Builders have been testing the viability of floor trusses, and they are now considering the results. Many of them switched from I-joists only because they were forced to do so. Either they couldn’t get them, or they didn’t want to pay double the price. Yet today, as they are seeing the increased availability and softer pricing of I-joists, will they stay with trusses? Did they experience enough advantages? Now is the time to solicit feedback from the field and to highlight the gains realized. While the open web advantage of trusses has been partially offset by the holes being drilled into I-joists, trusses are easier to connect, they make better use of material, and they are amazingly adaptable to jobsite conditions. [For all images, See PDF or View in Full Issue.]
Ease of Connectivity: A regional builder switched a townhouse job from I-joists in June 2023, built several buildings with trusses in the summer and fall, and was framing the 20th unit on a 22° winter morning. Each unit contained two levels of floor trusses framing into an LVL stair beam, which was, in turn, carried by a girder truss. The plies of the girder were easily tied together with FTC clips. However, after installation, their connection didn’t look much different than the doubled I-joists in earlier units. However, not visible in those units was a 2-1/8” wide filler member almost twenty feet long that was supposed to be nestled between the doubled I-joists and attached from both sides with an onerous number of nails. In addition, the framer benefitted, especially in the cold weather, from the simpler LVL to girder connections and the hanger-less top chord bearing details elsewhere on the job.
Material Optimization: Rarely emphasized is the advantage derived from trusses being custom-designed to optimize the use of materials. In general, truss lumber grades and cross sections can vary considerably along their span, to best resist design stresses. In this example, expensive DSS-grade material is only used in part of the bottom chord while less expensive #1 and #2 grades are used on the remainder of the chords. If an I-joist were substituted, the entire length of both the top and bottom flanges would need to have expensive material (usually LVL). In addition, the use of a partial double chord greatly strengthens this single-ply girder, while I-joists in this situation would have to be doubled, adding more cost and handling labor to the job.
Another considerable advantage of special truss design details is the use of short blocks derived from culls and off-fall. In the above truss, for example, twelve short blocks are utilized, and they were essentially “free.” I recently visited a large truss plant in Texas that installed a floor truss machine simply for the purpose of consuming the scrap generated from their two roof lines. In another large roof truss and wall panel plant in Maryland, so much scrap is generated in the roof truss shop that they can send pallets of short blocks to jobsites that might be better used in a floor truss operation.
Another example illustrates the advantage of customized truss design, in this case to provide for a drop in the floor deck, for either an entry into a handicapped shower or for a flush fireplace hearth. To frame this condition with I-joists (or solid wood joists) would require special designs and an extensive amount of hand-framing.
Adaptability: One final example recaps how the shape of trusses can be adapted to minimize additional jobsite framing. As shown, accommodation for two different bearing elevations can be built into the truss, while also providing a raised and narrowed cantilever soffit detail. No other product or system can come close to this adaptability.
As inventor Jack Schmitt observed in 1970 about floor truss construction, “Suitable lumber for this purpose is relatively inexpensive and difficult to obtain.” And today we might add, “and can be easily connected, optimally incorporated, and ideally shaped for the job at hand.” While builders may be attracted by reductions in I-joist pricing, they continue to seek alternatives to minimize the risk of being held hostage again by a handful of EWP producers.
Next Month:
Floor Finale
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