It is often said that temporary works designers need to get to grips with Eurocodes, and there are fundamental differences in the way temporary and permanent works should be designed. But this is too simplistic.
There is a huge variety of temporary works designs, just as there is huge variety in permanent works, and all codes have the problem that they are expected to be appropriate in a far wider set of situations than the writer of the code could cover. Steel and concrete don’t behave differently just because the works are temporary rather than permanent, so there is no fundamental difference in the way they should be designed.
There are problems with Eurocodes because they are difficult to understand, unnecessarily complicated and expensive to buy. However, they are here to stay, so we had better get over it, and make good use of any design guides that teach us simple and straightforward ways of designing structures that comply with the codes.
The other big issue with regard to temporary works is the conflict between limit state design and the use of working loads. Safe working load (SWL) is an essential concept. If someone buys a 5t jack he needs to know that it can safely lift what it says on the box. The same applies on site, where people can be expected to calculate loads reasonably accurately and select appropriate equipment to support them based on stated capacities.
A manufacturer can design a prop for a load of, say, 10t using limit state design and Eurocodes. Nothing then stops them painting “SWL 10t” on it or printing that working capacity in their catalogue.
They may have introduced extra factors in their design to satisfy themselves that it can take an accidental overload or eccentric loading, or to allow for possible damage. That would increase the factor of safety, but the prop can still be described as designed to comply with the Eurocodes.
The problem then arises of how a temporary works designer can use this prop in a Eurocode compliant design. The answer is to assume that the prop was designed using at least a 1.5 factor for variable load. Hence, in any Eurocode-based calculation, the designer can assume that the ULS capacity is 1.5 times the manufacturer’s stated SWL, and utilise proprietary items on that basis.
If this were accepted, manufacturers could go back to using working loads in all their literature instead of the bewildering variety of terminology we are currently confronted with.
We have characteristic values, design values, ultimate values and others. Even if the manufacturers are all using these terms correctly, many of those who select the equipment will not know what they mean, and the loads given are usually not compatible with safe use.
So can we all accept that it does not matter much what code we use, as long as we understand what we are doing, and encourage a return to the safe practice of publishing working loads for items of proprietary equipment?
- Barry Johnson is a director at Bell Johnson