Estimation of fatigue life for long span suspension bridge hangers under wind action and train transit

The main causes of induced fatigue in the cables and hangers of suspension bridges are wind- and traffic-induced vibrations. Because of the high flexibility and reduced weight (in relation with the whole dimension of the structure), during their life-cycle, suspension cable systems of this type of bridge experience a large number of tension-cycles with significant amplitude. In this paper, the fatigue problem regarding hangers of bridges is treated: these members have been considered subjected to axial fatigue effect that can be faced with simplified damage laws and fatigue curves referring to experimental data. Diffusive stress regions at the end of the hangers are not considered here. The motivation for such a study is that it represents a typical framework for a preliminary bridge design or for a feasibility analysis for a proposed structural scheme: in all these cases, it is necessary to explore and to compare different solutions in an efficient and ordered way. A system approach is then suitable, as used in other structural engineering branches. Attention in this paper is focused on the hangers since they are considered as vulnerable components with respect to the fatigue effects. This point of view is also important for the maintenance strategy and for the consequently possible traffic disruptions. Another aspect of discussion, concerning the subject of this paper, is the interaction mechanism between train and wind actions. While fatigue analysis of hangers of suspension bridges is the specific topic of this research, the paper begins with a general formulation of the design of such complex structures. In the author's opinion this is important since in this way one can coherently frame all the aspects related to the structure, its parts and the appropriate safety and functional requirements. Consequent to this point of view is a system engineering technique that breakdowns all the aspects of the structural problem, with specific attention to the organisation of the structural modelling activity. In the whole paper, constant reference is made to the experience gained recently in the performance-based design of an extreme long span suspension bridge (Bontempi, F., 2006, Baois of design and expected performances for the Messina Strait Bridge. In: Proceedings of the international conference on bridge engineering-challenges in the 21st century, 1-3 November, Hong Kong, Civil Division, The Hong Kong Institute of Engineers [CD ROM]) with the skyline shown in Figure 1, with a main central span of 3300 metres, intended to support both highway (3+3 lanes) and railway traffic (1+1 track).