The Development of the Truss Plate, Part I: The Perfect Storm

Issue #14277 - August 2022 | Page #10

By Joe Kannapell

A nearly perfect confluence of circumstances combined to create the truss plate in South Florida. The wartime home-building hiatus, the post-war baby boom, and the northward (from Cuba) and southward (snowbird) migration to Florida all combined to skyrocket the demand for new housing. But what made the most impact, literally, was the force of hurricanes, and the competition to design a system to withstand them. [For all images and photos, See PDF or View in Full Issue.]

Four consecutive years of devastating hurricanes hit South Florida beginning in 1947 when 140 mph winds hit Miami-Dade County head-on. In 1948, a second one with 110 mph winds hit from the opposite direction, and in 1949 a third storm came ashore, continuing the carnage. If those weren’t enough, the worst one hit in 1950, the first year names were given to hurricanes. This one was appropriately called “King” and its 150 mph winds damaged some 21,000 homes. That’s why the Dade County building department had no choice but to raise roof loading to 55 psf, the highest in the country (while the state of Florida did not require adherence to a building code until 1974).

By the early 1950s, residents were retreating inland from the coast, onto the edges of the Everglades. There, foundations had to be more substantial than simply concrete slabs over sand, creating a strong incentive to avoid interior footings, and encouraging the use of clear span trusses. But Dade County Officials, in order to make sure these newfangled wood trusses (and their connections) would stand up to severe wind loads, developed a truss-testing criterion by working with Cal Jureit, the Chief Engineer at a local testing lab. As a result, building permit applicants specifying trusses were required to submit full-scale truss tests, certified by an engineer, to back up their design methods. Critical to passing truss tests were the connections.

The simplest connector tested was a metal plate with holes punched about one inch on center. In the test shown here of a 32 ft., 3/12 pitched truss, the heel joint alone required 45 nails to be hand-driven into each face of the truss. The entire truss required a total of 400 nails, at a time before pneumatic tools were available. For an order of 26 of these trusses to frame a modest 1500 sq. ft. home, more than 10,000 nails had to be hammered.

Another connector considered was the split-ring, which had been successfully used for 20 years, especially on longer span trusses. While this connector outperformed alternatives in truss testing, it required multiple installation steps, including routing, drilling, and bolting. And because of the doubled thickness and protruding bolts and nuts, it required more than twice the space on delivery trucks. This never proved to be effective in South Florida.

An enterprising young architect, Carol Sanford, representing the H-Brace Company, brought flat plates with nail holes and wedge-shaped teeth to Jureit’s lab. Although the H-Brace passed the Dade County tests, its wrap-around design worked well only for a chord splice, but not for many other joint geometries. Realizing its impracticality, Sanford also brought a glued-gusset truss to the lab, which worked well but Dade County rejected it, doubting the glue would withstand the Florida heat. Finally, a persistent Sanford brought in a new design which had a combination of teeth and nail holes, like the H-Brace, but was formed from a flat metal plate. The resulting truss design, which required half as many nails, passed the Dade County test, and Sanford applied for a patent and gained it in 1955.

Cal Jureit witnessed these tests, but they must have left him unsatisfied, perhaps because even the latest Sanford design required hundreds of supplementary nails on a truss. Shortly thereafter he left the lab and became a partner in an engineering consulting firm. But he would return with a better idea…