For most of the 20th century, engineers did not study nor would they normally specify wood trusses. Wood was thought to be suitable only for houses and barns, so steel was the material of choice – except when there wasn’t enough steel. That was the situation the Army Corps of Engineers encountered when all of this country’s steel was going into the weapons production for World War II and they were challenged to construct many thousands of structures for military training bases. Especially after Pearl Harbor, they had no time to waste and needed to devise a solution.
One of the main reasons for engineers’ reticence to use wood was the lack of connectors that could carry as much load as the wood members themselves. After 1933, however, that drawback was overcome with the split-ring connector, which, combined with our nearly unlimited supply of wood, made wood trusses a viable solution for longer spans. So, the Army engineers approved one of the first massive uses of split-ring trusses, with the caveat that the structures incorporating them would be designated as temporary buildings that would be carefully inspected every two years (see also “The Development of the Truss Plate: The Split-Ring Connectors Prequel.”)
Camp McCoy, in Monroe County, WI, witnessed the construction of more than 10,000 wood trusses over nine months in 1942, even though doubts remained about their durability. Camp McCoy was briefly deactivated after World War II, reactivated during the Korean War, and then deactivated again until the 1970s when it was set to become a major training center and was renamed as Fort McCoy. In 1981, as the Army began upgrading it to the status of a major troop training base, the Corps of Engineering Reserve unit to which I was assigned was given the mission of certifying its trusses. By this time, of course, (plated) wood trusses had compiled a strong 20-year long track record, but little data was available on wood trusses’ longer-term performance, which my unit was dispatched to ascertain.
Of the trusses there, the preponderance were either nailed-together 20 ft. span or split-ring connected 40 ft. span trusses. Surprisingly, although they had sustained 40 years of northern Midwest snow loads, we found only minor wood splitting, which normally results from drying of the lumber. Shown are some of the remedies for splits prescribed by the Army Field Manual, although in most cases we were able to prescribe a simpler solution – drill a hole at the end of the split to arrest it and then close the crack by using a small-diameter stitch bolt in place of the yokes and clamps.
These sample repairs vastly understate the difficulty of repairing trusses with overlapping members. In addition, the bolts securing the split-rings themselves must be periodically tightened due to shrinkage of the lumber, which becomes an often-neglected maintenance issue. Yet, even with these challenges, the Army engineers likely realized that building trusses in advance greatly expedited the construction of this mammoth facility, especially by enabling buildings to be closed-in quickly in the winter months. And today, these 80+ year old trusses are still standing tall on the same buildings we inspected that house the 100,000 troops who have trained at Fort McCoy every year since 1984.
Although these structures stand as a testimony to the long-term performance, practicality, and economy of wood trusses, the complexity of the fabrication of split-ring connected joints stands in stark contrast to the ease of building plated joints. It could be said that these World War II experiences paved the way for acceptance of wood trusses by the engineering and construction communities, and that all that was needed was the invention of the truss plate to launch their stunning success that followed.