Alpine

18 May 2015 Structural Building Components Magazine www.sbcmag.info by Dave Brakeman, P.E., S.E. & Sean D. Shields onsider for a moment the basics of manufacturing a truss. Based on SBCA's 2012 Financial Performance Survey, lumber accounts for roughly 40 percent of the total cost. Plates account for about eight percent of the total cost. Design and production labor account for 30 percent, and delivery, sales and overhead account for the remaining 22 percent (these are rough industry averages). All other things being equal, if you could decrease your lumber costs by a few percentage points while raising your plate costs a small amount, would you take the trade-off? This is exactly the question component manufactur- ers (CMs) and plate suppliers asked themselves in the late 1990s. The solution they found, to account for the bending moment resistance of metal connector plates (MCP), may or may not be well understood. Figuring in the MCP moment resistance allows for more even distribution of that stress throughout the wood mem- ber, typically resulting in a lower maximum CSI for that lumber member. The impact of this redistribution is significant to truss design in that it means that, in certain cases, a slightly larger MCP will allow a lower lumber grade to be used. This article will attempt, through simple terms and a few examples, to explain how and why. Understanding Moment What are bending moment forces? One of the easiest examples may be to look at a peak joint of a kingpost truss (see Figure 1). As load is applied to the top chords of this truss, those chord members deflect and bend downward/inward. When the top chords deflect, the ends of the top of the chord members at the peak of the truss experi- ence rotation, causing the lumber ends to move away from each other at the top of the peak joint, creating gaps in the joint (see Figure 2). The top chords of that truss must be designed to resist this applied bending and rotational load. In this C MOMENT resistance Why Bigger Plates Are a Good Thing Consider how factoring joint stiffness could save you money. Figure 1. The peak joint of a kingpost truss. Figure 2. Bending moment forces as load is applied to the top chord of a kingpost truss. Reprinted with permission from SBC Magazine (www.sbcmag.info). All rights reserved.

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