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Benchmarking optimisation at conceptual design stage with morphological indicators

Thursday, 25 February, 2010 - 17:00
Campus: Brussels Humanities, Sciences & Engineering campus
Thomas Vandenbergh
phd defence

Within the framework of sustainable development we strive for structures with a minimum
volume of material. At conceptual design stage, a clear hierarchy among different structural
morphologies can be established with morphological indicators (or MI). MI are
dimensionless numbers that represent a property of a structure (its volume, its maximal
displacement…) and only depend on a reduced number of variables, providing user-friendly
and efficient tools to guide the designer during its exploration and comparison of different
structural types.

Up till now the theory of MI privileged resistance as the prior design constraint. But this
PhD work shows that when the limit on the maximal displacement is exceeded, this
approach does not always guarantee an optimal solution. Hence it is essential to provide
information about the most restrictive design criterion during the first design steps and
hereby avoid important a posteriori structural modifications due to the use of an
inappropriate optimisation methodology. In this perspective this PhD research introduces
new indicators, allowing the inclusion of additional design criteria.

A first extension concerns instabilities. An indicator is developed which enables one to
predict whether truss (arch) structures are prone to global planar instabilities. The second
important contribution handles vibration problems. Indicators and graphics are established
to assess the vibration response of beams, trusses and arches. The possible vibration
reductions achieved by tuned mass dampers are also treated and the author focuses more
in detail on the vibrations induced by pedestrians.

These new indicators and graphics enable to decide, without need for detailed calculations,
whether strength or stiffness is the design determining criterion. If a structure complies
with all the imposed stiffness constraints, minimisation of the indicator of volume leads to a
light, resistant and stiff solution. But if at least one of the stiffness constraints (on the
displacement, the global planar stability or the vibration response) is violated other
optimisation methods should be used. Some different optimisation methods, based on
numerical optimisation algorithms, are discussed and applied on a practical example and
confirm the high efficiency of MI when strength dominates the design but also emphasize
the need for alternatives when stiffness dictates it.