Strengthening of I-girder bridges : Fatigue endurance

Abstract: Structures have always been subjected to the detrimental consequences of the fatigue phenomenon. The consequences due to this damaging effects leads to crack initiation and the ultimate result being complete failure. Fatigue appears on structures that has to endure fluctuating loads over a long period. The crack initiation occurs irregular geometries such as in the vicinity of welds and bolts. A structure that suffers a lot due to fatigue are road bridges. In this thesis the structure under investigation is a composite bridge with a concrete slab on top of two I-girders. The recommended lifetime of road bridges in the Eurocode is 100 years.Bridges are exposed to daily and yearly traffic loads. Also the amount of traffic and therefore the total weight shows a continuously increasing trend over the years. This tendency is under no circumstances expected to stagnate. Eurocode has based its approach to verification of fatigue on applying adversely combined traffic loads, in both directions, on the bridge. In the procedure of double I-girder composite bridges verification there is an uneven distribution of the vehicle loads and the stresses, as the bridge is behaving as an open cross-section. The concrete slab is the only apparent part that is transferring the vehicle loads to the other girder and gives therefore an unequal distribution of the loads.Because of this uneven load distribution there appears to exist some potential in finding a means to strengthen the bridge by ways of making it work as a semi-closed cross-section. This will enhance the load distribution over to the non-stressed girder. In this thesis the focus was set on the stress relation between the center web stiffener to the lower flange, with and without the strengthening, for a real case section. Assessment were done for case studies where the Rokån bridge as well as four arbitrary cross-sections were investigated in search of retrieving an increased understanding of the mechanism of stress distribution.The strengthening approach is to introduce a truss assembly, in shape of a K, in the plane between the two I-girders bottom flanges. This approach reduced the stresses in the middle position of the Rokån bridge by approximately 8.3 %. In addition, the parametric study of the arbitrary cross-sections, with the same truss system, where made. Reductions there showed stress values in the interval of 5.9 % and 16.6 %. Based on this study it was found that it is possible to establish a relation between the length on the bridge and the distance, w, between the girders. With regards to this study in applying this K-bracing, indicates that given stress reductions can prolong the bridges technical life to about 2.5 times.In comparison to strengthening ways available on the market, such as CFRP sheets and an additional concrete slab, there are advantages and disadvantages, such as self-weight, total cost, new fatigue details and the most important stress reduction.While the reductions tends to be quite high, there still exists a FE-model uncertainty regarding the coupling between the K-bracing junctions and the bottom flanges where the fusion of the elements was done with a node to node attachment. In a reality attaching the K-bracing onto the flange will be done by means of a bolted, welded or friction connection, which may lead to another fatigue category and increased/decreased stress concentrations at the new joints.