The Physics Behind Dove-Tailed Joints, and Force Equations. History The dove-tailed joint has been around for thousands of years. According to Appalachian joinery, "Some of the earliest known examples of the dovetail joint are in furniture entombed with mummies dating from the First Dynasty of ancient Egypt, as well as the tombs of Chinese emperors". In the last couple hundred years, the dovetail design has become most popular with Appalachian Indians. The Appalachian even created their own style, called the Appalachian style, which is the most common style in use today. The Appalachian style makes use of spaces between the logs for moss or other types of chinking to be used for insulation. The older dovetail building were built very sturdily by skilled craftsmen, and have stood the test of time, and quite a few of them still stand today. Physics The dove-tailed joint uses some simple laws of physics in order to snugly join the logs together, in such a way as to keep them from wanting to separate. The intricate design of the dove-tail could best be compared to a combination of two wedges stacked on top of each other, with the top wedge rotated ninety degrees to the first wedge.The dove-tail is a double-compound joint that is cut so that every log is placed on a downhill slope of the other log with the downhill slope towards the center of the building. They also have a vertical flat spot at the start of the dovetail which allows the stacked log to rest against, so that the logs have a backstop to prevent slipping towards the center of the cabin. The dovetail uses the physics of gravity to snugly pull the walls together from the weight of the walls, roof, and snow load, since the weight is acting to bring the logs down the slope. With the logs sloped towards the center of the building, with a backstop, they are not inclined to pull apart.               Photo courtesy of the author The above photo's are taken by the author from the house he built using dovetail construction.  The first picture is a close up of the dovetail corner of one wall which shows the double-compound angle that is utilized to keep the corners together. The picture also shows the gaps between the logs. The gaps will be utilized to run electrical wires in, then they will be filled with insulation, and the logs will be permachinked to fill the gaps. The second photo shows a dovetail jig that is being utilized to cut a dovetail joint with a chainsaw. The jig gives the perfect angle for the dovetail, and saves countless hours of laying out and cutting the joints by hand. The sawmill that is shown is the workhorse that is necessary to cut all of the beams for the cabin. The third picture shows the authors dovetail log cabin under construction with the roof partially done. Force equations When designing the various members of a dovetail log cabin, it is important to be able to analyze the various forces acting on certain members, such as the roof line and eaves, so as to be able to tell how strong to engineer them. There are a couple of basic force equations that come from Newtons Second Law, which states that for every action there is an equal and opposite reaction. The common way that engineers find forces in house members which are in static equilibrium, is by drawing a free-body diagram which replaces members with forces and then gives directions to those forces relative to an axis system. Similarly, a kinetic diagram is used to show forces on objects that are moving. The next step to analyzing the forces on a member of the house (assuming static equilibrium), is to use the three standard force equations. Those three equations state that the sum of forces in the x-direction equals zero, the sum of forces in the y-direction equals zero, and the sum of moments equals zero (Statics & Dynamics P.113). Very often however, the forces are not all lined up on a given axis, so vector decomposition is necessary to break the vectors into their respective X&Y directions. The following picture shows a good example of some members on the roof that are replaced by a free-body diagram. Free-body diagram forces in the top log. Drawing was produced by the author using an auto-desk educational product. Home Sweet Home Next Page Previous Page