Structural deficiencies in Railway steel bridges are usually the result of deterioration caused by ageing, corrosion, fatigue, and higher load demands. In this context, steel bridge girders are the structural members prone to corrosion which implies a substantially reduction of their flexural capacities. As a result, a large number of steel railway bridges are in need for strengthening or retrofit. In this thesis, experimental and analytical investigations are conducted to predict the reduction in the flexural capacity of existing deteriorated steel girders under static loading and several retrofitting schemes are developed in the light of strengthening the girder cross-section. The experimental study covers the use of two Carbon Fibre-Reinforced Polymer (CFRP) composite types, namely, normal modulus sheets (NM-CFRP) and high modulus strips (HM-CFRP). A total of thirteen medium-scale W-shape steel beams with a span of 1.6m were tested under four-point bending setup. The thirteen beams were divided in four groups such as: i) Group G1 consisted of four beams with different percentages of cross-sectional area reduction without any strengthening; ii) Group G2 consisted of four notched beams strengthened with bonded NM-CFRP sheets. Herein, two out of the four strengthened beams, were bonded using saturant epoxy, while the other two were strengthened using high performance adhesive; iii) Group G3 consisted of two notched beams strengthened with bonded HM-CFRP strip with and without a wrapping system; iv) Group G4 consisted of three notched beams strengthened with unbonding NM-CFRP sheets and a ductile anchoring system. The results of the experimental study underline the effectiveness of the proposed retrofitting schemes in terms of flexural capacity increase and deflection control of the existing corroded steel girders. In addition to the experimental program, an analytical model was developed to set up a numerical method that is capable of predicting the elastic and post-yield behaviour of the unstrengthened and strengthened deteriorated steel girders. This numerical method can be used by designers to calculate the losses in the moment capacity of the deteriorated steel girders to an acceptable level of accuracy. The analytical model was validated using the experimental results that were presented in the experimental program.
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