The 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.
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. [For all images, See PDF or View in Full Issue.]
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.
Try as we may, we couldn’t discourage CMs from off-center duct chases, and fortunately we had recently picked up a copy of the newly released PPSA program in 1972. Yet the input was overwhelming, and we had to pour over the tabular output and limit the deflection in the chase, to guard against the possibility of dips in the floor. To simplify the analysis, TPI added another approximation formula, to account for the increased stress and deflection, if the chase was not too close to a bearing.
As top chord bearing floor trusses became increasingly popular, reaction limits were added to TPI, which were based on a maximum of ½” gap between the bearing and the adjacent web member. However, field conditions often resulted in a larger gap, which magnified stresses and local deflection, but is difficult to model with software. The amount of deflection may seem negligible, but not when a top bearing truss is adjacent to a bottom bearing truss. In the floor section shown, the F5s have extended top chords bearing upon headers over a doorway, while F4s bottom chord bear. An excessive gap between the header and the F5 webbing will exacerbate bending of the extended top chords, while the adjacent bottom bearing trusses will have little or no deflection in the same area, leading to a dip in the floor deck.
Although ANSI/TPI 1–2022 will not be effective until the 2024 International Building Codes (IBC and IRC) are adopted, the refinements summarized here are important enhancements in the 50 year evolution of floor truss design and add to the versatility of the product.
Next Month:
Floor Machines Adapt
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