Floor trusses have been golden for the component industry for over half a century, and their origin is worth recounting and celebrating. Because of their flexibility and utility, wood trusses dominate apartment construction and have taken it up to 5 stories. Getting there, however, has required the collective minds of CMs and plate manufacturers over these years, developing innovative details that expedited construction and lowered costs. And as robotic manufacturing technology comes on stream, they’ll be even more competitive. [For all images, See PDF or View in Full Issue.]
The predecessor to the wood-web floor truss was the steel-webbed TJL, developed by Art Troutner and Harold “Red” Thomas who formed Trus Joist in 1960. TJLs featured tubular metal webs connected to chords by steel pins. Their use spread quickly in the wood-friendly Northwest, and, as Trus Joist made deals with nationally franchised entities, they gradually made their way across the U.S. Trus Joist heavily promoted the TJL to architects and engineers through their highly effective representatives across the country, like Bruce Gordon, who later established Structural Systems in Maryland, and Steve Jones, who is now the proprietor of Structural Technologies in Virginia.
Another steel-webbed truss was introduced in 1968, by the inventive Carol Sanford, which provided CMs the opportunity to enter the business with minimal investment. Sanford’s innovation took advantage of the superior strength of steel in tension and incorporated it into a structure compatible with wood construction. His tension web was also easy to manufacture, since its flat ends did not contain teeth but only a series of holes that would receive the teeth of a conventional truss plate. His design also included steel reinforcement straps on both top and bottom chords to enhance their strength. These straps were essentially long truss plates that would be stacked in multiple layers as forces dictated: as shown here, none in the first panel, one in the second panel, two in the third panel, and three in the center panels.
While these steel straps would give truss chords incredible strength, the forces that resulted in the tension webs would increase substantially beyond their ability to be connected to single wood chords. It is not surprising that Sanford’s chief engineer, Bill McAlpine, did not sign on to this invention as he did others. McAlpine would have omitted these redundant straps, and so did the CMs who built these, like Hall of Famer Staton Douthit at Associated Truss in Texas. Another limitation of Sanford’s conception was the limited capacity of the top chord bearing detail shown here, which limited spans and made bottom chord bearing details preferable. Unfortunately, with bottom chord bearing end details comes the risk that trusses can be installed upside down, necessitating complete retrofitting. Despite these potential downsides, this product could be produced without a web saw, introduced the concept of a center duct chase, and was a valuable precursor to the wood-web floor truss.
Finally, in 1970, after Jack Schmitt gained control of Truswal, he introduced the wood-web floor truss based on the work of one of his early hires, Carl Virta. Not known is whether Virta or Schmitt were influenced by either the TJL or Sanford’s tension web. Yet both of these products opened the door for this great complement to roof trusses.
When plate manufacturers saw the (exaggerated) size and the large quantity of truss plates on Schmitt’s patent drawing, they rushed to develop their own offerings. Most of their existing design programs could handle simple span floor trusses by adding a fictitious diagonal at a centerline duct chase, but many struggled with off-center chases. Even the doubled chords shown on Schmitt’s end detail required design program adjustments, and we at Hydro-Air could not determine why Schmitt showed doubled chords at the end of a truss. We did prove the effectiveness of doubled chords that were located near center span where stresses were highest. And this is how the state-of-the-art advanced, design-by-design, at each design office until the industry know-how was aggregated in 1977 into Design Criteria for Parallel Chord Trusses (PCT–77).
During this same formative period, the main nemesis of the wood floor truss would also arise, the TJI. For many years, tract builders preferred the flexibility of open web design and the utility of wood web bearing details. However, the TJI and other I-joist products could be manufactured en masse, in a highly automated factory, with much lower labor costs than CMs could achieve. With the onset of automated processes though, this advantage can be lessened and the growth of floor trusses can be accelerated as they enter their golden years.
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