The design of temporary works gains importance in the civils’ and structures’ engineering industry as more challenging structures, tighter schedules and site footprints are required. Currently, the design standards for temporary works are based on safety concepts inadequately drawn from established norms (as for example the Eurocodes) for permanent structures, or simply introduced empirically. A more realistic safety concept for temporary works can be developed based on the actual likelihoods and consequences of failure, and the duration of the structure’s service life.
As temporary works, one can define any structures facilitating or enabling the construction (or demolition) and protection of permanent structures, or providing access to the construction site. They may or may not remain in place at the completion of the construction project. Without disregarding special types, a general distinction of temporary works can be reasonably made between above ground and below ground temporary works, which are correspondingly assigned with different types of risk exposures. Typically, below ground (and underground) works rely on the adequate recognition of ground properties and they strongly depend on the ground performance, while above ground works may be more susceptible to weather phenomena and earthquake excitations. In both cases, temporary works are associated with a typically very short design life; less than one year in general, and occasionally slightly longer for very large projects. Another characteristic of temporary works is that they are intended to be slender structures compared to the loads imposed, in order to allow space for construction works, but also to reduce construction budgets as reasonably possible. Finally, the structural system’s geometry and supporting conditions may continuously be changing throughout the construction phase. At the same time, these works can be associated with extremely high consequences in case of collapse, related to financial damage, damage to third party assets, and life, health and safety of the labour staff and the public. In many cases the financial loss can exceed the entire construction budget, i.e. collapse of temporary works can have greater consequences than a collapse of the completed structure. For underground works, it is evident that collapse at the construction stage has resulted to losses which are multiple to the initial project budget. Therefore, it is considered that temporary works should be treated in design with a customised, project specific, risk-based approach.
In the author’s view, such an approach includes a rationalized adjustment of the component’s service life, but also the disproportionally varying level of consequences in case of minor damage or full loss. As regards the service life of temporary structures (in other words, the acceptable risk level), one can possibly use safety factors that are lower than the safety factors used for permanent structures. However, the design safety level should be dictated by the possible exposure in case of failure: A large, external scaffolding system adjacent to traffic and pedestrians must be designed with a higher standard than a scaffolding used e.g. for indoors fit-out works. A deep basement excavation support, next to existing high-rises calls for a higher safety level than shallow foundation works for a small residential building.
Is this intuitive? Even obvious? Yes, and the temporary works design must reflect this.
Some references:
ISO, 2394: 2015–General principles on reliability of structures. International Standard
Holický, M., Optimisation of the target reliability for temporary structures.
Tanner, P. and Hingorani R., Thoughts on Construction Risk Mitigation and Acceptance
Spyridis, P., Adjustment of tunnel lining service life through appropriate safety factors.
