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CROSS Safety Report

Collapse of lifting tackle connected by threaded bar

Report ID: 692 Published: 1 October 2018 Region: CROSS-UK

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Overview

A reporter describes how a set of 3t chain blocks and a 1m length of 20mm diameter threaded bar failed and fell from a height of 5m.

Key Learning Outcomes

For construction professionals:

  • Quality assurance and competent supervision on site can help to ensure that the works are constructed in accordance with the design

  • Consider introducing a quality control procedure for the inspection of safety critical connections for temporary works to ensure they are installed correctly 

Full Report

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No one was struck or injured when a set of 3t chain blocks and a 1m length of 20mm diameter threaded bar fell from a height of 5m, according to a reporter. The lifting tackle (chain blocks) was suspended from a hook attached to a lifting anchor. This consisted of a threaded bar system that joined 2 bars together using a proprietary coupler in accordance with an approved design.

The top of the threaded bar was cast into a concrete slab above an arch. The bottom length of the threaded bar had become unscrewed from the top length causing the bottom bar and attached lifting tackle to fall. The blocks were not being used to lift at the time.

The reporter identifies several causes for the incident, including that the anchor was not installed in accordance with the design, and the supervisor made a decision, which was not checked or condoned by the engineer, to install the arrangement.

the anchor was not installed in accordance with the design, and the supervisor made a decision, which was not checked or condoned by the engineer, to install the arrangement

Several lessons learned

The reporter lists several key messages learned from the event:

  • Teams should make sure that details of engineering designs are understood and implemented in accordance with the designs
  • Teams should make sure that they have assurance arrangements in place to check compliance with installations of design
  • Teams should engage with the workforce to check whether there are ‘local’ arrangements in place to manage issues that they observe whilst undertaking works

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

This is a tension system, and any tension system has to be looked at very carefully indeed. Over the years we have had several tension system failures (from numerous causes) but one characteristic is that the system can lack ductility which can lead to failure. The lack of ductility in this case was that the thread was not fully engaged.

Secondly, the consequences of any tension system failure are gross (fall distance will most likely be considerable) whereas this is much less likely in a comparable compression system. Lock nuts, locking screws, or other secondary means of achieving security should always be considered in tension systems.

Temporary works situations are as important as permanent works in such circumstances. Poor communication must be avoided, and competent personnel utilised who understand and implement what is required.

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