Thomas Vandenbergh


Thomas Vandenbergh

PhD research

Alternative design strategies using morphological indicators: design for strength versus design for stiffness

Date2006 - 2010
SupervisorW. Patrick De Wilde
Within the framework of sustainable development we strive for constructions with a minimum volume of material. At conceptual design stage, this can be realised by using dimensionless numbers, called morphological indicators (MI). The first studies (limited to statically determined structures and simple load cases) show the potential of MI as real design instruments. Meanwhile, additional indicators have been developed allowing to meet the three basic requirements, which must be satisfied by every structure: the indicator of volume gives an indication of the necessary volume material to realise a fully stressed design (strength). The displacement indicator is a dimensionless measure for the maximum movement of the structure (stiffness) and the buckling indicator evaluates the buckling sensitivity of compressed elements in the structure (stability).

Although it is not explicitly mentioned in his book, Ph. Samyn opted for a clear hierarchy among these indicators: the dominant design instrument is the volume indicator, because it reflects the quantity of used material. The two other indicators are used as a posteriori control tools. This has gradually led to the idea that the only way to achieve lightweight structures should be based on a design for strength strategy.

Recent investigation does not criticise the intended objective but more the used approach. On the one hand this is based on the establishment that optimisation is an essentially non-linear process and that that the found minimum thus necessarily dependents on the followed way. On the other hand, optimization works in the domain of structures which are susceptible to resonance demonstrates that an optimum, obtained with design for strength often shows the same problem: the combination of mass and stiffness offers solutions with an unacceptable dynamic behaviour. This implicates the a posteriori modifications of the stiffness and/or the introduction of damping.

This research does keep the same objective but changes the used methodology. The results obtained by design for strength are compared to those based on design for stiffness. The basic idea of design for stiffness is simple: reverse the calculation order. Design for strength checks the stiffness of a design which satisfies the strength criterion, while design for stiffness examines the strength of a design which meets the stiffness constraints.

The impact on the optimal structural typology and topology is analysed. The (dis)advantages of contemporary high-tech architecture, in which the carrying exoskeleton is composed of triangular elements, instead of a classic beam-column system is also studied.

Conclusion: this research aims to optimize the use of the existing indicators (W and Delta) and the development of new similar (dimensionless) pre-design instruments for vibrations, connections and global instabilities to consider the main stiffness constraints.

Start Date: 1st January 2006

Expected End Date: 1st January 2010