Study On Fatigue Behavior Of Rc Bridges Under Environmental Actions And Vehicle Loads

As a significant factor inducing brittle failure of structures, fatigue has been paid much attention to by the structure industry. With the upgrading of the traffic volume and density, fatigue problems have gradually emerged in concrete bridges and accelerated with the combination of the deterioration caused by environmental actions. On such background, it’s of great value to conduct research on fatigue behavior of concrete bridges under both environmental actions and vehicle loads.Based on the investigation of the fatigue load models and calculation of the fatigue stress level of real bridges, experiments were conducted under the stress level of the design live load and the statistical live load to reveal the effect of the stress level. In order to investigate the effect of the environmental actions, fatigue experiments were conducted on corroded RC beams under the stress level of the design load. The fatigue life and the law of strain evolution against cyclic times under the two different factors were analyzed.A theoretical fatigue stress calculation method was established and programmed with the software Matlab, in which the damage to concrete and steel bar on the material level caused by fatigue and corrosion was considered. The strip method was included to reflect the difference of the stress calculation method on the member level from that on the material level,and to consider the interaction between concrete and steel. A coefficient was induced to consider the incongruity of the strain between the corroded steel bar and the concrete surroundings.Considering the damage to the constitutive law of concrete and the loss of the cross area of longitudinal steel bars caused by fatigue and corrosion, and inducing the deterioration of the bond between concrete and steel bars by the stress-slippage relationship of the spring element, the FEM software ANSYS was used to further investigate the fatigue behavior of the member as a whole.Finally, fatigue lives of the real bridges under both environmental actions and vehicle loads were primarily evaluated according to the statistical results of the experiments conducted both home and abroad.The conclusions of this study are as follows:By the calculation of the fatigue stress of the small- and middle-span concrete bridges, it was found that the fatigue stress of real bridges under the stress level of the design load agreed with the requirements of the design specification, while fatigue stress of real bridges under the stress level of the overweight live load surpassed the limit of the specification.The fatigue mechanism for RC beams on the member level showed significant difference from that recognized on the material level. The fatigue mechanism for RC beams on the member level showed 2 phases. The 1st phase, which accounted for 10% of the fatigue life, was controlled by the deterioration of the elastic modulus of concrete in a form of exponential curve, and the 2nd phase was controlled by the effective section area of steel bars linearly, and almost accounted for the whole cyclic life.Stress level accelerated the fatigue damage. The higher the stress level was, the shorter the fatigue life. But the extent of the deterioration of the member occurred before fatigue failure changed slightly. An increase of stress amplitude of 20% reduced the extreme cyclic times by more than 38%, but the growth rate of the stress in the member increased slightly, from 12.08% to 12.3%; and when the stress amplitude rose 70%, the the extreme cyclic times declined more than 66%, while the growth rate of the stress rose to 15.5%.Corrosion also accelerated the fatigue deterioration process and aggravated the fatigue damage, but the effect of corrosion showed as the initial injury, with the same deterioration speed under different corrosion rates. A beam with a corrosion rate of about 5% had the same strain growth rate with the un-corroded beam with the same configuration and under the same stress level, but the fatigue life dropped 34.9% more than that of the un-corroded one on average.According to the results of experiments conducted both in this paper and by other similar researches, the un-corroded bridges and the bridges which had a corrosion rate of 5% showed of no fatigue risk under the design live load, but the un-corroded bridges under the overweight load showed a significant risk of fatigue damage in the later periods of their lives. When both the two factors were considered simultaneously, the stress amplitude specified by the design code of concrete structures could not meet the needs of the real bridges.