Keywords: high-speed structures, ageing of structures, train car accelerations, galloping effect, degrading track geometry, alternative for bridge inspection.

As some high-speed railway lines have been in service now for several decades, inspection of the bridges and viaducts has become an important issue. However, detailed inspection of larger structures has the disadvantage that there is no early detection of possible failures or degrading. The installation and the monitoring of complex monitoring systems is a serious cost and its weakness of the system is the limitation to local monitoring, without acquiring data and assessment concerning the global behaviour with time, nor identification of the cause of measured values.

At least once every two months a test train is run on various sections of the high-speed network. This train is equipped with measuring devices, which mainly register track quality parameters. Among these is the vertical train car acceleration. The results from continuous measurements of vertical accelerations of test trains may also constitute a valuable alternative for detecting possible defects in structures. From analyzing these results local acceleration peaks can be explained. Hence, possible abnormal results should be the result of defects.

Recent (2011) data from test trains have been compared to older results (1997) from the time of commissioning a section of the Belgian high-speed network. This comparison shows that the average value of maximum accelerations has not changed fundamentally, although the dispersion of the results is large.

However, a more fundamental track-related phenomenon can influence the measured values. The test series of 1997 were carried out in early spring (mostly March), whereas the recent values were collected during summer. This means that rail stress has a larger compression part than at a lower temperature. Apparently, rail alignments are less consistent and singular points, such as viaduct piers, and consequently also transitions from elastic earth platform to concrete abutments, are detected as having a larger influence, compared with the condition of having general tension in the rails.

In addition, the test train results allow assessment of the galloping parameter for longer viaducts. This parameter permits a determination of whether train cars can accumulate kinetic energy during the crossing of longer structures. The results have shown that the galloping effect has decreased with time, which probably results from the spreading of local track discontinuities.

Data from test trains concerning more recent structures, such as the Prester Bridge, the Battice viaduct and a bridge at Leuven, confirm these findings, although these results require careful interpretation and local effects from track devices can easily trigger the use of acceleration diagrams as a tool for the assessment of structural health.

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