The images of hard-hatted workers, perched hundreds of feet above the growing cities of the early twentieth century have become iconic. Their work, joining the connections of skyscrapers, bridges, and tunnels, created what we recognize as modern infrastructure. The methods and the fasteners they used to do so have become as antiquated as many of those black and white images.
While rivets were once the quintessential option for connecting and erecting steel structures, they are no longer used in this vital capacity. Instead, TC bolts are the standard fastener option for major infrastructure applications. Why did this change occur? Are TC bolts simply better than rivets? The answers are a bit more complex.
Although they are not an infrastructure standard, rivets are still used for many other purposes. These fasteners are reliable, economical, and will connect permanent joints without adding a lot of extra weight. Rivets are still a common fastener in the aerospace industry. They are also used on various equipment and lighter-duty structural applications where a sleek profile is needed.
Rivets were not phased out because of any major flaw in their performance, but the process of installing them on structures like truss bridges and skyscraper frames requires special skill and a lot of labor. Structural rivets must be heated, hammered, and caught, which requires multiple workers who are all trained and efficiently acting as a team.
Although they provide a tight hold, do not require a precisely compatible hole size, and the result is a tidy esthetic, rivets cannot compete with a fastener like a TC bolt. This is why you will seldom see rivets on American bridges built after the 1970s and will instead see A325 tension control bolts or similar fasteners used in their place.
Tension Control Becomes The Go-To Choice
TC bolts became the go-to choice for many infrastructure connections because of their unique advantages. Tension control bolts or twist-off bolts are actually fastener assemblies. They are comprised of a dome-headed bolt with an end piece that’s called a spline or twist-off. They are partially threaded and include a nut and a washer. They are also pre-lubricated, which is integral to their installation, which is completed with an electronic wrench called a shear wrench or TC gun.
Compared to rivets, TC bolts are more expensive and complex to manufacture, but the process of installing them is faster and requires far less labor. Instead of requiring several riveters to handle the installation of a single rivet, TC bolts can be installed by a single worker using a shear wrench. TC bolts are also a favored component for fast installation because they remove any uncertainty on when the correct tension is achieved.
TC bolts are self-calibrating by the nature of their design. As the shear wrench torques the nut and the bolt’s spline in opposite directions, the spline will shear off, which indicates the correct tension has been reached. This can amount to a more consistent performance compared to rivets, which may have a slight variation in strength based on the compression force achieved during cooling. Galvanized tension control bolts take care of many priorities for modern infrastructure, from resistance to rust to long-lasting support.
The Legacy Of The Structural Rivet
Even if they are no longer used in many heavy structural capacities, rivets still have a legacy all across America’s bridges, tunnels, railroads, and skyscrapers. Because of their gradual phase-out in the 1960s, a simple way to identify historic bridges and other forms of early twentieth century architecture is to simply look for the rivets. They may be aging and they may not be an optimal fastener option for today’s rapid-paced developments, but they still get the job done.
