truss — A framework of triangulated forms in which all loads are carried by compression or tension in each member of the frame. It is time for the fire service to make peace with an old nemesis: the truss . For too long, the fire service has vilified this structural engineering marvel. Trusses...
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The truss that changed the construction world was patented in 1840 by William Howe, a millwright in Spence, MA. By reducing diagonal members to just one in each panel, the Howe truss was the first fully triangulated truss. The Howe truss relied on timber for its vertical compressive web members and iron for its diagonal tensile web members. (More on truss jargon later.) This famous truss design lasted well into the 20th century and provided a basic model for contemporary truss design. Like all trusses, it was popular for two reasons: ease of construction and low cost. The design of the Howe truss eliminated the need for skilled post-and-frame carpentry by using threaded iron rods for verticals and simple junction boxes as connections. For its time, it was truly a marriage of design and functional practicality. A legacy Howe truss could span up to 150 feet.
In 1844, another famous truss was patented by Thomas and Caleb Pratt: the Pratt truss. The original Howe truss used diagonal timber web members in compression and metal vertical web members in tension. The Pratt was engineered just the opposite: diagonal web members were in tension and vertical web members were in compression. Legacy Pratt trusses could span up to 250 feet. Another famous truss to emerge during the truss-patent frenzy of the 1800s was the Warren truss, patented in 1848 by James Warren and Willoughby and Theobald Manzani in Great Britain. What set the original Warren design apart from the Howe and Pratt trusses was the absence of vertical web members. The presence of just diagonal webs created a series of equilateral triangles between the chords. Each diagonal web member was alternately stressed in either compression or tension. The Warren Truss was the first to reveal true lightweight benefits: maximum strength with minimum material. Legacy Warren trusses were capable of spanning up to 400 feet. By the early 20th century, after the cost of iron had declined, variations of the Pratt had eclipsed the Howe and became the most popular metal truss bridge in the United States.
Although truss technology continues to evolve, just about all contemporary truss configurations are hybrid decedents of the Howe, Pratt and Warren designs. These legacy trusses relied on joint connections that were pinned with a single round "pin" (bolt, rivet) of some type. This hinge-like connection allowed the articulating pieces to flex (slight rotation) around the pin.
Plywood gusset plates appeared soon after World War II. (More on plywood gusset plates in a moment.) You can thank A. Carroll Sanford of Pompano Beach, FL, who in 1952 invented the metal "gusset plate." With the invention of the metal plate connected truss, the manufacture of lightweight wood trusses quickly became a mass-production industry.
Blame Chicago for truss proliferation. Actually, blame the railroad expansion during the 1800s; however, following the "Great Fire," Chicago was an architectural clean slate where the "vernacular" traditions of skilled craftsmen would be eclipsed by the theoretical analysis of the engineer. Emerging from design and construction techniques germinated during the design of railroad bridges, post-fire Chicago was ready to become America's first "engineered city." Railroad bridge engineers were key players during the design of America's first "skyscraper," Chicago's now-demolished, 10-story, steel-framed Home Insurance Building, which was constructed in 1884. The world continues the engineering and architectural evolution that was spawned by the need to get a train from one side of a river to the other and Chicago's Great Fire.
The triangle is the secret of truss strength and stability. A truss is simply a series of triangles. The triangle is a naturally rigid geometric shape that resists distortion such as bending. Because it is impossible to change the shape of a triangle without lengthening one of its three sides, a triangle is rigid, stable and strong.
Consider a four-sided geometric shape such as a rectangle. Because you can lean the rectangle to one side without having to lengthen or shorten a side, you can create a different geometric shape, specifically a parallelogram. Thus a four-sided form is unstable and has limited structural value. The net effect of triangulation efficiency means that a truss comprised of multiple panels essentially behaves as if the entire truss was the size of just one panel within the truss. Although it's not magic, truss engineering comes pretty close. Older conventional buildings were supported by mass, such as the excess wood within a heavy timber beam, and were supported by numerous columns in compression; a modern, lightweight building has reduced structural reliance on mass and compression by supporting structures with math — specifically geometry. A structural system that relies on mathematics produces a structure that relies heavily on tension.