Keywords: dynamics of bridges, load bearing capacity assessments, bridges, static and dynamic loading test, railway steel-concrete composite bridges, dynamic load factor, spectral analysis.

This paper presents an overview of the in-service performance assessments of an steel-concrete composite (SCC) short-span railway bridge superstructure. Field load testing and visual inspections for the assessment of the SCC bridge durability under an actual service environment were conducted. The test result indicates that the SCC bridge superstructure has no structural problems and structurally performs well in-service as expected. The results may provide baseline data for future field SCC bridge load bearing capacity assessments and also serve as part of long-term performance criteria for the SCC bridge superstructure. To investigate its in-service performance, field load testing was conducted under an actual service environment. Field load testing is an attractive tool for re-evaluating the capacity rating of bridges. For the first time, the capacity rating for an SCC railway bridge for an in-service environment is calculated and discussed with various existing methods for the rating factors such as allowable stress and DLF [1]. As the SCC railway bridge superstructure was instrumented, a real load test was conducted under similar loading and weather conditions as during initial field loading tests in the 2002. This was done to ensure the structure's integrity before opening it to the public, to establish base line conditions for a future in-service field load test program, and to compare the actual performance with theoretical calculations. After the initial field load test, the follow-up field load test was conducted to ensure that the SCC railway bridge structure was behaving satisfactorily and to check any signs of degradation. The SCC bridge superstructure was tested using conventional tractile locomotion E 662.2. The results of this test were later used to evaluate bridge in-service bearing capacity.

Based on the results presented in this paper the following conclusions can be drawn. The maximum deflection diference from two service load tests (SLT) (2002, 2004) was estimated to be 0.18mm. This is 9.57% lower than the maximum value SLT max w = 1.88 mm from both SLT. The maximum value SLT is 59.53% lower than the maximum theoretical value MKP. It means that the SCC bridge superstructure may be designed with a less restrictive design deflection. The dynamic responses in 2002, 2004 (monitoring) also show that the passage of the trains produces insignificant vibrations, the maximum dynamic deflection effective value wrms = 0.48 mm (2002) and wrms = 0.32 mm (2004). This is attributed to the difference between the natural frequency of the SCC bridge superstructure and the forcing frequency of the passing locomotive and trains. After two years of service, dynamics load factor (DLF) values of the SCC bridge are well compared with DLT values measured in the initial tests (2002). All experimental DLF values are lower than prescription by the Slovak standards DLF values. Therefore there is no need to post the load limit. The results indicate that the SCC superstructure structurally performs very well, and the capacity-rating evaluation for the SCC Bridge can use a rating factor from the existing methods for the conventional materials such as the allowable stress and load-factor. A simplified finite element analysis calculation was compared to the measured one. Despite both the complex structural layout of the bridge and simplifying assumptions of the model results showed good agreement for all experimentally identified (2002, 2004) damped natural frequencies in the basic frequency range 0 - 11 Hz. and that is good compared with the theoretical values.

1

J. Bencat, "Report on static and dynamic test results of the bridge on railway Zilina - Detmarovice v km 256,919", No. 6 - 3 - 19/SvF/03, ZU/Zilina, 2003.

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