Eurocodes and temporary works.
The majority of the structural design standards and all of the Eurocodes are aimed at the design of permanent works. The application of European standards to the design of temporary works may not always be appropriate, and could potentially lead to unacceptably low overall factors of safety without a full understanding of the applications of partial factors to both loads and resistances.
There are fundamental differences in the design process for permanent and temporary works. For example, temporary works loads tend to be less well defined and more variable in nature. Poor information in general, particularly in the case of below ground works, demands a conservative approach to design. It is therefore current practice for suppliers of proprietary equipment to include generous safety factors when quoting capacities to allow for this uncertainty, and to account for the rigours of usage that temporary works equipment tends to get subjected to.
Sledgehammer to crack a nut
The application of Eurocodes to small scale temporary works is definitely a case of using a sledgehammer to crack a nut. Due to the lack of clear direction and guidance in this area, there is a fair amount of confusion for temporary works designers in situations that have previously been second nature to them. This causes a dilemma when determining the suitability of a particular situation, given that previous experience and engineering judgment are lost now that unfamiliarity with complex design procedures exists.
Understanding Eurocode terminology is the first challenge for any engineer. For example, the concept of “working load”, in respect to proprietary equipment, has been and remains a convenient and easily understood concept in which the ultimate or failure values for equipment resistance are reduced by a lumped safety factor, typically of between 1.5 and 2, to derive working values. It is then a simple process for a designer or checker to compare unfactored load calculations with working resistances. Providing that actual or calculated (unfactored) loads are less than working load capacity for the piece of equipment in question, then it is strong enough to do the job, with an adequate factor of safety built in. It’s a universally understood system that has served us well for generations.
Explaining the concept of factor of safety in terms of partial factors applied to loads and resistances is not easy, and often falls on deaf ears. For most users familiar in dealing with the concept of working load, charts that define equipment performance data in terms of limit state design parameters may cause some uncertainty as how this data needs to be compared to the calculated actions to provide a safe solution.
The Temporary Works Forum (TWF) is acutely aware of these issues and a TWF working group is actively looking at producing guidelines for temporary works engineers. In the mean time, permanent and temporary works engineers will need to continue to work closely together and communicate effectively to provide robust and above all safe temporary works solutions.
- Tony Gould is technical director at Groundforce Shorco. Click here to go to his technical blog.