Structural-Grade, Self-Calibrating TC Bolts For Infrastructure Applications

TC bolts, also called tension control bolts or twist-off bolts, are a type of structural fastener that carry some very unique attributes. They are used as an integral solution for connecting metal parts in many infrastructure placements.

At their most basic, TC bolts describe a hex or round-headed bolt that is partially-threaded. They usually have shorter thread lengths compared to bolts that are not used in heavy structural applications. The bolt ends in a part called a spline or twist-off, which serves as a tension-control device.

TC bolts are often paired with a nut and a washer as part of a total fastener assembly, which comes pre-assembled by the manufacturer. They are also lubricated as part of the manufacturing process, which contributes to their self-calibrating capabilities.

To install TC bolts, the bolt should be inserted into the parts being connected. The washer should then be added to the opposite end so that it functions as a barrier between the nut and the installation material. The nut of the TC bolt assembly is then torqued. This is done with a hand-held electric tool known as a shear wrench or TC gun. This tool is specially designed to simultaneously torque the nut and the spline of the bolt but in opposite directions. Two things occur when this force is applied: the nut is tightened onto the threads of the bolt and the spline is sheared off the end. Once the spline separates, this indicates that optimal tension has been achieved.

In addition to this built-in capability for ensuring consistent and secure installation, TC bolts are also very fast to install. Just one person can complete the process. These and other advantages have made TC bolts a favored alternative to structural rivets, which were previously used as a standard in many of the same types of connections.

In general, TC bolts are considered high-strength, structural-grade fasteners. Their strength capacity and resistance levels will vary based on their type, material, and other factors. The most common types of TC bolts now fall under ASTM F3125 specifications, which are set by the American Society for Testing and Material organization. This spec is used to replace ASTM A325/F1852 and ASTM A490/F2280.

Previously, TC bolts were most commonly categorized as A325 TC bolts, which are standard duty and exhibit a tensile strength rating ranging from 105,000 and 120,000 psi. These bolts are also called F1852 TC bolts and are made from medium carbon steel, carbon boron steel, medium carbon alloy steel, or weathering steel.

Heavy-duty TC bolts are categorized as A490 TC bolts, which are made from alloy steel and have a tensile strength capacity between 150,000 psi and a max of 173,000 psi. They are also known as F2280 TC bolts.

Added corrosion resistance can be gained with galvanized TC bolts, which are coated with zinc to prevent rust. Not all TC bolts can be galvanized; those intended for heavy-duty, extended strength applications should not undergo hot-dipped galvanization in order to prevent risks of hydrogen embrittlement.

When TC bolts are required for construction jobs, they should always be sourced from a reputable structural fastener supplier or manufacturer. To install and perform reliably, TC bolts must be made and tested to many integrity standards. This is why their source matters.