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

Design of a lightweight trailer
used results from a previous project to estimate the mass and the acceleration.
Hans had a lot of trouble reconstructing what had been done in the previous
project. There were some typing flaws in numbers of the report. This made the
reconstruction of what loads were added to which other loads very difficult, for
example the loads exerted by driving straight always seemed to be added to the
other loads such as those for braking or turning a corner. In the finite element
calculations the complete trailer was modelled and the load they used for
calculating was the load of the sand. The by hand calculations begin with the axle
loads which was 9 tonnes as defined by regulations. Acceleration figures were
taken from a report of a previous project. The engineers tried to compare the
geometry of the trailer near the axles with standard geometries used to calculate
stresses and strains. For example, the moment of inertia of a certain
configuration was calculated by using standard geometries from a handbook. In
these calculations the engineers tried to decide what the material thickness and
the orientation of the fibres in the composites should be to get the strains below
the allowable strains for composites and the stresses below allowable stresses for
the aluminium parts. The engineers discussed how far the strains should be
under the allowable strains because holes need to be drilled in some of the load
bearing structures to connect them with other parts. Such holes lead to stress or
strain concentrations that are difficult to predict in composites. The strains were
required to be well below the allowable strains to allow for the strain
concentrations round the holes.
7.3.2 Misuse and overloading
The customer told some stories about misuse of the trailers in the meetings.
During transportation it is not uncommon for some trailers to be overloaded,
thus a load for the composite trailer might be much more than 32 tonnes allowed
in the design. As said before a truck and trailer combinations can be subjected to
spot checks and weighed but this does not always happen and drivers can
deliberately or unintentionally load a trailer with more than 32 tonnes. The
customer said that his former company had had a verdict from court in which it
was stated that a trailer producer should know that trailers are commonly
overloaded in use and that they should account for this in the design of a trailer.
It was not clear how much extra load should be included in the design. The
engineers had calculated a 32 tonnes load, but the customer indicated that the
load could sometimes be as high as 40 tonnes. Neither the customer nor the
engineers changed the requirements or suggested making calculations for a
trailer carrying more than 32 tonnes. In my interview with her, Liz said that it
was not obvious to her that she should account for overloading of the trailer in
the design. She had not noticed the remarks on overloading. Further, the
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