A Dropped Load
Several hundred pre-cast concrete reinforced panels had been craned and installed without any obvious safety issues when a panel detached from the basketed flat sling arrangement and fell to the floor below.
The slings had not failed, it was obvious that one sling had slipped towards the centre of the load allowing the load to slip out of the supporting flat slings.
This is a story of successive human and organisation errors, which resulted in the event and is about the knowledge, reliance and application of friction to secure lifting slings to loads.
The attached video supplied by the Department of Mines, Industry Regulation and Safety – Western Australia on “Friction” relates to this incident. https://vimeo.com/108303043
- Location of lifting points on the panel and lifting configuration were not specified by the manufacturer, it was left up to the rigging crew to decide.
- The surface of the panel had a shiny finished surface and rounded chamfered edges where the slings made contact.
- Two flat slings in a basketed arrangement were used to lift the panel. The slings were not “choked” around the load and primarily made contact with the smooth underside of the panel.
- The flat slings had a protective “heavy duty” plastic sleeve to prevent the sling being cut by sharp edges. We now had a “Smooth” on “Smooth” contact surface between the slings and load.
- The flat slings were attached to a four-legged chain configuration which were attached to the crane hook.
- The flat slings were not supported in a vertical orientation by the lifting chains but were oriented at an angle to the load contact surface – see attached DMIRS video as an example of the sling angle of attack.
- This resulted in the flat sling not making full contact with the load surface – with the outside leading edge of the sling taking up the load and the inner edge of the sling taking up little to no load. Due to the angle of attack – a force was being applied to each sling, which tended to pull the slings into the middle of the load.
- Several hundred panels were rigged in this fashion and craned into place. The rigging and crane crew were comfortable with this arrangement as nothing had previously happened and there were no direct indicators of potential trouble.
- As the lifting chains were heavy to manage, it was decided to use a lighter weight and shorter set of chains, as it was determined that this would make it easier for the rigging crew to manage the lifts. The shorter set of chains decreased the flat sling angle of attack between the sling and load, increasing the force pulling the slings into the centre of the load.
- We now had a “Smooth” on “Smooth” basketed flat sling in a non-choked rigging configuration with an increased horizontal force on the slings due to the use of shorter lifting chains. It was a “Hair Trigger” situation, waiting for a slight change in circumstances.
- Several panels had been lifted using the different rigging configuration when “Bang” it all happened during a crane lift, one of the flat slings slipped and the load fell out of the slings to the level below.
There are several obvious points in this sequence of events, where human intervention would have prevented the incident from occurring.