Field testing of a 30-year-old composite Preflex railway bridge

Within the framework of the French National Research Project MIKTI dealing with ‘composite steel-concrete footbridges and bridges for tomorrow', a loading test was performed in Belgium on a railway bridge deck consisting of simply supported composite ‘Preflex’ beams. The bridge was 30 years old at the time of the test so the time-dependent stress redistribution between the steel of the girders and the concrete was almost stabilised. The bridge deck was loaded progressively by two heavy diesel-electric locomotives. A 4 m long zone situated at the mid-span of the deck was instrumented and monitored during the loading test. The following measurements were taken: the mid-span deflection; the slopes in five locations; the length variations of 16 extensometers of 250 mm length situated near the bottom fibre of the beam and acoustic emission. All of the measurements showed that the behaviour of the bridge deck under the test load was linear elastic, the concrete was not cracked before the test and no cracking appeared in the concrete during the test. The concrete part of the cross-section fully contributed to the stiffness of the beam, confirming the excellent long-term behaviour of this kind of railway bridge, even after a very large number of live load-cycles applied on them during more than 30 years of service. These observations are in very good agreement with the values of stresses computed by a step-by-step method, applying the principle of superposition of linear viscoelasticity for modelling the time-dependent stress redistribution in these composite beams.