A while ago I wrote about method statements and the ability of the person receiving such statements to read and comprehend them. Often they are neither read nor comprehended but simply kept on file. In this case, however, the personnel concerned have sufficient knowledge and experience to judge whether the methods proposed appear viable and adequately safe, and to enquire further if they are in any doubt.
For just such a reason I recently had a call from one of them about an operation involving a four-point lift utilising the building structure for support. Several matters were cause for concern. Not least of these was the proposal by the contractor to connect to the structure by the rather crude method of burning holes through the beams.
Using a building structure for support can be a perfectly acceptable practice, provided it is done correctly and the necessary checks are made. It is essential to check that the structure is adequate for the purposes of lifting. However, because of the other loads which are imposed, a simple calculation based on the load to be lifted or a proof load test is not adequate. The following procedure is therefore recommended.
A theoretical check on the structure should be carried out by a structural engineer or other suitably qualified person. To do this, the engineer will need to know the position, direction and amount of the load to be imposed and the method of connection. The connection between the lifting appliance and the structure should be properly designed for lifting purposes (a beam clamp, for example), and tested and certified. The user should obtain written confirmation of the adequacy of any such structure and this should be kept with other lifting equipment records. It is also advisable to mark the structure for future reference so that the position can be clearly identified.
Assuming that the structure was adequate, my next cause for concern was the rating of the equipment. The load weighs nearly 6t and they proposed to lift it on four hand chain blocks each of 2t capacity. Clearly there is likely to be some difference in loading between the four blocks and, at first sight, an excess capacity of over 33% might seem quite adequate. Indeed, whilst lifting, it probably is a sufficient margin of safety. The real problem comes when lowering.
The reason, quite simply, is that the effort required on the hand chain to lift a load is directly related to the load. So, when lifting, if one block gets ahead of the others, the operator of that block will notice that it needs more effort and will naturally slow down.
Equally, the blocks left behind need less effort, and their operators will speed up. Thus a reasonable share will be maintained.
When lowering, the situation is reversed. If a block gets ahead, the effort it requires gets less. In effect the block gives its share of the load to the others. With a four-point lift of a symmetrical load, only two blocks across one of the diagonals will be loaded. In this job that would mean nearly 6t on 2 x 2t capacity blocks, an overload of nearly 50% on each. The minimum rating of the blocks for this job should be 3t and that assumes an equal distribution of load between any pair across either diagonal. If that cannot be guaranteed or monitored in some way, then a further allowance is required.
The other consideration is the effect on the load. Why is it being lifted on four points? Is it because the load needs such support? If so, will it be distorted by its own weight if it is lowered on only two? If the four point lift has been chosen because it is being lifted from beneath the centre of gravity, then stability may be an issue when lowering. Care will be required to keep sufficient tension in all four blocks to avoid the load suddenly rocking from one side to the other, imposing shock loads on the equipment and structure.
The same situation occurs with jacks. Hydraulic jacks generally have a relief valve within the pump mechanism. This is set to the pressure required for about 110% of the safe working load and prevents excessive overload when operated to lift. However, that will not prevent overload when the load is imposed externally as is the case when the pressure is released on another jack in a multi-jack operation.
Ensuring that equipment is adequately rated for the share of the load imposed on any component or item should be a routine part of the planning process. Usually, sharing the load is not too great a problem with a multi-point lifting beam. The beam will have a degree of flexibility sufficient to accommodate small tolerances of leg length etc. However, the subject of another recent technical query was a combination of a rigid load and an equally rigid lifting beam designed to lift on four chains, one at each corner of the 17.5t load.
In this case, the perceived problem was to a large degree self inflicted, as the client had specified that the beam be capable of taking the full load on either diagonal. The beam as delivered met their stringent requirements but then they expressed concern about the sharing of the load between the chains and the rating of the chains. A trial lift was arranged with load cells to check the tensions in each chain. Despite both load and beam being rigid, there was enough deflection to share the load adequately. But – guess what – the load weighed in at 20t, not 17.5! So the real problem was rather more fundamental and yet again demonstrates the importance of getting the basic facts correct. To design, manufacture and test for the higher load at the outset would have cost relatively little more. In the event it was an expensive error to correct.
