Series Provides Fundamentals of Dealing with Historic Trusses
Under a PTT Grant titled "The Truss Form in Vernacular Carpentry in the Eastern United States, 1660-1850," experts from the Timber Framers Guild examined almost two dozen historic buildings containing heavy timber roof trusses.
After probing, photographing and documenting representative examples of four truss types, the researchers have published a series of monographs showing the overall configuration, transverse and longitudinal views, exploded joinery at the connections, and iron work. The studies include structural analyses that illustrate loads, stresses and displacements on truss members and connections.
The four truss types included are scissors, kingpost, queenpost and composite/raised bottom chord trusses. Trusses of each type appear in thousands of historic meetinghouses, churches and civic structures throughout the eastern U.S. From the 18th century through the 1930's these buildings, plus factories and mills, required large column-free spaces.
Truss roofs were extensively used since they were much more efficient at spanning long distances than single large timbers, and historically were the main way to produce monumental enclosed public space. Wood was the cheapest building material available at the time, and its versatility allowed it to be cut to any shape or size. The study promotes a better understanding of the structural dynamics of each timber truss type and typical methods of analyzing defects and identifying potentially damaging problems.
The principal investigator was Jan Lewandoski, an author and timber frame specialist who has supervised the reconstruction of many historic churches, meetinghouses and covered bridges. Structural analysis was provided by Ed Levin, architectural and structural designer and consultant. Drawings were provided by architect, author and timber framer Jack Sobon. All were founding members of the Timber Framers Guild, which publishes Timber Framing, the quarterly journal where the monographs first appeared. Editor Ken Rower completed the project team by assisting in the research and production of the monographs.
Since many historic trusses are inaccessible and their connections obscured, this study is invaluable for those responsible for the care of these buildings and provides models from which to infer information that would otherwise be unobtainable through inspection.
The study also addresses misconceptions among architects, engineers and preservationists about how historic timber trusses really work. The vagaries of wood and lack of contemporary fasteners often resulted in trusses that don't conform to the criteria of modern trusses today.
The series of monographs is available for downloading free of charge at the Timber Framers Guild website at www.tfguild.org. The Guild also plans to publish the series in a single volume.