TC Bolts: A Structural Miracle Bolt

What is that broken part of that TC Bolt?

At first glance, it may seem as if someone may have destroyed a perfectly good TC Bolt.

But that fragment you may be looking at has just solved one of the biggest problems in structural engineering.

In the structural fabrication of buildings, bridges, and other steel structures, merely tightening down those nuts on those bolts is just not enough.

The bolts in those applications have to stretch by a precise amount, thereby creating what is called in engineering terms a “Preload”. A preload is a huge clamping or tensile force that locks steel plates together like a compressed spring.  In other words, the stretching of the bolts creates a force capable of compressing two materials together.

Why is Preload Important?

If there is too little preload the very connection made can loosen. It can loosen with vibrations and heavy loads- making you structure unsafe over time.

Preload also stops or minimizes Joint Gapping – it keeps structural plies tightly sealed, preventing the joint from opening up or separating under dynamic loads.

By keeping the joint compressed it helps to Avoid Fatigue Failure by minimizing the cyclic stress fluctuations on the bolt itself, vastly increasing the lifespan of the connection.

Too much of the preload and bolt can permanently stretch or even fail. Of course too much stretch will negate the clamping force needed.

For decades engineers and fabricators alike struggled to tighten every single bolt on a structure with exactly the same force- until the invention of Tension Control or TC Bolts.

Using a special tool, the splined end is gripped while another wrench or socket tightens the nut.

When the bolt reaches its engineered preload, the spline snaps off at the pre-designed weak point.

It does not break off because something went wrong, it breaks because it was designed to do so when the pre-load force is met.

So that fragment of metal off the bolt is actually a one-time mechanical indicator , instantly informing the fabricator installer the bolt has reached the correct clamping force without the need to use any special gauges, repeated torque checks or even guesswork.

With that in mind, sometimes the safest engineering solution is designing a mechanical indicator as part of the system which allows for that splined fragment to snap off to let you know the bolts was securely fastened.

By no means, or intent that the information presented here is the only factors to consider in the structural metal engineering and fabrication trade. Those well informed in this industry must be aware of other factors such as Rod/Stud Stress and the equations used to come up with the accurate calculations based on the other types of Bolting Hardware and other Individual Component Effective Length Values etc. – not discussed here

Sources: Calhoune Super Structure (https://calhounsuperstructure.com/resources/blog/use-turn-of-nut-for-accurate-pre-loading-of-bolted-joints/) ; Medium (https://bananajutsu.medium.com/bolted-joint-preload-9ead1f81511b) ; Nord-Lock Group (https://www.nord-lock.com/nb-no/innsikt/bolting-tips/2025/what-is-preload-and-why-is-it-important/); US Metal Designs (www.usmetaldesigns.com)