Vibration behaviour of trusses at conceptual design stage, under pedestrian loading and using morphological indicators

Abstract eng:
Within the framework of sustainable development we strive for constructions with a minimum volume of material. Only considering criteria on resistance and buckling, at the stage of conceptual design a clear hierarchy among the different structural topologies can be established with Morphological Indicators (MI). MI are dimensionless numbers that represent a property of a structure (e.g. volume). This allows one to compare the efficiency of structures objectively, with only a reduced number of variables to consider. Up to now, dynamic effects were not considered when designing constructions with MI. However, an optimum obtained by minimising the volume only considering strength, often results in solutions with problematic dynamic behaviour. To avoid these problems the stiffness, mass and/or damping characteristics must be modified, implying a volume increase, additional apparatus and as a consequence a cost rise. With an optimisation process that considers/predicts dynamic behaviour at the stage of conceptual design, an optimum can be obtained without the necessity to alter the (strength-optimised) construction drastically afterwards. This study presents a basic analysis methodology which allows predicting whether vibration norms/guidelines ask for a modification of the design or not, at conceptual design stage. The paper focuses on the prediction of the eigenfrequencies and peak accelerations of statically determined trusses under pedestrian excitation. Application of the Swedish norm Bro 2004 [1], the British standard BS5400 [2] and French guidelines by Sétra [3] on an existing pedestrian bridge illustrates that the different norms/design guidelines propose fundamentally different load models, yielding different acceleration levels. This lack of uniformity can yield contradictory decisions for the acceptance of the vibration level of a given design. A unified model is necessary, based on experimental validation. Moreover, the theory of MI -as presented in this paper- allows predicting the vibration behaviour of structural archetypes at conceptual design stage. Hence, it provides a user friendly tool during the first steps of the design process for the architect/structural designer.

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