A
Th e
Component Manufacturing dverti$ dverti $ er
Don’t Forget! You Saw it in the
Adverti$$er
August 2023 #15289 Page #10
Floor Truss Technology, Part III: Floor Design Intricacies By Joe Kannapell
he advent of floor trusses gave us new insights that continue to be refined today. They also gave us a peek into whole house design with the promise of a fully componentized house. For the first time, they challenged us to build something that people could live on top of, rather than just underneath. And this human factor gave us new design responsibilities that sometimes smacked us in the face in the early days. Fortunately, our TPI codes have developed the tools over the years that we’ve needed to properly design floor trusses, and there are even more in ANSI/TPI 1–2022.
T
The idea is hard to grasp: that any shallow parallel chord truss has much higher forces than a triangular roof truss of the same span. That was why the framer on the Jiffy Lube we supplied couldn’t believe that his 32 ft. flat roof trusses required much more lateral bracing than common 6/12 pitched house trusses. And, as floor trusses are much skinnier than flat roof trusses, the forces are proportionally higher, requiring upgraded lumber and plates. Then there are special conditions, like extended top chord bearings and duct chases, which amplified stresses, but especially deflection. Gradually, beginning with PCT–77, the initial Parallel Chord Truss criteria, design requirements began to account for these conditions. And today, ANSI/TPI 1–2022 section 7.6.2.3 includes enhanced deflection checks for floor trusses. Of primary importance is that occupants be satisfied with floor performance. This led to rafts of physical testing, with careful monitoring of deflection. Since early software could only handle fully triangulated trusses, a diagonal web had to be inserted across the duct chase, and an additional time-consuming iteration of the analysis had to be performed. This prompted TPI to allow use of the simple beam deflection formula for clear span floor trusses, which was backed up by physical testing. This formulation also underscored the 63% greater stiffness of floor trusses over solid joists, its main competitor at the time, but also an 18% advantage over one of the highest grades of today’s I-joists. However, this formula was limited to simple spans with centerline chases, which proved too limiting. Continued next page
PHONE: 800-289-5627
Read/Subscribe online at www.componentadvertiser.com
FAX: 800-524-4982