Advanced composites have become one of the most popular methods of repairing and/or strengthening civil infrastructure in the past couple of decades. While the use of Fiber Reinforced Polymer laminates and sheets for the repair and strengthening of reinforced concrete structures is well established, research on the application of FRP composites to steel structures has been limited. The use of FRP material for the repair and rehabilitation of steel members has numerous benefits over the traditional methods of bolting or welding of steel plates. Carbon FRPs (CFRPs) have been preferred over other FRP material for strengthening of steel structures since CFRPs tend to posses higher stiffness. The emergence of high modulus CFRP plates, with an elastic modulus higher than that of steel, enables researchers to achieve substantial load transfer in steel beams before the steel yields. This research investigates both analytically and experimentally, the bond characteristics between ultra high modulus CFRP strengthened steel members and the flexural behavior of these members. A series of double strap joint tests with two different CFRP strip widths are carried out to evaluate the development length of the bond. Both ultra high modulus and normal modulus CFRP laminates are used to compare strengthened member performance. Steel plates reinforced with CFRP laminates on both sides are loaded in tension to evaluate the load transfer characteristics. Debonding under flexural loads is also studied for ultra high modulus CFRP strengthened steel girders. Flexural tests are carried out under 4-point bending on several small scale wide flange beams. This study also introduces the novel ultra high modulus CFRP plate strip panels for strengthening of steel bridge girders. The first field application of ultra high modulus CFRP laminates in strengthening steel bridge girders in the United States is also carried out as part of the research. Full scale load tests carried out before and after the strengthening are utilized to measure the degree of strengthening achieved and checked against the expected results. A finite element model is developed and calibrated using data obtained from the field testing of the bridge. The model is then used to evaluate the behavior of the bridge under different conditions before and after the strengthening process.

Bridge strengthening is frequently associated with costly, long-term reconstruction. In some cases the bridge load capacity is sufficient for standard traffic, but requirements for an extremely heavy vehicle can exceed the maximal bridge load capacity. These vehicles normally weigh hundreds of tons. Moreover, well-known bridge strengthening technologies are not efficient and economical for this special load case. This doctoral thesis presents a short-term bridge strengthening method using a temporary support structure for the assistance of the existing bridge carrying structure. Although this strengthening method is rather old, it is rarely known. Approaches from 1980, are still often used, rely on simplified principles and approximation models. This approximation causes restricted efficiency of the strengthening system and creates a mathematical model that is not necessarily safe. The introduced new short-term bridge strengthening theory is based on monitoring of real bridge structures and temporary support structures during the crossing of extremely heavy vehicles. A mathematical model, which represents behaviour of real structures suitable, is introduced. The major effects, which have the greatest influence on this strengthening method are discussed. Further more a safety hydraulic element invented at Brno University of Technology, which can be used with the strengthening system is shown. The application of this system ensures required boundary conditions and therefore it guarantees the proper usage of the strengthening system.