The collapse of the temporary footbridge in Espoo's Tapiola has understandably caused a lot of concern and also spawned incorrect interpretations and information in the media. The most important thing is the prevention of similar accidents in the future, and the Eurocode offers a solution for this, among other things. The Eurocode should generally be used when the structure may pose a danger to personal safety.
Eurocodes are European standards for the design of load-bearing structures. The basis of the reliability level of the Eurocode system used in the design of structures is the penalty class defined for each structure. The penalty category is chosen primarily based on possible loss of life and secondarily on the basis of economic, social or environmental damage. In licensed (building permit) construction, the concept of difficulty class is also known.
Based on the sanction and/or severity category, the magnitude of the loads, the designer's qualification requirements, the level of inspection of the plans, the contractor's qualification, the level of construction supervision and various implementation-related categories are determined, among other things.
Some of the structures remain in the gray area and without a host
In licensed construction (house building) and infrastructure construction of public roads (e.g. bridges), the choice of penalty class is somewhat straightforward and the use of the Eurocode is justified. On the other hand, due to the confusion of the ministries' areas of responsibility, it is more difficult to find a justification for the use of the Eurocode in infrastructures built by municipalities and private individuals. However, this is not a very big problem, because it is generally understood to use the instructions of the Norwegian Railways Agency, even if no authority actually requires it.
For example, in the case of wharves, port structures and other shore structures, it is difficult to find the responsible authority, even though RIL and the Norwegian Railways Agency have drawn up some guidelines for these structures and they are implemented annually. Similarly, many construction sites related to industry (silos, tanks, chimneys, masts, structures supporting cranes) remain in the gray area and without a host. There may be an authority to supervise these structures, but the focus of supervision is elsewhere than on load-bearing structures.
In Finland, many projects have been built or at least are underway, where a wide-framed hall is implemented above a public road or train track. The level of requirements in fairway design is higher than in house construction, and this inevitably results in a conflict regarding which guidelines are used to design each structural part. Such cases can usually be solved with the cooperation of the authorities, preferably as early as possible in the project. The ownership parties can also vary, which makes it difficult to maintain the structures.
The tools to solve the issue exist
A situation like the accident in Tapiola, where a temporary passageway in public use is taken to a construction site, is more difficult. Occupational safety at the construction site is regulated by a government decree, and it is supervised by the regional administrative agency. However, a public thoroughfare is compared to a light traffic thoroughfare, in which case the suitability of the pedestrian bridge should be ensured in a different way.
The tools to solve this already exist. The Ministry of the Environment's regulation on load-bearing structures limits its scope to the design and implementation of load-bearing and stiffening structures of buildings, as well as the design and implementation of structures that are significant in terms of structures and operational safety, when their possible damage may cause a danger to personal safety. In this case, the regulation first defines the use of the Eurocode system as mandatory in practice.
Second, a penalty class should be defined for each structure. Here too, possible loss of human life is taken into account. On the basis of the penalty category, personal qualifications and the levels of plans and inspections are determined, as explained at the beginning of my article.
Third, in every more demanding project, a thorough design basis should be written so that each designer uses the correct loads and reliability factors and identifies the critical points for the safety of the structure. By precisely defining the areas of responsibility for planning and construction, better opportunities are also created to avoid covered areas. The design principles can also take a stand on the means by which continuous collapse is prevented.
Third party verification is required
Whenever the building structure is in the highest penalty category, a third-party inspection of the plans should be used. This has been recommended by the Accident Investigation Center for the first time already decades ago, and now it has been part of the legislation for some time. Requiring it is easy for building control, but still too rare. A third-party audit is a peer review where the most glaring errors come to light and some of the smaller ones as well. Regarding bridges on public roads, the requirements for inspection are presented relatively unambiguously in the guidelines of the Norwegian Railways Agency.
The accident in Tapiola was reported when the footbridge collapsed. It was certainly true in itself, because it was a bridge that was built over a transverse channel. On the other hand, it was not a bridge in the sense that it did not seem to meet the requirements for a bridge in terms of safety. And on the other hand, designing a temporary footbridge as a final bridge structure can be oversized (and expensive). Perhaps the toolkit should have a customized level of requirements somewhere between a construction site bridge and a general light traffic bridge. This would be justified especially when there are many users on the bridge, such as next to a metro station.
Hopefully, the legislators and the authorities will take a stand on the starting points for the planning of the unclear interfaces listed above and the structures that remain completely without official supervision.
I wrote the text on the evening of the day of the accident, and not all details were available at the time of writing.
The article has been published In Rakennuslehti, 12.5.2023 May XNUMX.
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