| To rehabilitate or strengthen bridges in service is a feasible way to deal with the contradiction between the decreasing bearing capacity and the increasing demand, which is consonant with the development strategy of traffic infrastructure in our country. This paper gives a study on the strengthening issue of concrete bridges, and the main contents are as follows:1. Based on comprehensive reading of related literature home and abroad, researches on bridges strengthening technology are reviewed.2. A trapezoidal non-isoparametric element is presented to promote the algorithm of numerical integration method, which can avoid the plenty integration calculations in traditional way through the analytic solution given by derivative constitution functions.3. The effects of relative compression height and preexist strain on strengthening efficiency are studied. A equivalent reduction coefficient is suggested to consider the decaying effects of prestress strands after strengthening, and the proper value is estimated through theoretic deduction and parameter analysis. Then an explicit method for calculating FRP volume of RC, PC girder strengthened with externally boned FRP is present.4. A new ductility index is proposed for members with complicated load-deformation curves such as PC girder, strengthened RC or PC girder with externally bonded FRP system, and the index can give the same evaluation crierion for those different systems.5. Strengthened members should be ductile. In this paper, a limit relative height is put forward to ensure the essential ductility, and the proper values are suggested for several usual strengthening cases such as externally bonded FRP sheet or plate for RC, PC girders. Moreover, the relationships of feasible capacity improvement verse relative compression height and the limit relative height for ductility are studied.
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