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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 75
PROCEEDINGS OF THE SIXTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY
Edited by: B.H.V. Topping and Z. Bittnar
Paper 73

The Application of Fatigue Models to Railway Bridges

R. Gallagher+, D.W. O'Dwyer+ and M. Hartnett*

+Department of Civil Engineering, Trinity College Dublin, Ireland
*Department of Civil Engineering, National University of Ireland Galway, Ireland

Full Bibliographic Reference for this paper
R. Gallagher, D.W. O'Dwyer, M. Hartnett, "The Application of Fatigue Models to Railway Bridges", in B.H.V. Topping, Z. Bittnar, (Editors), "Proceedings of the Sixth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 73, 2002. doi:10.4203/ccp.75.73
Keywords: railway, fatigue, dynamic, bridge, rain-flow, assessment.

Summary
This paper describes work that is being carried out to develop an approach for assessing the effects of dynamic loading on ageing metal bridges. The overall project involves work in a number of different areas: the development of bridge and rolling stock models and their interaction; the development of loading data to model variable track alignment and wheel profiles; the measurement of bridge responses to rolling loads; the calculation of cumulative fatigue damage and the evaluation of remaining fatigue life.

This paper illustrates the approach taken in assessing the fatigue loading applied to the bridge. It discusses the methodology and mechanisms of fatigue analysis. The procedure presented involves the implementation of the rain-flow algorithm. This method involves decomposing the stress history of each location on the structure into a series of stress cycles. This approach enables the application of traditional methods such as the Palmgren-Miner hypothesis. Considerations such as material properties, stress concentrations and previous loading history are also considered and addressed.

Under repetitive load applications, steel will fracture at a stress level well below the ultimate stress. This phenomenon is known as `fatigue'. Approximately 80-90% of failures in metallic structures are related to fatigue and fracture [1]. Cyclic stresses firstly initiate and then propagate cracks in structural members, eventually leading to failure. This phenomenon occurs when the cyclic stresses are above the fatigue limit. In steels, the fatigue life steadily increases with the decreasing stress until the stress level of the fatigue limit is reached. Below this limit the fatigue life apparently becomes infinitely long and existing cracks do not grow any further. The fact that not all metals show such a limit may indicate that possibly it is a fictitious quantity and that some metals simply have not been tested to a large enough number of cycles [2]. There are several different types of fatigue and fatigue mechanisms. Two or more of these types of fatigue and/or failure mechanisms may act together to cause effects greater than would be expected by their separate action, that is, there is a synergistic effect [4].

In order to attempt to evaluate the cumulative damage undergone by a bridge several pieces of information must be known:

  1. An estimate of the frequency and axle weights of the trains.
  2. Properties of the bridge material, particularly tensile strength and fatigue data.
  3. Accurate survey carried out of the bridge.

When the detailed survey of a structure is being carried out there are several pieces of information that need to be established. The first is to determine the locations of any stress concentrations. Then the locations of corrosion, pitting or loss of section due to environmental corrosion need to be identified. In a detail survey it is very important that consideration is made for more than one damaging mechanism to be active at a particular location, for example, fretting fatigue and corrosion occurring at the bridge bearings.

In variable amplitude fatigue, cycle counting methods are used to reduce the random load history into a series of discrete events which can be analysed and compared with laboratory data obtained for constant amplitude fatigue loads. The rain-flow counting method is the most popular method of stress cycle counting [4]. In order to consider the fatigue loading of railway bridges certain computer models had to be developed. These include bridge, rolling stock and track models. The computer program developed models the bridge, track and rolling stock separately and consider their interactions explicitly. This enables the interaction between the wheels and the track to be modelled non-linearly. The computer program gives the stress history at each joint in the bridge structure and enables a fatigue analysis to be carried out.

The research work being carried out will form an integral part of Iarnród Éireann's bridge management programme, which will prioritise both bridge members and bridges for repair and renewal. The objective of this research is technology transfer, so that knowledge gained in the academic environment is passed onto the work environment.

Approach taken in this research work;

(a)
Determine previous traffic and axle loading.
(b)
Model bridge structure and rolling stock vehicles and their interactions to obtain the stress history of critical locations on the bridge.
(c)
Measure the strains in the bridge members on-site and validate the computer model.
(d)
Implement rain-flow algorithm.
(e)
Sum stress cycles and use the Palmgren-Miner hypothesis.
(f)
Carry out a detailed survey of the bridge.
(g)
Estimate cumulative fatigue damage and hence remaining fatigue life.

References
1
The Committee on Fatigue and Fracture Reliability of the Committee on Structural Safety and Reliability of the Structural Division, "Fatigue Reliability: Introduction", Journal of the Structural Division, Proceedings of the American Society of Civil Engineers, 1982, 108 (ST1), pp 3-23.
2
N. Taly, "Design of Modern Highway Bridges", Prentice Hall. New York, 2001.
3
N.E. Dowling, "Mechanical Behaviour of Materials", Prentice Hall, Englewood Cliffs, NJ, 1993.
4
B.I. Sandor, "Fundamentals of Cyclic Stress and Strain", The University of Wisconsin Press, Wisconsin, 1972.

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