CROSS Safety Report
Fall of bridge deck support due to bolt over-tightening
This report is over 2 years old
A 7t steel frame designed to temporarily support part of a bridge deck during bearing refurbishments work fell from height as it was about to be lowered some 21m to the ground.
Key Learning Outcomes
For the construction team:
Consider introducing a quality control procedure for the inspection of safety critical connections
Connections can often be the weak link in structures and attention to detail is required
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A 7t steel frame designed to temporarily support part of a bridge deck during bearing refurbishments work fell from height as it was about to be lowered some 21m to the ground. An operative who was standing on the frame at the time of the dangerous occurrence suffered minor injuries, inflicted by his safety harness as it arrested his fall.
The frame was supported on two air hoists, each of which was supported from a stub cantilever. The stub cantilevers were bolted to the web of a steel beam. It was the failure of this bolted connection that the investigation by the Health and Safety Executive (HSE) and others identified as the principal cause of the accident.
The detailed investigation concluded that the principal cause of failure was the accidental over-tightening of the bolts using an impact wrench which damaged the threads. In testing carried out on behalf of the HSE, the damaging of the threads and eventual stripping of an M16 Gr 8.8 bolt was repeated, using an impact wrench. It is noteworthy that the impact wrench in question has a quoted bolt range of M16 to M22.
The detailed investigation concluded that the principal cause of failure was the accidental over-tightening of the bolts using an impact wrench which damaged the threads.
The torque graph from the impact wrench manual indicates that the appropriate torque for an M16 bolt can be achieved in less than a second. It was also noted that although the design considered the bolt to be a non-preloaded bolt, operator experience was solely relied upon to achieve the correct torque.
For design, the correct torque settings for safety critical connections should be stated clearly on the drawings. Furthermore, the HSE says that critical connection designs should be checked by an external consultant and that critical connections must use high strength friction grip (HSFG) bolts with load indicating washers or tension control bolts (TCBs).
Expert Panel Comments
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The report states that this was a grade 8.8 bolt that failed. Such bolts should not be torqued because control is difficult and indeed they are not designed for this action. If pre-loading is required, the correct bolt to use is an HSFG bolt which has a heavier head and appropriate thread with a heavy nut to assure against thread stripping failure.
The report also states that the connection was through a beam web. That suggests the bolt grip length was very short and as such, any over torquing would have imposed a high plastic strain on very short length. Additionally, the torque tension relationship in bolts is very unreliable since the amount of friction within threads is uncertain.
If there is too much friction, then over torquing can cause serious damage. It appears that there was a failure to follow recommended procedures and there was a lack of experience. Good site control together with the proper use of HSFG bolts should prevent such occurrences.
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