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November 2020 #12256 Page #78
The Enduring Problem of Truss Partition Separation
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Don’t Forget!
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HINGS WO
By Frank Woeste, P. E.
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or more than 40 years, truss partition separation, often loosely referred to as “truss uplift,” has been a practical (and annoying) issue for homeowners and homebuilders. While it was first investigated at the field level and researched in the laboratory in the 1970s, an October 2020 JLC Online post, “Crown Detail Hides Truss Movement,” reminded me of the issue and its history.
Can Truss Movement Contribute? As a PhD graduate student at Purdue University in the 1970s, I had the opportunity to be involved in possibly the first structural analysis of this phenomenon with the type of truss software used today by plate companies and component manufacturers (CMs). The late Professor Don Percival at the University of Illinois had observed trusses that had separated from a partition in the winter months up to ¾-inch and shared this information with the late Dr. Stan Suddarth. In some cases involving major separation, Don observed ice/frost on the top chord and bottom of the roof sheathing. Knowing it was during the heating season and having the presence of ice/frost on the top chord and sheathing, we theorized that the very “odd condition” was due to expansion of the top chord and shrinkage of the insulated and warm bottom chord, due to it being dried or kept dry during the heating season. It was then my job to simulate different levels of chord expansion and shrinkage in Stan’s pioneering computer program (known as the PPSA). By making a range of assumptions which dictated the amount of chord expansion and shrinkage that was possible, for one case the program predicted “arching” up to ¾-inch which matched a reported maximum amount. The maximum arching was predicted by assuming 100% juvenile wood in one chord, coupled with assuming a maximum differential in moisture content of the top and bottom chords. The structural model assumptions that predicted the ¾-inch arching represented a “worst case scenario”, possible but not likely. While the longitudinal shrinkage rate of lumber is low for “mature” wood, it can be up to 10 times more for juvenile wood. The innermost growth rings are referred to as juvenile wood, whereas the outer rings are referred to as mature wood.
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