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STRUCTURAL FRAMING

Many elements of a home work together as a system to resist lateral and axial forces imposed on the above-grade structure and transfer them to the foundation. The above-grade structure also helps resist lateral soil loads on foundation walls through connection of floor systems to foundations. Therefore, the issue of system performance is most pronounced in the above-grade assemblies of light-frame homes. Within the context of simple engineering approaches that are familiar to designers, system-based design principles are addressed in this Chapter. The design of the above-grade structure involves the following structural systems and assemblies: • floors; • walls; and • roofs. Residential Structural Design Guide 5-1 Chapter 5 – Design of Wood Framing Framed Home FIGURE 5.1 Components and Assemblies of a Conventional WoodEach system can be complex to design as a whole; therefore, simple analysis usually focuses on the individual elements that constitute the system. In some cases, “system effects” may be considered in simplified form and applied to the design of certain elements that constitute specifically defined systems. Structural elements that make up a residential structural system include: • bending members; • columns; • combined bending and axial loaded members; • sheathing (i.e., diaphragm); and • connections. 5-2 Residential Structural Design Guide Chapter 5 – Design of Wood Framing The principal method of design for wood-framed construction has historically been allowable stress design (ASD). This chapter uses the most current version of the ASD method (AF&PA, 1997), although the load resistance factored design method (LRFD) is now available as an alternative (AF&PA, 1996a). The ASD method is detailed in the National Design Specification for Wood Construction (NDS) and its supplement (NDS-S). The designer is encouraged to obtain the NDS commentary to develop a better understanding of the rationale and substantiation for the NDS (AF&PA, 1999). This chapter looks at the NDS equations in general and includes design examples that detail the appropriate use of the equations for specific structural elements or systems in light, wood-framed construction. The discussion focuses primarily on framing with traditional dimension lumber but gives some consideration to common engineered wood products. Other wood framing methods, such as post-and-beam construction, are not explicitly addressed in this chapter, although much of the information is relevant. However, system considerations and system factors presented in this chapter are only relevant to light, wood-framed construction using dimension lumber. Regardless of the type of structural element to analyze, the designer must first determine nominal design loads. The loads acting on a framing member or system are usually calculated in accordance with the applicable provisions of the locally approved building code and engineering standards. The nominal design loads and load combinations used in this chapter follow the recommendations in Chapter 3 for residential design.