A structural system can be defined as complex if its behaviour is influenced from nonlinearities, uncertainties or interactions. A long span bridge in this sense is a complex structural system that interacts with the surrounding environment and users for whom it is destined and presents exceptional characteristics, needs, and performance [1]. For such a complex system, a system engineering approach should be used in its design, build and management.

System engineering is a robust approach to the design, creation and operation of systems [2]. It focuses on the precise specification and goals of the system structure and behaviour, the activities required in order to develop an assurance that those specifications and goals have been met, and the evolution of the system over time. As a result, starting from the identification and the quantification of system goals and requirements and by fixing the performances, the correct and robust design can be implemented.

For a long span suspension bridge, it is consistent to comply with precise levels of safety and serviceability. In both cases, the knowledge of the loads and the consequent response of the bridge become important. In particular, since a superstructure of this kind is extremely flexible, certain levels for the deformations and the geometric and dynamic characteristics of the bridge deck need to be maintained.

With these in mind, becomes apparent the need to implement an advanced structural health monitoring system that is able to monitor both the various loads that the structure is subject to, such as natural (ambient), antropic and entropic, as well as the consequent structural response to these loads (Table 4.1), while in the long time, the entropic actions (decay of the materials) that influence the structural behaviour should be assessed.

In this manner, it is possible to use the information obtained from the various monitoring systems, in order to determine the real-time condition of the bridge and its interdependencies [3].

It is clear that the concept of structural monitoring is no more to be intended as the passive exploration of deterioration or damage states, consequential of ambient conditions, design deficiencies or material scarcities, which originate from observations on the structure. On the contrary, the innovative approach focuses on the principle of controlling the structural system in a proactive way, in order to reveal, almost instantly, the presence of any undesired consequences. The whole process is enchased from the recent introduction of monitoring systems which are based on novel and advanced technologies such as fiber optic sensors, wireless data transmission and GPS [4].

Finally, the information obtained can be applied in planning and implementing inspection and maintenance activities, and in long term the monitoring system will contribute to lower costs and superior engineering quality.

1

F. Bontempi, L. Catallo, L. Sgambi, "Structural analysis and design of long span suspension bridges with regards to nonlinearities, uncertainties, interactions and sustainability", Proceedings of the Second International Conference on Bridge Maintenance, Safety and Management (IABMAS'04), 18-22 October 2004, Kyoto, Japan.

2

National Aeronautics and Space Administration (NASA) 1995. "Systems Engineering Handbook", Available online at: http://ldcm.gsfc.nasa.gov/library/ Systems_Engineering_Handbook.pdf

3

K. Gkoumas, (in Italian) "Metodologie per il Monitoraggio di Sistemi Strutturali Complessi con particolare riferimento al caso dei Ponti", Graduate Thesis, Department of Structural and Geotechnical Engineering, School of Engineering, University of Rome "La Sapienza", 2003.

4

K.-Y. Wong, K.-L. Man, K-L., W.-Y. Chan, "Monitoring Hong Kong's Bridges: Real-Time Kinematic Spans the Gap", GPS World, 12 (7), 10-18, 2001.

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