Influence of stiffness constraints on optimal design of trusses using morphological indicators

Within the framework of sustainable development we strive for structures with a
minimum volume of material. When we only consider criteria on resistance and
buckling, Samyn [1] and Latteur [2] prove that even at the stage of conceptual
design a clear hierarchy among the different truss typologies can be established.
Up to now, stiffness constraints - such as the upper limit on static displacements
- were not considered. However, an optimum obtained by minimising the
volume, only considering the strength criterion, often results in solutions which
violate the stiffness constraint(s). To avoid large displacements a stress level
reduction can be imposed. However, this comes at the cost of a significant
volume increase. With an optimisation process that involves the stiffness
constraints at the stage of conceptual design, an optimum can be obtained
without the necessity to alter the structure drastically afterwards, which partly
annihilates the main objective of minimal use of material. This approach,
compares the different truss types on a new priority scale, generating new
optima. This implicates a non-negligible change in the truss choice at conceptual
design stage. The solutions are logically depended on the displacement
criterions. This approach forms a first step to a new design philosophy that
considers all the stiffness constraints (static displacements, resonance, local and
global buckling) at conceptual design stage and is called design for stiffness.