Timber Frame Construction
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Tiimbermber FFramerame CConstructiononstruction ARCHITECTURALLY EXPOSED TIMBER By Ben Brungraber and Jim DeStefano Figure 1: Traditional Timber Framing imber frame construction, con- sisting of heavy timbers joined with interlocking connections, dominated construction in Eu- Trope and Asia for over 2000 years. But nowhere did it fl ourish as it did in Ameri- Timber ca. The demand for new buildings and the virtually unlimited forests in colonial Amer- ica drove the technology of timber framing to new heights. America became a melting pot of English, Dutch and German styles of timber framing. Unleashed from the restrictions of medieval European guilds, early American timber framers were free to Frame innovate more effi cient methods of laying out and cutting timber joints. Two signifi cant technological develop- ments in the mid nineteenth century resulted in the decline of timber frame construction. The circular sawmill, powered fi rst by water Construction and later by steam, made producing small and uniform dimension lumber very effi cient. The wire nail machine dramatically reduced the cost of the iron nails that were previously produced individually by blacksmiths. With dimension lumber and inexpen- Figure 2: 3D CAD drawings are used for design and shop drawings sive common nails now readily available, light frame wood construction soon industry, serving a few select clients who fi nished timber frames will look. replaced timber framing. Early light frame appreciated handcrafted buildings. The colonists who built the fi rst American wood construction was called “balloon In 1989, This Old House® rebuilt a timber timber frames would have been thrilled construction” because the structures appear- frame barn in Concord, MA on national simply to have our electric handsaws and ed so light and fragile. Carpenters with far TV. The demand for timber frame structures drills at their disposal. These tools are what less skill than that required to build a timber exploded almost overnight, and the industry modern timber framers used fi rst, but they are frame structure could build balloon frame has not been the same since. Timber framing just powered versions of venerable tools that structures quickly. By 1900, timber frame has become a mainstream building technology, the original framers would have recognized. residential construction had all but died out and the small backwoods timber framers Over the last twenty years, there has been and, by WWII, was even replaced in barns. have grown into large businesses building tremendous progress in some very specialized commercial and public buildings using high- and powerful new tools. There are nearly a tech design and fabrication methods. The Revival of Timber Framing dozen manufacturers, world-wide, of heavy Timber frame construction has experienc- stationary saw platforms that can cut a tenon ed a rebirth. It started in New England in in a single pass. the 1970’s with a group of young carpenters But what has really revolutionized the exploring alternative building technologies. fabrication of timber frame structures is They had become disenchanted with the the numerically-controlled timber cutting sterile nature of mass-produced stick frame machines that can do a lot of the tedious, houses, and were drawn to the handcraft- yet critical, work in crafting timbers. The ed character of timber frame construction. German-manufactured Hundegger timber The timber frame revivalists built frames cutter has become the tool of choice among that emulated and reproduced the forms the established timber frame companies. and joinery of early American frames. Some With a Hundegger, the automated fabrica- of these pioneers even scorned the use of tion of timbers fl ows seamlessly from the 3- power tools and worked exclusively with Figure 3: Computer controlled timber cutting D shop drawings. This huge upgrade in tool hand tools. machine sophistication means that much of the framers’ Over the next decade, the industry matured time with tape measures, squares and pencils as timber framers became more profi cient Timber Framing Goes High-Tech has been translated into time in front of a at the craft and learned more about the Today, most timber frames are engineered terminal, tweaking the cutting instructions. traditional technology. The Timber Framer’s structures. Many of the established timber The big timber cutters also do an exquisitely Guild of North America formed to share and framing companies now use computer- accurate job of removing much of the wood in disseminate information about the craft. Power generated 3-D shop drawings when describing traditional joinery designs. tools were developed and adapted from other their frames. These drawings are particularly There is still a lot of handwork and countries for cutting timber frame joints. But useful for both defi ning their scope of work craftsmanship left to do, though. Even timber framing remained a small backwoods