If there is any lasting image of how large steel structures used to be built, it is probably the hot riveting process. You’ve probably seen grainy, old black-and-white movies of an intriguing gang – men universally dressed in bib overalls, with no safety equipment other than cigarettes, heating rivets to red heat in a forge and throwing them at the riveters with a pair of tongs. There the rivet is caught by a metal funnel, or even a gloved hand, slid into a waiting hole in the flange connecting a beam to a column, and beaten into submission by a pair of men with air hammers.

Although the work was dirty, hot and dangerous, hot riveted joints were a practical and proven way of joining elements together in steel structures, and any commercial building from before the 1960s is likely to have at least some riveted joints. . However, times are changing and technology is advancing and riveted connections are largely becoming obsolete in the construction industry, being replaced by bolted connections. Riveting teams of three or more people were replaced by a single ironworker making hundreds of predictable and precisely tensioned connections; This resulted in better connections at lower costs.

Bolted joints torqued to spec with an electric wrench may not look like angry rivets flying around the job site, but there’s definitely a lot of engineering behind them. And as it turns out, the secret to making bolting a one-man job mostly lies in the bolt itself.

A Table with a View

My first encounter with tension control bolts started when I got really lucky at work. In the early 2000s my department moved and I somehow managed to get a desk with a real window. It was great to be able to look at the world from the outside, but one day the company started building an additional building right outside my window. This was a mixed bag for me; True, I would lose my view once the six-story structure was built, but in the meantime I would be able to watch its construction from the comfort of my desk.

I watched in amazement as the steel frame rose upwards, the ironworkers quickly and efficiently bolting the columns and beams together. One thing I noticed was that bolting seemed to be a one-man job; A single ironworker was tightening the nut with an electric wrench without needing to support the head of the bolt on the other side of the joint. This surprised me; How does the bolt not rotate in the hole?

I got my answer when I saw something fall out of the wrench after the blacksmith removed it from the tightened joint. From my perch by the window, it looked like the end of a grooved shaft, and I could see that one end was clearly sawn off. That’s when I noticed that all the bolts that hadn’t been tightened yet had the same threads leading out of the nut, and they all clicked into place: the thread should fit into a socket inside the wrench, which tightens the nut and is coaxial with the socket holding the bolt. so the nut can be tightened. Moreover, it was obvious that you could use this scheme to automatically torque the connection by designing the spline to cut at the required torque. Genius!

Stretch and Snap

While I wasn’t exactly right in my analysis, I was pretty close. I only learned much later (for example, while researching this article) that the bolts used for structural framing are called tension control bolts, or TCBs, and that there is a lot of engineering involved in them. But to understand them, we must take a look at bolted joints and learn how they work to prevent everything from buildings to bridges from collapsing.

we bought an in-depth look at bolted joints before, but the short story for the TL;DR set is that the bolts are actually really strong springs. When you tighten the nut on a bolt, the bolt flexes a bit, which provides a clamping force on whatever is trapped between the bolt head and the nut. The degree of tension and therefore the amount of tightening force depends on the strength of the material used to make the bolt, the size of the bolt and the amount of torque applied. Therefore, most bolted assemblies have a specific torque for all bolts in the connection.

For structural steel, connections between frame members are carefully designed by structural engineers. Numerous calculations go into each joint, resulting in detailed bolting plans. Some connections have multiple bolts; sometimes 20 or more depending on the application. The hole pattern for each member is determined before any steel is cut, and each framing member usually comes from the manufacturer with the exact number of holes specified on the plan. The plan also specifies what grade of TC bolt will be used in each connection (more on that below), as well as the diameter and length of each bolt.

When ironworkers build the frame, they first use a cotter wrench to line up the bolt holes in the two members they bolt together. A clevis wrench is a large, open-ended or adjustable wrench that tapers to a point on a long handle. The shank is used to align the bolt holes while the ironworker inserts a TC bolt into other holes. The bolts are initially tightened by hand only, but this is followed by a critical part of the assembly process called clamping or pre-tensioning.

Snugging is defined somewhat loosely as tightness achieved with “a few blows” of an impact wrench, or “with all the effort of an ironworker” using a standard nut driver. Everything related to winding is highly subjective, as the number of “ugga-duggas” that count as several impact wrench bursts varies from user to user, and ironworkers alike can apply a wide range of force to a wrench. But the goal is to bring the framing members into “tight contact,” which usually means about 10 kps or “kips,” which means 10,000 pounds per square inch (about 70 MPa).

After all the bolts in the combination are pre-tensioned, the final tensioning process is carried out. The tool I saw ironworkers use on TC bolts years ago goes by many names; the most common being the “cutoff switch” or “TC gun”. It is also known as the “LeJeune gun” after a major TC bolt and tool manufacturer. Some cut-off switches are pneumatically operated, but more are electrically operated and cordless guns are becoming increasingly popular. The final tightening cycle begins by threading the TC bolt wedge into the inner slot and the nut into the outer slot. The outer socket tightens the nut to a certain torque; At this point the slipper clutch shifts the power transfer from the outer socket to the inner socket, reversing the direction of rotation in the process. This applies sufficient torque to the spline, rotating it to clear the TC bolt at its weakest point (the narrowed neck between the thread and the threaded portion of the bolt). This ensures that the bolt is properly tensioned and shows the correct amount of thread.

Trading Tools

TC bolts generally come in two grades: A325 and A490. Both are based on ASTM International standards, with A325 bolts covering the tensile range of 120 to 150 kps (830 to 1,040 MPa) and A490 bolts covering the range of 150 to 173 kps (1,040 to 1,190 MPa). Most TC bolts have a round head because there is no need to hold the bolt by the head end. This provides a smoother surface on the head side of the joint, reducing the chance of damage during installation. Depending on the application, TC bolts can be treated to prevent corrosion by galvanizing or passivating.

If a TC bolted connection needs to be disassembled, it is a problem if the spline is already broken. To overcome this, a special accessory for the switch is used, known as a reaction rod. This is essentially an inner socket sized for the nut and an outer ring with a solid torque arm welded to it. The lever tightens against and adjacent to the nut, providing the reverse rotation required to loosen the nut.

Lot testing is also very important for code compatibility. This involves randomly selecting TC bolts from each batch to test on a Skidmore-Wilhelm machine, which hydraulically measures the tension on a bolt. Strict procedures are followed for the pre-tensioning and post-tensioning of each bolt, and the results are recorded as part of a structure’s engineering records.