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T E C H N I C A L N O T E <br />Fastener Loads for <br />Plywood – Screws <br />A P A <br />The Engineered Wood Association <br />Number E830C <br />December 1995 <br />Introduction <br />The integrity of a structure is frequently <br />dependent upon the connections <br />between its component elements. For <br />maximum strength and stability, each <br />joint requires design which is adapted <br />to the fastener type and to the strength <br />properties of the individual structural <br />members. Included in the following <br />tables are ultimate withdrawal and <br />lateral loads for plywood joints <br />fastened with wood and sheet metal <br />screws. These load values are based <br />upon tests conducted by APA – The <br />Engineered Wood Association. <br />Test Results <br />Plywood-and-Metal Connections <br />Self-drilling, self-tapping screws are <br />commonly used to attach plywood up <br />to 1-1/8 in. thick to steel flanges up to <br />3/16 in. thick. However, since threads <br />are usually provided on only a portion <br />of the fastener shank, it is important to <br />specify the appropriate fastener length <br />for a given plywood thickness. This <br />precaution ensures that the threaded <br />portion of the shank will engage in the <br />steel framing. Several lengths and styles <br />are available. Additional details for <br />these types of screws may be obtained <br />from specific fastener manufacturers. <br />The following data apply to wood <br />screws and sheet metal screws. <br />Lateral Resistance: <br />Performance of plywood-and-metal <br />connections is dependent upon the <br />strength properties of all three elements. <br />a) Plywood-critical joints are <br />characterized by a shearing of the <br />plywood veneers oriented parallel to <br />the direction of the applied force. <br />Veneer plugs, whose width equals the <br />diameter of the screw shank, may <br />shear loose and protrude at the <br />loaded end. <br />b) Fastener-critical joints are <br />characterized by a shear failure of the <br />screw shank. As shown in Figure 1, <br />once localized crushing of the wood <br />has occurred, resistance to fastener- <br />head embedment into the metal causes <br />the screw to become a shear specimen <br />and joint behavior is dependent upon <br />the shear strength of the fastener. Shear <br />failure of the screw shank occurs at the <br />wood-metal interface. <br />c) The metal-critical joint may fail in <br />one of two ways. Failure occurs when <br />the resistance of the screw head to <br />embedment is greater than the resis- <br />tance of the metal to lateral and/or <br />withdrawal load, and the screw tears <br />away from the metal framing. Failure <br />also occurs when thin metal in a <br />metal-to-plywood joint crushes or <br />tears away from the screw. <br />Tables 1 and 2 present ultimate lateral <br />loads for wood- and sheet-metal-screw <br />connections in plywood-and-metal <br />joints. Loaded end distance in these <br />tests was one inch. Plywood face grain <br />was parallel to the load since this <br />direction yields the lowest lateral loads <br />when the joint is plywood-critical. All <br />wood-screw specimens were tested <br />with a 3/16-in.-thick steel side plate, <br />and values should be modified if <br />thinner steel is used. <br />FIGURE 1. <br />Failure of Laterally <br />Loaded, Single-Shear <br />Metal-to-Plywood Connection <br />Shearing of screw shank <br />Crushing of plywood <br />TN,E830.0 4/13/00 10:11 AM Page 1 <br />Page 29 <br />© 1995 APA - The Engineered Wood Association <br />2130 N Grand Ave <br />10/1/2024