WITH the rise of modularised skids on sites and the ever increasing focus on safety within the mining sector, adequate cranage and lifting devices are still one of the key safety considerations while on site.
The correct selection of lifting sets is essential to be able to lift both safely and cost effectively.
CDMS senior structural engineer Blake Thomson says that today, most lifts are performed with dedicated lifting lugs rather than wrapped chain.
“This provides a well-defined load path as well as preventing ‘squeezing’ of the lifted item as the chain tightens during the lift,” he said.
The two major lifting orientations are single point lifting and lifting with a spreader bar, both of which have their advantages and disadvantages.
Single point lifts are often preferred for their ease of use on site due to the lower number of elements, when compared to spreader beam lifts.
However despite being easy to use on site, they are more difficult to analyse. Diagonalling is a common issue for single point lifts, since it is impossible for all cables to be cut exactly the same length.
When the frame is lifted one of the cables can go slack, this causes the frame to balance until two opposite slings are holding 100 per cent of the load between the two.
As these loads are only present in two of the four corners of the frame, additional bending moments are generated within the frame from the component loads.
Spreader beams have the ability to provide even load sharing between the lifting points. Subsequent spreader beams used in the set allows not only even distribution but it removes the lateral component of force which is present in single point lifting.
This means that the load path is often similar (to an extent) to the item while it is in service, therefore increasing the likelihood that the item will be structurally sound during lifting.
The disadvantage with using spreader beams is the increase of the rigging height and subsequent costs related to transporting the additional items and their manufacture.
These systems are often utilised if the equipment obstructs the sling path for a single point lift, or for items that are rarely lifted, where the higher forces and a more complex load path of a single point lift could cause failure of the structure.
The lifting lug itself is often a point that requires considerable attention as it is the main interface between the rigging and the structure.
Some critical items for consideration when designing lifting lugs are practical clearances for the rigging and lug orientation.
Often lifting lugs are designed without correctly considering the working point of the lug during operation, which can lead to unintended bending moments entering the structure, and if the unit is already highly utilised during lifting this can possibly lead to failure.
Mr Thomson said many sites boasted a variety of lifting specifications which required compliance, including weld configurations, non-destructive evaluation or minimum dynamic amplification factors (DAF) that are required on site.
“CDMS has had vast experience in lifting all kinds of items, from dedicated baskets, site plant such as pressure vessels, modularised skids, preassembled bolted structures and large fuel storage tanks using both single point and spreader beam designs,” he said.
“If you need any assistance with particular lifting compliance challenges, I recommend using an experienced design engineer who understands the lifting specifics.”
Content provided by CDMS