Ethical issues in engineering design - 3TU.Centre for Ethics and ...

Design of a bridge
certain codes. If new codes are developed professional associations of engineers
organise courses for engineers to learn about these new codes. 4
There are special codes for the loads on concrete bridges and steel bridges
[NEN 6723, 1995] and [NEN 6788, 1995], respectively. The problem is that these
codes are still based on codes formulated in 1963. The predictions of traffic flows
over bridges in these codes are not realistic. Axle loads and frequencies are
predicted based on these traffic predictions, and as a result, the predicted axleloads
and frequencies are too low, leading to heavier actual fatigue loads than
those used to calculate the bridge. The use of the traffic predictions of these old
codes to design new bridges has led to the need to completely renew large parts
of a bridge because fatigue damage made this necessary, the Van
Brienenoordbrug. This bridge was opened in 1990 and steel parts of the bridge
supporting the road had to be renewed after large fatigue cracks were found in
1997 [Barsten in de brug, 1997]. Some of the fatigue cracks needed to be
repaired immediately. The situation was dangerous because a heavy truck might
have caused a local failure of the bridge and for example have gotten a wheel
stuck in a hole in the bridge deck.
Within a few years there will be a European code for bridges. This code will
be incorporated into the Dutch code system as NEN-EN 6706. This European
code was developed in the nineties and the fatigue loads are more realistic and
have been determined statistically. This European code is now available in a not
yet definite version. At the time of the case-study there was a “preliminary green
version”. This meant that comments were being solicited on this version. After
incorporation of the comments it will become a “green version”. The green
version of the European code can be used and compliance with the building
decree is assumed, however, the old NEN codes can also be used. Engineers and
companies are also asked to comment on the green version. After a given period
of time the green version will be changed taking into account the comments, at
this point it becomes the definite version. The definite version of the European
code will replace the NEN codes in use at present.
During the design for the IJburg bridge a choice had to be made to use either
the NEN or the European code. The European code has realistic fatigue loads.
This might lead to more material being required to be used in the bridge. If
fatigue is important then more attention needs to be paid to detailing: sharp
edges and corners are not recommended under fatigue loads. The safety factors
are a bit lower in the European code than those in the NEN codes, because the
loads are determined with less uncertainty. Lower safety factors may decrease
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4 There are several professional associations for the different disciplines in construction. There
is, for example, an association for engineers who work with steel (Bouwen met Staal), there is
also a research centre for civil engineering in the Netherlands called the CUR.
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