To demonstrate the method by which existing bridges may be assessed and, where necessary, strengthened.
SUMMARY
A large number of existing steel bridges for road and rail are riveted structures built more than 100 years ago.
Many of these old bridges have been repaired or strengthened several times following damage in the World Wars or due to changes of service requirements. The safety of these bridges for modern traffic loads throughout their remaining service life needs to be assessed.
A classical method for the assessment of the remaining fatigue safety of existing bridges is given. It is noted that the S-N lines referred to in this method do not comply with the S-N lines given in Eurocode 3 for new bridges. Also, the safety assumptions used in this method are different to those specified in Eurocode 3. The method may, however, be easily transferred into the Eurocode system. Though the method described here is suitable for railway bridges, similar principles apply to highway bridges.
General methods for strengthening bridges are outlined and a case study is presented.
The nature of bridge behaviour is such that the structure should be subject to regular inspection and maintenance. Old steel bridges that are subjected to growing traffic density and traffic loads may need a general safety check to evaluate them to determine their residual safety and remaining service life. This may include:
experimental determination of the stresses
assessment of condition
strengthening of elements
replacement of elements
speed reduction
traffic reduction
partial or total closing of the line or highway
Methods for evaluating the remaining service life of existing structures are becoming increasingly important as the number of structures exceeding their design life is growing exponentially. This growth is related to the bridge construction boom which began over one hundred years ago. Few structures need to be replaced when they reach their design life because the design life was not defined scientifically in the past. Most structures are able to endure fatigue loading well beyond their original design life.
The design life is that period for which a bridge is required to perform safely with an acceptable probability that it will not require repair.
Generally, little is known of previous loading, structural modifications, or possible crack location in an existing bridge. Starting with simple conservative assumptions and proceedings in steps, an acceptable decision can be taken regarding the safety of the structure.
A classical method for the assessment of the remaining fatigue safety of existing railway bridges is given. This method is based on the procedure which was used by the German railways.
It is noted that the S-N lines referred to in this method do not comply with the S-N lines given in Eurocode 3. Also, the safety assumptions used in this method are different to those specified in Eurocode 3 for new bridges.
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