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20 May 2015 Structural Building Components Magazine www.sbcmag.info Moment Resistance Continued from page 19 Recognizing that MCPs provide joint stiffness and resistance to rotation, testing of plates was undertaken in the late 1990s to determine their ability to resist moment forces. According to the ANSI/TPI 1-2014 Commentary: The results of the tested specimens were compared against three theoretical models used to predict the ultimate moment capacity of the steel-net sections. Through that comparison, a formula was developed to account for a MCP's ability to resist moment forces. That formula was included for the first time in Section 8.7 of the ANSI/TPI 1-2002 edition. Soon after this, plate manufacturers incorporated the formula into their truss design software. While this occurred over a decade ago, the practical application of the formula wasn't fully realized until years later. In general, CMs didn't begin utilizing this equation until their local building codes referenced ANSI/TPI 1-2002, which began with the 2006 International Residential Code (IRC). Further, the formula has gone through some revisions. Again, according to the ANSI/TPI 1-2014 Commentary: The equations in Section 8.7.1, originally included in the 2002 specification in a slightly different form, are developed from the most accurate model from this research as validated by testing. Subsequent use and further research showed the need for modi- fication of this method to recognize the interaction between axial compression and moment stresses and to recognize the effect of plates located off center. Application of the Formula So what does this formula mean for CMs from a practical standpoint? Looking at the peak joint again, the stiffer that joint is, the more it is able to resist the rotation from the bending moment force. From a design perspective, the maximum critical force of the top chord member is then reduced because it is redistributed between the chord and the MCP. Figure 4 shows a graphical depiction of the force that the lumber has to resist when a MCP provides no stiffness. Figure 5 shows a graphical depiction of the much smaller forces that the lumber has to resist when a MCP provides rotational stiffness. In summary, because the moment force formula is now incorporated into the design software, from a design perspective, the lumber chord member no longer has to resist the applied load all by itself. By factoring in the MCP joint stiff- ness, the CSI of the top chord is reduced and a lower grade of lumber may be sufficient to resist the applied load. Let's look at two case studies to see the impact this has. Case #1 Kingpost Let's continue to look at a kingpost truss. If the moment force formula is ignored and the joint is treated as it theoretically and historically was, the truss joints would be designed using a hinge model (providing little to no rotational stiffness) and results in the truss design seen in Figure 6 on page 22. Using a 4x4 plate at the peak, 2.5x6 plates at the heels, and a 2x4 plate at the D joint, the top chords would need to be con- structed of 2x4 2700f – 2.2E MSR SP, the bottom chord would be 2x4 SPF #1, and the kingpost would be 2x4 SPF stud grade. If the moment force formula was used, and MCP stiffness was factored into the design, the truss would be designed using partial fixity and would result in the truss design seen in Figure 7 on page 22. If the plate sizes of the peak and heels were increased slightly, the top chord material needed to resist the applied loads could be reduced to 2x4 SPF 1650f – 1.5E, while the bottom chord and kingpost material would remain the same. This reduction in the grade of top chord material represents a significant cost savings to a CM. Let's look at another example. Case #2 Modified Queen Here's a fully triangulated queen truss. Again, if the moment force formula is ignored and the joint is treated as a hinge, the truss design would result in the truss design seen in Figure 8 on page 22. Using a 5x5 plate at the peak, the T2 and T3 segments of the top chord would be constructed of 2x4 SPF 2100f – 1.8E, while the remaining top chord and bottom chord material would use 2x4 SPF 1650f – 1.5E, and the webs would be 2x4 SPF stud. Figure 4. The force that the lumber has to resist when a MCP provides no stiffness. Figure 5. The much smaller forces that the lumber has to resist when a MCP provides rotational stiffness. Continued on page 22

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