What is a Tension Control Bolt?
A tension control bolt is a pre-assembled bolt, nut, and washer used especially in steel frame construction. Also known as a TC bolt, they possess a domed head rather than a hex head because they are installed from a spline on their other end. This spline connects with a special power wrench for installation. Other names for this fastener set-up include twist-off bolts and LeJeune bolts.
Tension control bolts come perfectly pre-lubricated by the manufacturer. They are ready to be installed with a shear wrench. This wrench has an inner and an outer socket. The inner socket rotates the spline clockwise and the outer socket rotates the nut counterclockwise. Once the correct tension has been achieved, the spline breaks off.
Pros of Tension Control Bolts
TC bolts install easier than heavy hex bolts, DTIs (direct tension indicators), and other construction fasteners. There is just one lot number involved in the tension control bolt assembly and they come precisely lubricated. They are regulated by the AISC, American Institute of Steel Construction, and RCSC, Research Council on Structural Connections.
The single most unique feature of the tension control bolt is that it is installed with a precisely calibrated shear wrench that takes the guesswork out of achieving the correct preload (tension created in a fastener when it is tightened). This wrench connects over the spline of the bolt and once the correct tension has been reached, the spline breaks off.
Many feel that tension control bolts are more economical because they require only one person for their installation and inspection. Therefore, installation goes faster and with fewer mistakes.
Cons of Tension Control Bolts
Tension control bolts must adhere to specifications listed by the RCSC, as mentioned before and any degradation of the lubricant on the bolt will cause it to fail inspection. Once the bolts make it to the job site, they must be stored properly (away from outside elements) to protect their lubrication. Galvanized bolts have a more durable lubricant on them as compared to the water soluble lube on the plain bolts, but you still must store them correctly too. If something happens to your tension control bolts out in the field, you cannot just re-lubricate them yourself. They must be sent back to the manufacturer for them to make the corrective measures and retest them.
Something to be aware of with these bolts is that often times the installers like to put all the bolts in and then go back later to tension them. This is inadvisable because the bolts can lose their lubrication in as little as three days. Another thing to avoid is covering the bolts with a tarp in the outside elements. Condensation can form causing the lubrication to completely wash away. On the other hand, hot and dry weather makes the lubrication evaporate. It it seems the bolts are salvageable after being stored out in the weather, one must send them back to the manufacturer to have them re-lubricated and retested.
Some would argue that another con about tension control bolts is that they more expensive than both heavy hex bolts and DTIs (direct tension indicators). On top of the costlier bolt assembly, the shear wrench used for their installation runs at least $1700.
Just to wrap up some of the above points on the pros and cons of tension control bolts, here is a summary. They are easy and efficient to install. You must pay attention and store them properly. And lastly, it is arguable over whether they are more expensive or more economical than their heavy hex bolt and DTI counterparts.