| Summary: | There are many beautiful old structures around the world, some of which were designed for completely different purposes than their current applications. For example, Swedish railway bridges were only designed to carry axle loads up to 200 kN in the beginning of the 20th century, while modern loads can be twice as high. The traffic intensities have also increased dramatically and the velocities are now higher than ever before. In order to maintain old structures while the loads increase, upgrading of their load carryingcapacity may be needed. Administrative upgrading refers to increasing their nominal capacity to withstand stresses beyond original limits by refined calculations, using real material data, geometry and loads. This sometimes allows bridges to be upgraded with little or no physical modification. Upgrading by strengthening refers to physical alteration of the structure.The objective of the studies this thesis is based upon (reported in detail in five appended papers, designated Papers I-V) was to evaluate several strengthening systems by assessing their in-situ effects on existing bridges.First, a novel strengthening method involving internal post-tensioning of bridge slabs was developed and examined in a laboratory test (Paper I). The material used in the test consisted of two 1:3 scale trough bridge specimens, and the purpose was to study effects of the method in a controlled (laboratory) environment. The results were encouragingand the method was subsequently applied to a real railway bridge in Haparanda, Sweden. To assess the method’s ability to increase load capacities, the bridge’s response to a train load were monitored before and after strengthening (Paper II). The results showed how the bridge’s tensile strains from the train load were completely counteracted by the posttensioning.Next, an assessment procedure, consisting of curvature monitoring wasproposed for double-trough bridges. The proposal was based on results of the field test in Haparanda (Paper III).In addition, the effects of two ...
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