Keywords: cable stayed, structural control, multi-objective optimization, integrated design.

Applying control to civil engineering structural design is a growing topic. Structural dynamic properties can be improved by using active or passive devices. Active devices have proved to be more efficient in reducing the dynamic response in structures than the passive devices but so far they have a relatively high investment cost. In addition, due to the very few applications the structural design of active structures remains a relatively unknown subject.

A literature review is conducted on the current investigation of integrated structural and control design. Also the principal guides and regulations of footbridge design are taken into account.

The optimum design consists of standard steel profiles. Polynomial functions are used to fit the geometric properties as continuous functions to avoid discrete optimization. This regression model is accurate enough to be used efficiently for the optimization.

In this paper a parametric two dimensional model of a cable stayed footbridge is developed. Both passive and active control strategies are employed. Bridge geometry, deck and tower sectional properties, cables prestress and control factors are dealt with as design variables. An optimization algorithm based on the maximum entropy principle is chosen to provide a least cost structure which guarantees safety and serviceability.

The service and ultimate limit states loading conditions are given. The dynamic loading consists in a crowd of joggers during a running event. The time history loading of a jogger is shown and the dynamic analysis evaluates the deflections and accelerations undergone by the structure.

The optimization procedure is carried out to find the optimum passive and active design and the results are presented. It is shown that the algorithm is efficient in increasing safety and reducing the cost in both designs. The dynamic properties are improved, mainly the modal frequencies and damping factors.

Both designs converge to a safe and stable solution while reducing significantly the cost. The optimum passive solution is 17% more expensive than the active, the chosen design being dependent on the overall cost of the active design (which must include the active tendons cost and maintenance). The active design requires low power consumption, so the operating costs proved to be negligible.

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