Alpine

19 May 2015 Structural Building Components Magazine www.sbcmag.info simple example of a kingpost truss, the span is a significant factor, as well as the strength of the lumber used in the top chords and the ability of the metal plate at the peak joint to resist these bending and rotational (bending moment) loads. Let's take a small step back and look at the joint. Another way of thinking about the MCP's ability to resist the rotation of the end of the wood member is to talk about it in terms of stiff- ness. If stiffness were plotted on a spectrum, at one end there would be ultimate stiffness, and on the other, zero stiffness (see Figure 3). Ultimate stiffness could be achieved if there was some way to weld the ends of the top chord and kingpost members together, as if they grew that way. At the other end of the spectrum would be something akin to the absence of stiffness, like a hinge. If all the ends of the chord and kingpost members were allowed to rotate freely, there would be little to no stiffness. Obviously, a MCP provides stiffness or resistance to the bend- ing moment rotation. It's also clear that the MCP is closer to the "weld" end of the spectrum versus the "hinge" end, but where exactly in this spectrum does it fall? This is an important question because, instead of having to design a truss by relying entirely on the strength or stiffness of the wood used in the top chord, the MCP's stiffness can, and should, be accounted for in helping the lumber resist the rotational loads in the truss design. Accounting for Moment Prior to the publication of ANSI/TPI 1-2002, a MCP's ability to resist bending moment (rotation) forces was not typically factored into the design of a truss. While plate suppliers and CMs who conducted their own proprietary testing could fac- tor in this resistance under TPI 1 Section 1.3.2 (see sidebar below), which enables anyone to use their own test data to establish alternate designs as equivalent to the standard, this was not widespread. At this rAte, it won't be long before mAnuAl sAw feeding is covered with dust. Compatible with any linear-feed saw, the Wood Runner ™ automated lumber retrieval system increases production by 30%, while reducing labor by half. Wood Runner sets the pace of the operator, and ensures accurate lumber selection every time. Visit www.apexmw.com or call 507-322-3700 to learn more today. 008_WTS_wood_runner ad_7.123x3_v3.indd 1 3/7/15 11:34 AM 1.3.2 Alternate Provisions. 1.3.2.1 Materials, Assemblies, Structures, and Designs. This Standard does not intend to preclude the use of materials, assemblies, structures, or designs not meeting the criteria herein, when they demonstrate equivalent performance for the intended use to those specified in this Standard. The use of such alternate provisions shall be indicated on the Truss Design Drawing. Continued on page 20 ULTIMATE STIFFNESS NO STIFFNESS WELD MCP HINGE MOMENT Figure 3. If stiffness were plotted on a spectrum, at one end there would be ultimate stiffness, and on the other, zero stiffness.

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