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

Ethical issues in engineering design
There are also load scenarios for strength calculations for the more extreme
situations that might occur, such as a fully loaded trailer driving too fast over a
pothole.
In this design process the load scenarios were not known. Liz had experience
with designing trailers but this was her first assignment to design a trailer
without a roof that could be loaded with sand. The engineers did not know how
the sand would behave when the trailer turned a corner. The sand might, for
example, shift and push, with its total mass behind it against the side panels. The
engineers could refer to previous projects for some load scenarios, but at the start
of the design project Liz and Hans tried to reason out what the loads would be
using educated guesses and an aerospace engineering method.
An example of an educated guess is that Liz thought that the torsional
stiffness of the trailer should be somewhat higher than that of an existing
aluminium trailer. At the time of the design period there were problems with
aluminium trailers, with the welds used to connect the side panels to the front
panel. Fractures in these structures were probably being caused by movement
and displacement of the side panels causing extreme stresses in the welds. A
more torsional stiff trailer or a more flexible connection between the front and
side panels would solve this problem.
The aerospace engineering method included the following ideas: calculations
were done using limit and ultimate load. In aerospace engineering limit load is a
load that the structure will experience with a certain chance during its lifetime. A
limit load may lead to some elastic deformation but never to permanent damage.
An ultimate load scenario was used for strength calculations. An ultimate load is
a load that may damage a plane and lead to permanent (plastic) deformation but
will still allow the plane to land in reasonable safety. As a design rule the ultimate
load is 1.5 times the limit load. This 1.5 is called the safety factor.
This aerospace engineering method was combined with a dynamic factor
common in automotive engineering. This dynamic factor was used to account for
extra forces on the trailer, when for example, a pothole is encountered. This
dynamic factor was 2. 6 The ultimate load was then 3g (1,5*2g) times the mass of
the cargo in which g is the gravitational constant. According to Liz, trailer
producers use a total factor of 3, as established for a previous trailer project.
All calculations were done using a maximum mass of 32 tonnes of sand and a
maximum mass of 9 tonnes on the axles. This maximum load for the sand was
calculated taking into account the allowed maximum total mass of 44 tonnes for
a truck, trailer and load combination. The trailer will have a mass of about 5.5
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6 It is indeed mentioned in a handbook that a dynamic factor to account for rough roads should
be included but there a factor of 3 is proposed [Fenton, 1996 , 44].
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