. This being the less than little discussed and oh so under realized – Dap.
And part of that just might maybe be, that a Dap is a joint so simple that the term the uninitiated use, that term most “lay-people” use to describe timber joinery may fit it almost perfectly. A Dap is a but a “notch”, that and that alone, a strait forward and simple notch.
Though in most instances a Dap’s seeming simplicity is multiplied and compounded towards complexity and the solving of structural requirements in multiple ways, (often quite literally, imparting and conveying loads in multiple directions, or providing a load path to turn a corner, the Dap is often the change of direction load path) by being used in reciprocal fashion. Two Daps, each receiving an opposite other, in a sort of simpatico structural handshake.
Though not exclusive to Chord connections, the most common use of Daps in bridge framing is in Post to Chord connections, those very connections which convey loads from one truss panel to the next panel in the chain. In this circumstance they hold a secondary structural bonus in quite literally keeping the joint and all wood to wood contact to an absolute minimum, (typically the depth of reciprocal Daps are dictated by the square inches of interface needed to convey the loads asked of it while avoiding crush in the side grain surfaces within the joint) and simultaneously and to huge advantage maxing air flow around joined timber in the truss-work. This a particularly important advantage at the Bottom Chords where gravity typically draws the errant and occasional leak in a bridges cladding. Minimized contact and maxed airflow help insure a very minor leak, fails to compound into a problem requiring complex repair.
Daps are likewise also occasionally used to convey tensile loads in Counter Braces, both in area variant Long Trusses, and most strikingly in Paddlefords, both examples have been explored and expounded on here on the Bridgewright Blog.
In more typical joined timber constructions simple Daps are and were, at least in my opinion, sadly underused.
Two notable exceptions to this rule are the striking polarities of historic English and French roof framing systems. To somewhat oversimplify and nutshell these, English roof systems often include Common Rafters over Butt Purlins joined to Principal Rafters – The French tradition (likewise only often) holds Purlins over Principal rafters. In both traditions the over-riding pieces join those that carry them with shallow Daps, or occasionally a two tiered variant Dap often known as a Cog & Clasp.
Strangely and almost regrettably, when mother country typologies influenced framing tradition here in New England, (see Overnight Turn on a Paradigm) neither Common Rafter nor Common Purlin roof systems override their support structure with shallow Daps. Both systems are found here in regional pockets, and in both instances, be it Rafters or Purlins both are framed co-planer with the Principal Rafters or Purlins which support them. And this is where the regrettably plays in, these framing traditions were explored in a number of books which have heavily influenced the timber framing revival, and these have in turn driven a predilection for co-planer framing in our revival.
This is almost paradoxical in that designing timberframe constructions with secondary systems overriding primary carriers, be they Rafters and Purlins or Tie Beams and Summer Beams, helps to avoid the three and four way connections many work to sidestep as we design. And it does so with architectural elegance. Beyond that framing with overriding, and thusly continuous Purlins and Joists always also makes for more robust and stable frames – And is this not what we all work to do as we join timber?