Category Archives: Historic Fabric

Waxing Philisophical

One Size Fits All – Does this phrase fit any reality anywhere?

From this Preservation Carpenter’s perspective this beyond silly never appropriate, never fits turn of phrase, works no better with Preservation philosophies than it does with articles of clothing.

Circumstance reasonably drives what is appropriate in everything we do and in every choice we make. A circumstance sometimes overlooked in the preservation of historic wooden structures is oddly enough the type of wooden structure being preserved. And I am not speaking to how or when a construction was framed, I am speaking to what its utilitarian purpose was and is, and accordingly, what forces and loads it will be asked to bear as it continues on in time in its intended purpose.

I was reminded of this while recently in attendance of the Timber Framer’s Guild conference in Saratoga Springs New York. This reminder came in the form of a parallel perspective I had somehow never before considered, a perspective driven home to me when sitting in on a presentation Jim Kriker of Rondout Woodworking gave on his ongoing work in preserving the famed Hudson River Sloop Clearwater. ( I happily experienced an in-process tour of the Clearwater several Springs ago while helping Rondout with a mill project  – I’d helped Rondout  with another Up & Down Mill in years past, this go, my involvment was in part to free up a Rondout team member to work on the Sloop in its limited off-season) Something Jim said during his presentation drove home how partial replacement of any section of a wooden boats framing is seldom the choice made, pieces are most often replicated and replaced in total and only occasionally by adding new joinery to the mix. This being, in particular, the case in wooden boats which serve as passenger vessels which are subject to the inspection regime that service demands.

This differs greatly from most preservation efforts where seen as chief among the aims in preserving any given historic construction and returning it to structural soundness is minimizing the amount of “historic fabric” removed to achieve that end. And towards that end, individual framing members often see bad sections removed and these are pieced back together, this often at effort and expense greater than what might be required to replace that individual piece in total. That additional expense is seen as worthwhile in that it keeps the structure as close to its original state as is possible. This both to honor the structure and the people and circumstance which has left it to us.

I have often been involved in such efforts, stitching compromised framing back together in replacing rotted section with intricately fit Dutchman and other joined carpentry repairs, whole replacement ends Scarfed onto sound segments of timber components and even removing rot in hollow sections and replacing such lost section with a matrix of rods and epoxy. All this to both honor the building and the Carpenter whose work left us the building we work to leave to the future.

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Here is seen a small section Dutchman in a compression member / Arch Leaf – One captured by the adjacent Post and its sister Arch and the Bolt that will couple all three pieces – Circumstance used to ensure that bending under load and stress on the bond in this repair is completely contained, this with a measurable expectation that this Arch will convey loads as it always has

In most typical constructions, houses and barn’s – The deciding factor in such approach is one of budget, a building’s significance or how structural repairs might affect a historic buildings value. Seldom if ever is structural purpose and behavior taken into consideration when considering approach and philosophy.

I’ve occasionally seen those who hold “Fabric” as the everything and the end-all measure in any Preservation efforts success, point at wooden truss restorations in contempt for failing to retain potentially savable fabric, or in measuring success or a perceived lack thereof, in percentages. And with the same eye they would measure such an effort on any other type of timber construction with no regard as to how one structurally works and behaves as opposed to the other.

This is neither reasonable nor is it in any way even sensible.

The reason for this is Tension Joinery and the almost complete lack of it in most typical timber-framed constructions. True tensile joints ( those intended to deal with constant tension ) are almost always limited to Tie Beams and are meant to deal with roof thrust. ( thrust that is also typically in part resolved through other means within the framings configuration ) The only other tensile loads seen in typical framing are not constant but only cyclical and are borne of shifting wind loads and the Wind Bracing emplaced to resolve this and how it effects the adjacent framing.

This would also be because by far, most of our standing stock in “typical” historic framing, lack any Clear-Span Trusses, such Trusses are almost completely limited to public buildings, such as Town Halls and Churches.

Bridges, however, are by definition Clear-Span Trusses, and in most Truss Types many of the pieces within a Truss’s framing are in tension. And the loads they are subjected to are all of those asked of the typical frame with multiple direct load paths to the ground, ( shifting snow and wind loads and the moving live loads they are constructed to house and bear) and additionally the completeness of their own weight / dead load and the heavy rolling loads a Through Truss is designed to convey.

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There are exceptions to every rule – Here is seen a new tension splice in one of a set of paired Posts – Tension varies from panel to panel in a bridge truss, the load increasing in every step away from midspan and towards shore / Truss Terminus – In this instance the Engineer of Record determined this one’s position allowed for this Scarfed replacement end – All but one of the other compromised Posts in this effort were replaced in total

Replacing lost section in a tension member of a truss is far different circumstance than is that of replacing such section in a compression member in a typical timber-frame. In most instances, it simply cannot be done without diminishing the capacity of the Truss. As an impossible to ignore example, Bottom Chords are the primary tensile member within a truss’s configuration and are made up of multiple laminae, each spliced together to create the length necessary to complete the required span, this almost without exception means there is a tension-splice in one of the lamina in every panel in the truss. ( the exception to this being the four end Panels ) Meaning, if one were to only remove rotted section, this would require adding two tension-splices to the mix and doubling the number in two panels, this both giving up redundancy ( in two lamina truss types giving up all redundancy ) and without question diminishing capacity.

Though the loading of the framing of Wooden Boats and Wooden Through Trusses is almost wholly different behaviorally, their service shares an obvious commonality. And to my mind that commonality and the required fail-safe in safety that is necessarily interwoven with their service, requires a similarity in philosophy in approach to the preservation of both.

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