Every now and again a new client approaches our practice with a clear vision in their mind: they want to live in a structure that utilizes a true timber-frame approach. Often this request is infused with a sincere love for the beauty of wood. In other cases, clients have had close contact with a true mortise-and-tenon structure, perhaps hundreds of years old.
There are several approaches architects and builders can take with timber-frame structures. One approach is to fashion a structure from big timbers and connect beams and column supports utilizing ‘time honored’ mortise-and-tenon joinery. Another approach is to replicate more industrial era techniques utilizing large and strong metal connecting plates with bolts. In both cases, architects and engineers must design timber-frame structures to meet modern-day structural and energy codes.
MMA has had the pleasure to pursue and build expertise in both approaches, including knowledge about timbering wood, sustainable forestry, how wood dries and weathers with age, and methods for applying and protecting the finish on large timber frames.
A Mortise-and-Tenon Barn in New England
As part of a comprehensive building program for a Vermont estate, MMA was asked to design a sophisticated mix-used horse barn (Vermont Barn) with living quarters for family and friends. MMA found the right partner in Bensonwood in New Hampshire, possibly the nation’s premier builder of highly-crafted heavy timber-frame homes, barns, and complex structures.
For our project the client wanted the true feeling of a centuries-old New England barn. Bensonwood’s structural engineer Ben Brungraber led a team that directly combined three key structural components: (1) Douglas-fir heavy timbers, (2) traditional mortise-and-tenon joinery, and (3) structural insulated panels (SIPs). The approach unified 18th century building techniques with 21st-century energy-efficient technology.
Part of the appeal of old barns is the weathered nature of old wood. As wood dries out over time timbers develop checks and splits, increasing the timber’s character. For our mortise-and-tenon barn structure, the raw Douglas-fir frame material was given a period of time to lose a percentage of its moisture within the Bensonwood shop prior to fabrication. When wood gets out into the exterior environment—as in a cold and wet New England winter—it can regain moisture in its fibers. Managing moisture content in wood members is an important step. The age-old techniques of mortise-and-tenon joinery account for the eventual shrinkage of wood as it loses its moisture over many years. At the same time, the use of the SIPs system brings a level of rigidity and tightness to a timber-frame structure.
Industrial Era Redux: Crafting Tailored Timbers and Trusses at the Marshside
While a true mortise-and-tenon timber frame structure may meet the needs of a modern barn, in some cases a building’s dimensions, plus the desired aesthetic of a structure, dictate a different approach. Such was the case of the Marshside Restaurant project in East Dennis, on Massachusetts’ famous Cape Cod.
During the 20th century large structures started to combine heavy timbers with metal connections. This type of modern metal joinery offered a different set of benefits, such as increased structural strength, rigidity, and simplification in joinery. These benefits applied to our Marshside Restaurant project, which needed a roof structure to span 45 feet while providing the lateral resistance to higher wind loads applicable along coastal sites.
To help us, MMA turned to G. R. Plume of Washington state, one of the most innovative timber-frame shops in the world. Like the Vermont Barn project, we knew from the beginning that Douglas-Fir timbers were well suited to the task, but we wanted a more tailored experience with the metal-to-wood joinery. Having developed an expert process in developing highly-crafted, and often artful wood-to-metal connections, G. R. Plume provided MMA and our structural engineer expertise and knowledge highly suitable to our goals.
Connections and Materials
In order to address the structural requirements of our project, it was decided early that MMA would employ all Douglas-fir Select Structural FOHC (free of heart center) lumber from British Columbia, Canada. Moreover, unlike the shop time-dried timber for the barn project in Vermont, our lumber was radio-frequency kiln dried (RFKD) inside a low-vacuum microwave kiln the length of a football field. Whereas the partially green timber for the barn project was cut only after it arrived at a moisture content of 20-25 percent, the RFKD lumber leaves its kiln at 15 percent across all fibers of the wood, regardless of their position within the lumber.
With this greater dimensional stability, G. R. Plume was able to provide remarkably accurate and dimensionally stable cuts and mortises for our wood-to-metal joinery that was both structurally effective and artfully articulated, lending the timber frame trusses and columns in the project a refined industrial look that was befitting of the overall aesthetic of the restaurant’s interior and level of ambiance.
The process of each connection involved three critical steps. First, MMA developed the desired look and arrangement of the steel connectors, including bolt placements and counts. The next step was for designs to be reviewed by the structural engineer and modified as necessary for strength considerations. The final step was submission to G. R. Plume to review CNC routing, general cutting, and erection sequence requirements at each joint. This final step was the most important because of the nature of the inset ‘mortised’ metal plate connectors. With such connections there is often only just one direction or axis for the wood and metal to come together and each member had to build up in sequence to form the final trusses and truss to column assemblies.
Regardless of method of timber framing construction, MMA has worked on several projects now using both methods of mortise-and-tenon and more modern metal connector joinery. The results for each are beautiful in their own way, offering a different set of aesthetic and structural considerations.
The below links offer additional resources of interest on timber frame construction, including links to the aforementioned timber frame shops above.
Timber Framing History – wikipedia