Two new building codes are now in the pipeline
Within the next year many local and state jurisdictions will start to adopt and enforce the 2009 International Building Code (IBC 2009) and the 2009 International Residential Code (IRC 2009). These new codes reference, and thus require, the use of a new standard called ANSI/TPI 1-2007 (TPI-07). The most significant changes required by the new standard are in plating, but there are changes that affect lumber design as well.
Because of the new and modified plating checks, some plate sizes will increase, although this will be greatly influenced by the type of trusses you design. One estimate is that plating square inches will increase an average of 3% to 5%, and lumber costs will increase perhaps 1% over current design standards. Changes to individual truss designs may vary greatly.
Why should plates get bigger at all?
If you are an old truss designer like I am, you might assume that “everything there is to know about the analysis of trusses is known.” But that isn’t the case. Knowledge about plating and lumber design grows over time. As this happens, and the standards are reviewed, there are areas where certain assumptions of the past are replaced by the new knowledge gained. Is that ‘new knowledge’ “right,” and the previous knowledge (standards) wrong? Not necessarily – but both are based on ‘the best available knowledge at the time,’ and governed by the fact that each standard represents a consensus of those that participated in the creation of the standards. Each new standard therefore “tightens” areas that were based on assumptions, and in doing so frequently results on more stringent rules. I'm not sure it is widely understood that the development of new standards is not a “closed” process. In fact, plate companies, component manufacturers, academia, building officials, model code officials, consulting engineers and other groups in the US with an interest have participated.
What Type of Joints Will be Most Affected?
Because of more stringent checks in TPI-07, joints that plated previously (under the TPI-02 standards) may require a larger plate and in some cases may no longer plate. Those most affected are corner joints, splices, and peaks. Here are some suggestions to help:
· A corner joint, such as a raised heel with vertical, may benefit from running the vertical through to the bearing or having a 2x6 (as opposed to a 2x4) end vertical.
· A design option that analyzes the pitch breaks using a semi-rigid model will help a lot with some trusses. (In MiTek Engineering software, this is located in Design Info, Fixities.) This feature reduces the amount of moment at the perimeter break joints for which the program is designing. Using Semi-Rigid Joints will generally increase the stresses on the lumber while decreasing the moment on the plates. This will typically decrease the plating requirements, but it may increase (we think this will be minimal) the lumber size or grade.
· With splices, an option to “Use solid bar tension values” (MiTek = Plating Options) will help. The downside to using this feature is that it requires stringent quality control. You must make sure the solid bar section of the plate (as opposed to the section through the plate with slots) is centered over the splice line. There is only an eighth of an in. (1/8-in.) positioning tolerance along the plate's length and the rotational tolerance is very limited (about 2 degrees). Although not recommend to use this feature as a rule, it may allow you to plate some joints that you could not otherwise.
· Reduce the length of a panel that is next to the peak or hip joint. Doing so reduces the moment at the joint so that a smaller plate may be used.
· Expanding your plate inventory to include more 18g high strength plates.
The use of these techniques may eliminate a large amount of the plating increases that we would see from TPI-07 using “out of the box” defaults.