and for helping clients to visualize how the modern timber framers still use chisels to STRUCTURE magazine • March 2005 27 Most modern timber frames are clad with to support one timber while not unduly Structural Insulated Panels (SIP) that are weakening the other. Consideration must installed immediately following the frame also be given to how the joint’s geometry will raising. SIP’s create a very energy effi cient change when the timbers dry. building envelope and allow the timber frame There are still circumstances where to be completely exposed to view on the inside architectural constraints or large reactions of the building. mandate the use of steel hardware and fasteners. Even using cranes to raise timber frames, it While there are a variety of proprietary is still customary to celebrate the topping out concealed connectors that can be used, the of a frame with an evergreen bough secured hardware is more commonly custom designed to the ridge pole, a tradition that has endured and fabricated. Considerable ingenuity over 1,000 years. is sometimes needed to engineer a viable connection with no visible parts. Timber Joinery Modern timber joinery has evolved with the aid of engineering knowledge and A structure is only as strong as its automated fabricating machinery. Timber connections. This is especially true with framers no longer limit themselves to using timber frame structures. The joinery design traditional joints based on archeological Figure 4: Cranes are used to erect frames and detailing is often the most challenging analysis of historic structures. By combining aspect of engineering a timber frame. elements of traditional European-based timber frames, with even more ancient square out housings – although they are likely techniques from the Far East, modern to be exquisite examples of these ancient tools, timber framers have been able to generate carefully made in Japan. In this shrinking a remarkably rich new set of joinery designs world market, American timber framers are and details. Perhaps the biggest single also likely to use power chamfer planers made component of this inclusive attitude has in Spain, stout French chisels, and heavy been the wide acceptance of the through- power planers made in Germany. Carvings, spline joint. Replacing a pair of tenons embellishments, and shop fi nishes are still (as brought over from European examples) crafted with hand tools. pegged into opposite faces of a supporting post, the through-spline joint (as found in Timber Frame Raising many Asian frames) offers a lot of advantages Most people have a very romantic image of to the modern timber framer. a timber frame raising (erection). It calls to mind images of an Amish barn being raised and sheathed in a single day by hundreds of neighbors while the women cook and sew quilts and the children run underfoot. American hand raisings are actually infrequent, and are generally limited to projects where the raising becomes an event to celebrate the construction. Timber frames are generally raised with a small crew and a crane, much the way a structural steel building is erected. Tapered hardwood pegs are used instead of high- Figure 6: Modern timber joinery strength bolts and sharp chisels are used for fi eld adjustments rather than cutting torches. Figure 7: Spline joint detail Mortise and tenon joints are most commonly used to connect timbers. Hardwood pegs Tension is usually the most challenging hold the joints together but are seldom relied loading for timber connection designers. A on to transfer loads. The safe load on a 1-inch big advantage of through-splined connections peg in shear is approximately 1 kip, making a is that all the pegs are inducing parallel-to- peg a poor substitute for a bolt. the-grain forces when the joint is loaded in The challenge of engineering a timber tension. This makes the same pegs both joint is to confi gure the geometry of the stronger and stiffer, while the post size no joint so that structural forces are transferred longer limits end distances beyond the pegs in bearing rather than through fasteners. with necessarily-short tenons, allowing the Traditional timber joints rely innately on designers to use more pegs than would fi t removing material from one member, in into any reasonable post. If the spline is order to support another. This accentuates not even pegged to the post, less damage is Figure 5: Roof framing has been assembled the art of compromise in designing these on the ground being done to the post; it is no longer being joints, creating a joint that is strong enough pulled apart by the opposing tenons. 28 STRUCTURE magazine • March 2005 can make the log seem to can do wonders for the engineer working be growing from the fl oor. with the methodology for the fi rst time. This elegant detail, espe- Even the bigger design/build timber cially popular in western frame companies have not been around lodges, also complicates the so long as to become insufferable in the acquisition, transporration, certainty of their experience.