Have you ever tightened a bolt? How tight is tight enough? Is it possible to tighten it too much? Whereas these questions might not matter as much when you assemble an IKEA side table for your living room, they sure do matter when you are tightening bolts in a member connection for a steel building. One convention is to follow the “snug-tight” method where bolts are tightened until first sign of resistance and then given another quarter turn.
Another convention is to use direct tension indicators. Direct tension indicators are washers that have small protrusions on them. When the bolt is tightened, the gap between the unturned element and the washer decreases. The size of the gap is then measured and when it reaches the desired size, the bolt is pronounced tightened. It is often required to have all the connections in a building inspected to ensure tightened adequacy. But to go and climb up to every connection and hand check each bolt is both extremely time consuming and still subject to human error, especially when there are a lot of connections and thus a lot of bolts to inspect.
Then a special machine is used to tighten the nut without turning the bolt. But here is where the ingenious part of the design comes in. When it has been tightened to the required torque, the small spline pops off! So that is when you know that the bolt is tightened just right and, as an added bonus, it makes for a very quick, easy, and thus efficient connection inspection, you just have to look for all the bolts to have their splines popped off.
Snug-tight conditions and direct tension indicators are still widely used today, so tension control bolts are just another option to use in construction, but a pretty innovative option!