Twist-Off Bolts
This method usually defaults to "twist-off" bolts, sometimes called "tension-control" bolts. These assemblies function by calibrating the torque needed to twist off a splined extension manufactured into the bolt shank. Made correctly, the "twist-off" will occur at a bolt tension above the minimum required.
The main advantage of "twist-off" bolts is that they can be tightened from one side by one person, although bolt installers now realize that regular hex bolts and DTI's can also be installed one-side, one-man, too.
There are a number of disadvantages to the "twist-off" system:
- Special wrenches are needed.
- Special connection clearances must be detailed for wrench access.
- Frequently more expensive than hex bolts and DTI's.
- Galvanized twist-off assemblies are in VERY limited supply.
- Field relubrication is prohibited by code.
- Compacting plies must be accomplished prior to twist-off.
- Deterioration of the thread condition for any reason will change the torque-tension relationship, and Kulak has shown in an ASCE paper that in as little as three days out of protected storage, one-third of all the twist-off bolts he tested did not develop the required tension at break-off.
