Experimental and Numerical Study on Fatigue Behavior of RC Beams Strengthened with U-Shaped Steel Casing

To achieve a rapid and great enhancement in the flexural performance of reinforced concrete beams, this study proposes a new strengthening method using U-shaped steel casing and focuses on studying the fatigue performance of strengthened beams. In this paper, ten specimens were tested under fatigue loads. Testing results demonstrated that no obvious damage was observed from the strengthened beam after 2 million fatigue cycles compared with the reference beam which failed at 58.97 x 10(4) fatigue cycles, indicating the good joint performance of the U-shaped steel casing and the RC beam. The strengthened beams generally failed by the sudden rupture of the bottom steel plate accompanied by concrete crushing. The mid-span deflection and material strains all increased rapidly at the beginning, then steadily developed during nearly 80% of the fatigue life, and grew rapidly again after the weld toe cracking until the strengthened beam failed. The fatigue life was significantly extended with the increase of either the height of the steel box or the thickness of the bottom plate, while it was dramatically shortened with the increase in the fatigue load amplitude. To facilitate the use of the outcomes, a simplified formula for predicting the concrete residual strain of strengthened beams was derived. The predicted results matched the experimental ones well. The derived formula is theoretically only applicable to strengthened beams with dimensions same as the specimens in the experimental tests under similar loading conditions. Reasonably good agreements were found between the finite element analyses and experimental results. The developed finite element model can well simulate the failure mode under static load and the damage evolution under fatigue load. Based on the stress analysis results, the fatigue lives of the strengthened beams were predicted relatively accurately using the Fe-safe software, with the deviations mostly less than 15%.