The Development of the Truss Plate: The Split-Ring Connectors Prequel

Issue #14276 - July 2022 | Page #10

By Joe Kannapell

For most of history, wood structures had been constrained by their connections. Nailed or bolted joints could only carry about half of what the wood could. As a result, our most abundant natural resource went mainly into homebuilding, where spans were short and stresses were low. That began to change, however, when the Timber Engineering Company (TECO) imported the split-ring from Germany in 1933. Their connector dramatically changed wood truss technology, in similar fashion to the truss plate twenty years later. For the first time, a joint designed with the split-ring could carry “100% of the safe strength of wood members,” enabling “…structures (that) may be prefabricated at any distance from the site,” and quickly becoming “The most significant development in timber framing since the advent of bolts and nails.”[1] After passing rigorous tests at the Forest Products Lab, the split-ring proved its viability by being installed in trusses which framed hundreds of government buildings throughout the balance of the 1930s. But it really came into its own in the build-up to World War II. [For all photos, See PDF or View in Full Issue.]

From 1940–1945, split-ring connected wood trusses formed the primary structural elements of the tens of thousands of “temporary” military buildings erected in the War effort. These trusses were constructed onsite for barracks and ancillary buildings through which ten million men would cycle. By using wood almost exclusively, this massive construction effort was able to proceed at break-neck speed without compounding wartime steel shortages, even while being an exacting labor-intensive process.

Perhaps the multiple steps required to install these connectors did not intimidate the experienced carpenters of that day. First, a drill press had to countersink a circular groove on each of the opposing member faces that would receive the split-ring. Then, when all joints were prepared, the rings would be inserted in the bottom member and the top member would be fitted onto the rings. Finally, a hole was drilled through the overlapping members at the centerline of each connector and a bolt with special large washers was installed. The fabrication proceeded outdoors, often under adverse conditions. Yet this tedious process enabled the first mass production of wood trusses, used an abundant domestic resource, and quickly and efficiently provided vast quantities of urgently needed shelter. The split-ring connector was incorporated in the National Design Specification for Stress Graded Lumber (where it is found today) and became the primary structural connection method into the 1950s. Its strength and durability has been attested to by the fact that hundreds of these structures are still in use. However, even with today’s technology, such as a Hundegger Saw to prepare joints, the assembly of truss members and bolting of connectors would be exceedingly difficult to automate.

Beyond the labor-intensive fabrication, there remained engineering challenges with split-ring trusses. For example, this typical 50-foot sloping flat truss, found in post theaters, warehouses, and maintenance structures, was more than 10 inches wide in cross-section and weighed more than one thousand pounds. When a series of these trusses over-deflected, engineers found significant splitting at the bottom chord splices emanating from the split-rings.[2] Their very detailed analysis uncovered some serious structural limitations of split-ring connectors. This, combined with their labor-intensive fabrication, their limited transportability, and jobsite handling challenges, set the stage for a better connector to be developed. And just as the military was abandoning wood-framed construction in favor of steel, a perfect storm was brewing in South Florida.