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Material Properties It is essential that a residential designer specifying wood materials appreciate the natural characteristics of wood and their effect on the engineering properties of lumber. A brief discussion of the properties of lumber and structural wood panels follows. Residential Structural Design Guide 5-3 Chapter 5 – Design of Wood Framing 5.2.1 Lumber General As with all materials, the designer must consider wood’s strengths and weaknesses. A comprehensive source of technical information on wood characteristics is the Wood Engineering Handbook, Second Edition (Forest Products Laboratory, 1990). For the most part, the knowledge embodied in the handbook is reflected in the provisions of the NDS and the NDS Supplement (NDS-S) design data; however, many aspects of wood design require good judgment. Wood is a natural material that, as a structural material, demonstrates unique and complex characteristics. Wood’s structural properties can be traced back to the material’s natural composition. Foremost, wood is a nonhomogeneous, non-isotropic material, and thus exhibits different structural properties depending on the orientation of stresses relative to the grain of the wood. The grain is produced by a tree’s annual growth rings, which determine the properties of wood along three orientations: tangential, radial, and longitudinal. Given that lumber is cut from logs in the longitudinal direction, the grain is parallel to the length of a lumber member. Depending on where the lumber is cut relative to the center of a log (i.e., tangential versus radial), properties vary across the width and thickness of an individual member. Wood Species Structural lumber can be manufactured from a variety of wood species; however, the various species used in a given locality are a function of the economy, regional availability, and required strength properties. A wood species is classified as either hardwood or softwood. Hardwoods are broad-leafed deciduous trees while softwoods (i.e., conifers) are trees with needle-like leaves and are generally evergreen. Most structural lumber is manufactured from softwoods because of the trees’ faster growth rate, availability, and workability (i.e., ease of cutting, nailing, etc.). A wood species is further classified into groups or combinations as defined in the NDS. Species within a group have similar properties and are subject to the same grading rules. Douglas Fir-Larch, Southern Yellow Pine, Hem-Fir, and Spruce-Pine-Fir are species groups that are widely used in residential applications in the United States.