Trusses are economical, manufactured systems that are carefully engineered using small wood members connected by flat metal plates with small teeth that are pressed into the wood. A “web” structure of triangles is created which balance tension and compression in the members and can span great distances. Sometimes trusses are built on the job site using plywood and nails instead of the metal plates, but most often they are built in factories according to the design and specifications prepared by licensed engineers. The variety of truss designs are enormous, both in the ‘web’ design and the styles of roofs that can be created. The design of trusses is best left to professional engineers, but the designer must be aware and account for the truss loads passed to the supporting structural members below. Since the goal of trusses is to span long distances, usually from outside wall to outside wall, the loads generated are often considerable. This will mainly impact window headers as they will receive double (and sometimes more) the load generated in conventional construction with interior bearing walls.
Wood and steel, if compared pound for pound are similar in strength. Steel is denser, requiring a smaller amount of material to do the job, but it weighs nearly the same as wood. Steel is manufactured in different grades with different strength characteristics and is homogenous throughout. Wood consists of directional fibers, like grains of rice that lay together in the same direction. The fibers are strongest in their long direction and therefore the design of wood members requires evaluation of how the member is oriented to the loads. Wood has more attributes that refer to the direction of the wood fibers. The direction of fibers is especially important when designing fasteners and connections. Also, the design properties of wood vary with the wood species, how it is graded at the lumber mill, its moisture content, and how it will be used within the structure.
STRUCTURAL LUMBER Structural lumber is visually graded when sawn at the mill. The grading process takes into account which portion of the tree the member came from and any natural defects such as knots and checks. It is also classified by its size and intended final use. For special use, some members are tested by machine and re-graded into a separate category called mechanically evaluated lumber, MEL. See your local Wood Products Association literature or the NDS® for a full listing of sawn lumber sizes, species groupings and design values Note: terminology in wood design is not always consistent. Thickness or width will usually equate to the “breadth” if the member is used upright on its narrow edge and height will equate to the “depth”. Be consistent when describing a member’s size. The breadth comes first, followed by the depth.
NOMINAL AND NET SIZES When lumber is sawn to a particular size, such as 6 inches x 12 inches, it will shrink in size when it dries or when it is “surfaced” or “dressed” (smoothed through a planer). The final, or actual size of the finished member is about 1/2” smaller in width and depth. The member is still called a “6 by 12” but its actual, net size will be 5-1/2 inches x 11-1/2 inches. Dimension lumber loses 3/4 of an inch in its wide dimension. A 2 x 10’s net size is 1-1/2 inches x 9-1/4 inches, except a 2 x4 which is 3-1/2”. Note: The actual or net size is used when designing structural members, but the nominal size is used for specifying members. NEW NATIONAL DESIGN SPECIFICATION 1991 and 1997 NDS® In Grade Testing Program Building departments across North America have adopted a new building code with fundamental changes in the way the structural wood members are designed. These changes include not only new wood design properties, but a change in the design method as well. The USDA Forest Products Laboratory in cooperation with the North American softwood timber industry, have been conducting a comprehensive scientific research program known as the In Grade Testing Program. Back in the 1920’s, theoretical assumptions about the properties of domestic lumber were derived from a few limited tests. This time, over 70,000 pieces of visually graded dimension lumber were pulled directly from the shelves of U.S. and Canadian mills and tested to destruction. Evaluation included all the basic properties of structural lumber such as bending strength, stiffness and compression. Analysis of revealed a wealth of information about how and why lumber performs in different situations.