Modern steel manufacturing techniques make it possible to produce steel with the nominal yield strength up to 1300 MPa for structural purposes. However, the application of very high strength steels is still limited in the civil engineering structures due to lack of knowledge about the effects of the manufacturing process and experimental results regarding the structural behaviour of the material. Moreover, in a fatigue loaded very high strength steel structure absolute and relative stress variations will be higher compared to stresses in structures made of lower steel grades. Accordingly, the fatigue issue will be one of the most important design criteria for very high strength steel structures. In this current study, V-shape welded specimens were manufactured from S690 and S890 rolled steels and cast steels with similar yield strengths. Fatigue cracks were created in the weld toe of the specimen under a fluctuated loading and subsequently the fatigue damaged specimens were repaired by the removal of the cracks with subsequent welding. The fatigue strength curves of repaired specimens are compared with the detail categories of EN 1993-1-9 (2006) and the fatigue strength curves of the test results in the aswelded condition from literature. The fatigue strength of the fatigue damaged connections was completely recovered by the established repair procedure.
Full-scale fatigue tests were performed on two retrofitted orthotropic bridge decks (OBD).
12The retrofitting systems consist of adding a second steel plate on the top of the existing deck. The aim is 13 to reduce the stresses at the fatigue sensitive details and therefore extend the fatigue life of the OBD by 14 stiffening the existing deck plate. Two retrofitting systems have been studied. The bonded system 15 consists in bonding a second steel plate to the existing deck by vacuum infusing a thin adhesive layer (2 16 mm) between the two steel plates. The sandwich system consists in bonding the second steel plate 17 through a thick polyurethane core (15 mm). The aim of the study was to assess the fatigue performance 18 of both retrofitting. No fatigue damage was detected in the retrofitting layers during fatigue tests after 3 19 million cycles of wheel load. The stresses close to the deck-plate-to-stiffener welds reduce by at least 20 55% when using the bonded steel plates system and 45% when using the sandwich steel plates system.
21Both systems proved to have sufficient fatigue life to withstand traffic wheel loads running on 22 orthotropic bridge decks and help extending the fatigue life of the existing OBD. 23 24
The assessment of the structural safety of existing br idges and viaducts becomes increasingly important in many countries owing to an increase in traffic loads. Most existing standards, however, are developed for the design of new structures. For this reason, an assessment method for determining the actual safety level of highway bridges and viaducts has been developed. The method focuses on the determination of the effect of traffic actions and consists of a number of levels. The first level requires the least work from the engineer but is the most conservative. Each of the next levels is less conservative. Some levels require actual measurements at critical parts of the structure. This paper consists of two parts. The first part explains the basis of the safety assessment me thod and the second part shows the step-by-step application of the method to an existing highway bridge in Th e Netherlands.
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