Alpine

22 May 2015 Structural Building Components Magazine www.sbcmag.info Moment Resistance Continued from page 20 If the moment force formula was used, and MCP stiffness was factored into the design, the truss could be designed as seen in Figure 9. If the plate size of the peak was increased to 5x6, the T2 and T3 segments of the top chord could be constructed of 2x4 SPF #1/#2, while the remaining top chord and bottom chord material would still use 2x4 SPF 1650f – 1.5E, and the webs would be 2x4 SPF stud. Again, this reduction in the grade of T2 and T3 top chord material represents a real cost savings to a CM. Overall Material Savings It's important to note the moment resis- tance formula is active and running behind the scenes in the design soft- ware (assuming your local code refer- ences ANSI/TPI 1-2002 or later). The software automati- cally takes advan- tage of the stiffness the MCP provides. The result is that, in some cases, the software will specify larger plate sizes than it would have traditionally. This is a good thing, not because the plate sizes are bigger, but because the MCP now, in most cases, helps to lower the lumber and overall truss CSI by redis- tributing the maxi- mum stress through- out the lumber members. Again, this redistribution should allow for a lower grade of lumber to be used in a given application. As mentioned at the beginning of this article, with lumber rep- resenting roughly 40 percent of the cost of a truss, saving money on lumber will have the biggest impact on the total cost of producing that truss. In some cases, factoring in the stiffness of the MCP can also allow designers to eliminate some low force webs. Alternately, increasing plate sizes does increase the price, but at only eight percent of the total cost, that cost impact is not as significant. Will it provide a benefit in every case? No. In some cases, like valley sets or other short span trusses, the increased plate size now specified by the software will reduce the necessary lumber grade of wood members to a grade lower than the lowest grade of lumber carried by a CM. As a result, even though there is a design benefit, the CM may not be able to take advantage of a lower grade because they don't keep it in stock. Taken as a whole, CMs benefit from taking advantage of the moment resis- tance formula through overall material cost savings. SBC Dave Brakeman is the Engineering Director for Alpine, an ITW Company. He has served as chairman of the TPI 1 project committee for the last three editions of this standard. Figure 6. King post truss with joints designed using a hinge model. Figure 7. King post truss with joints designed using a partial fixity model. Figure 8. Modified queen truss with joints designed using a hinge model. Figure 9. Modified queen truss with joints designed using a partial fixity model.

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