Fall 2024
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The Role of Diaphragms and Shear Walls in Creating Resilient Structures
When designing a building for lateral loads such as those generated by wind or earthquakes, engineers have several options. Lateral loads may be transferred to the foundation via braced frames or moment frames; diagonal rods, or flat diagonal bracing (including let-in bracing for wood frame construction), among other methods. Where wood structural panels are used for the roof, floors or walls in a building, lateral loads can be resisted using these ordinary wall elements. This type of construction is adaptable to conventional wood-frame construction used in residences, apartment buildings and offices. Buildings can be designed to resist the horizontal loads introduced by extreme wind or earthquakes by applying diaphragm design principles to your walls, floors and roofs.
Diaphragms and Shear Walls Defined
A diaphragm is a flat structural unit acting like a deep, thin beam. The term “diaphragm” is usually applied to roofs and floors. A shear wall, however, is a vertical, cantilevered diaphragm. A lateral force-resisting system results when vertical and horizontal diaphragms are tied together to form a structurally stable unit. (See Figure 1.) When diaphragms and shear walls are used in the lateral design of a building, the structural system is termed a “bearing wall lateral system.” Shear walls transfer reactions from the roof and floor diaphragms, sending the forces into the foundation.
Panel diaphragms have been used extensively for roofs, walls and floors for new construction and rehabilitation of older buildings. A diaphragm acts like a deep beam or girder, where the panels act as the beam’s “web,” resisting shear. The diaphragm edge members perform the function of “flanges,” resisting bending stresses. These edge members are called “chords” in diaphragm design and may be joists, ledgers, rim board, bond beams, studs, top plates, etc. A shear wall is simply a cantilevered diaphragm to which load is applied at the top of the wall, and that load is transmitted out along the bottom. This creates a potential for overturning. Any overturning force is typically resisted by hold-downs or tie-downs at each end of the shear wall.
Diaphragms vary considerably in load-carrying capacity, depending on whether they are “blocked” or “unblocked.” Blocking consists of lightweight framing, usually 2x4s, spanning between the joists or other primary structural framing to support the edges of the panels (See Figure 2). Another form of blocking for purposes of shear transfer is with metal framing screwed to adjacent panels to provide shear transfer between panels. Unblocked diaphragm loads are controlled by buckling of unsupported panel edges, resulting in such systems reaching a maximum load above which increased nailing will not increase its capacity.
Advantages of Wood Diaphragm Design
Structural wood panel diaphragms use wood’s capacity to resist sudden and cyclic loads. They maintain high strength and, if pushed to their ultimate capacity, yield gradually while continuing to carry the load. By considering the strength and stiffness of wood structural panel skin, engineers can eliminate almost all expensive and inefficient diagonal bracing that might otherwise be required. Wood diaphragms are easy to build and connect different portions of a structure. The system's primary components are structural panels and lumber, nails and metal connectors.
Panel diaphragm design has been proven through some of the most extreme weather and seismic events. Wood lateral systems enable engineers to produce a building designed to resist high wind and seismic loads at minimal extra cost. Learn more about the critical roles diaphragms and shear walls play in building design in APA’s free Diaphragms and Shear Walls Design and Construction Guide. This 37-page publication has numerous design examples, illustrations and formulas for calculating deflection and other vital details.
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Getting Technical
On-Demand Diaphragm Design Guidance
By distributing lateral loads to shear walls, diaphragms play a vital role in a building’s lateral load path. Discover how important and versatile they are in this on-demand webinar.
APA’s Designing Engineered Wood Diaphragm Systems webinar is available for on-demand viewing and provides guidance on the proper design of engineered wood diaphragm and subdiaphragm systems. Participants will learn best practices as they are guided through simple design examples. Viewers can earn AIA and ICC credit.
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Inside the Circle
Meet Engineered Wood Specialist Van Wilfinger
With over 30 years of experience in the construction industry, Van has cultivated specialized expertise thanks to roles such as building inspector, plan reviewer, building official, and most recently, chief building official for Clatsop County in Oregon. Van's knowledge covers questions related to building codes and regulations. Van services the Pacific Northwest region, including Alaska, Hawaii, Washington, Oregon, Idaho, Montana and Utah. Beyond his professional endeavors, Van and his wife enjoy outdoor activities such as hiking, camping, fishing and discovering new destinations. Van works from his residence in Oregon.
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