On construction sites there have been many incidents of structural fasteners failing due to incorrect torqueing (tightening).
These failures have occurred with many types of fasteners, including cast-in and drilled concrete anchors, as well as structural bolts in steel work and on mechanical plant.
Fastener failure has resulted in structural collapse, plant malfunctions and objects falling from heights, all of which can result in death or injury.
A contributing factor is often the incorrect use of rattle guns, also known as torque guns or impact wrenches, which results in fasteners being over-tightened or under-tightened.
Rattle guns (including those that are battery powered) may have torque ratings that can greatly exceed the maximum allowable torque specifications of fasteners.
Most rattle guns are not torque calibrated and, if fitted with adjustable settings, may have a limited range of adjustment.
Torque can also be reduced if:
- an incorrect or worn socket is used
- the battery is old, or
- battery charge is low.
Fasteners that are not tightened to the manufacturer's specified torque have the potential to vibrate loose or break.
Most manual torque wrenches will not show if a fastener has been over tightened and will only indicate if the fastener is under torqued or has achieved the specified torque.
Recommended control measures
Where a fastener needs to be tightened to a specified torque, this should be done using a calibrated torque wrench and not a rattle gun, as this often leads to over-tightening or under-tightening.
Ensure adequate training
The installer must be instructed or trained in a safe system of work for fastener installation, including:
- inspection and use of tools (e.g. rattle gun, sockets, spanners, manual torque wrench)
- specified torque requirements for fasteners
- acceptable methods for pre-tensioning fasteners - if a rattle gun is used its output torque should be lower than the final setting torque of the fastener or a torque limiting impact extension should be used for high torque rattle guns.
- the method for final torqueing of the fastener using a calibrated manual torque wrench.
Use appropriate tools
The builder/principal contractor is responsible for ensuring the installer has the appropriate tools (e.g. appropriate calibrated manual torque wrench and correct size sockets).
If a rattle gun can be adjusted and calibrated to deliver the required torque then it can be used to tighten the fastener instead of a manual torque wrench. However, if a torque controlled rattle gun is used, fasteners should be periodically tested to verify that the torque specified by the designer or fastener manufacturer has been achieved and not significantly exceeded.
Always refer to the manufacturers of fasteners and rattle guns for specific torque specifications and torque ratings.
Note: This guidance is based on an archived WorkSafe Victoria safety alert, originally published July 2014.
- AS 3850.2:2015 Prefabricated concrete elements, Part 2: Building Construction