In 2015, the ASTM consolidated the standards for structural bolts. Prior to the change, there were separate standards for high strength bolts including A325, A325M, A490, A490M, F1852, and F2280. Because these all fall under the high-strength structural bolting umbrella, these were changed to grades under the F3125 standard. Therefore, the newer standard of F3125 includes both heavy hex structural bolts and “twist-off” tension control structural bolting assemblies (including the nut and washer). In addition to the standard consolidation, certain technical aspects of the fasteners were also updated. Minimum tensile strength is now separated into a 120 ksi strength class and a 150 ksi strength class across the standard, and tension control bolts now have a maximum 1-1/4” diameter than the previous 1-1/8” diameter.
Though the standard has changed to F3125, a few basic things remain the same. Bolt head markings will remain as they were (see our bolt head markings chart), and Type 1 and Type 3 remain the same way to distinguish the makeup of the steel. Let’s go over the two types of F3125 bolts and their differences.
F3125 Heavy Hex Bolts
As you may remember in our blog on the difference between A325 and A490, Heavy hex bolts are the industry standard for structural bolting applications. Heavy hex bolts have a larger bearing surface to distribute the load. These structural bolts have a shorter thread length and a longer shank length. In structural bolting applications, this allows the bolt's threads to not be in the shear plane (where the two plates meet up). Generally, you don't want threads to be in the shear plane, because it can reduce the strength of the connections.
F3125 TC Bolts
Tension control (TC) bolts are pre-assembled fasteners with a rounded head and a splined end that shears off when the nut is properly torqued and the manufacturer's desired torque value is achieved. A great advantage to TC bolts is simpler installation and the amount of labor it saves. The installation tool is called a shear wrench, which is a lightweight tool that is quick, can be used by a single worker, and reduces fatigue. Birmingham Fastener distributes tension control bolts from 5/8” diameter to 1 1/4” diameter.