PSI - Issue 5

Ivan Baláž et al. / Procedia Structural Integrity 5 (2017) 1057 – 1064 Ivan Baláž / Structural Integrity Procedia 00 (2017) 000 – 000

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The bridge superstructure consists of a steel orthotropic two-cell box girder supported by a single plane fan cable system. Nevertheless, along the length 86 m + 54 m = 140 m, the cross section was originally a single-cell box steel girder because the middle web of the box girder was omitted from the point of the last, most-inclined cable anchorage till the Bratislava end of the bridge (Fig. 1). Cables in one vertical plane along the bridge middle axis are used. The bridge has four traffic lanes. There are two pedestrian walk ways of 3 m on both sides of the bottom flange. The steel box girder has a width of 21 m (Fig. 2). The orthotropic deck plate is mainly of 12 mm thick and increased to 16 mm, 20 mm, 25 mm, and 30 mm at the areas of the cable anchorages. The plate thicknesses are rather small due to preloading. The bottom flange plate thickness varies between 12 mm and 22 mm and three vertical web plates are 12 mm thick (Fig. 2). Cross frames are spaced at 3 m. The system of longitudinal stiffeners is as follows: closed trapezoidal ribs with 600 mm spacing made of 6 mm and 8 mm thick plates are used in the bridge deck, while the three vertical web plates (spacing 6.3 m, height 4.6 m) and the bottom flange of the box girder are stiffened by L profiles.

Fig. 2. Half of the SNP Bridge cross section used in calculation.

3. Imperfection measurements

In 1984 measurements of deflections at the centre of subpanels and deflections of longitudinal stiffeners of bottom flange of the SNP Bridge were made (Chladný, Baláž, Nádaský, 1985); the deflection taken at mid -point of the gauge length. A mechanical system consisting of a bar carrying two fixed probes and a central dial gauge was used for measuring ripples in plate panels. The gauge length were b and 2b , where b is the spacing of the longitudinal stiffeners. A builder`s level was used for measuring ripples of the longitudinal stiffeners. The gauge length was a , where a is the spacing of the transverse stiffeners. Initial deflections in the unloaded were computed from the well known formula of linear theory, in which signs “–“ ( “ + ” ) are used for a panel in compression (tension).   cr measured o w w   / 1   (1) The results of imperfection measurements indicated that the deflections of the bottom plate of the SNP Bridge are much lesser than on the other bridges which were investigated in Czechoslovakia and Germany (Massonnet, 1980; Chladný, Baláž, 1990). There were altogether 2288 measurements of plate panels imperfections using gauge length b = 787.5 mm and 1560 measurements of plate panels imperfections using gauge length 2 b = 1575 mm. Both measurements consisted of 17 sets differing in: a) plate panel thickness 12, 14, 16, 18 and 20 mm, b) kind of stresses in plate panel: compression or tension stresses, c) existence or not existence of transverse field butt weld in measured section. The obtained values were compared with the value b/200 = 3.94 mm. This value was exceeded only in the set TW 12-12, where 95 % fractile (176 measurements) was 5.825 mm ( b / 135) and in the set TW with 5.303 mm ( b / 149). T means panel in tension, W weld in panel between 12 mm and 12 mm parts of panel. TW means in all tension panels with welds (352 measurements). And there were altogether 92 measurements of positive imperfections of