| The great progress has been made in construction of the concrete box-girder bridges in our country recent years. But most of them present many cracks after servicing a period. Cracks weaken carrying capacity and durability of bridges. Therefore it is important to study the reliability of the cracked concrete box-girder bridges and to make a decision of maintainance and strengthing of the bridges by reliability index. This thesis studies the probabilistic model of material strength and size, the calculation model of resistance and the reliability based fuzzy failure criterion about concrete box-girder bridges with cracks. The main work is as follows. 1 The crack situation of two concrete box-girder bridges in China is investigated. And the distribution law of main cracks in box-girder bridges is analyzed. Then the reasons of cracks which formed in different location are studied. 2. The theory of fractal geometry is recommended to describ cracks in the concrete box-girder bridge, and the fractal dimension of cracks in box-girder is used to denote the degree of box-girder crack. Based on above theory, the probabilistic model of strength for cracked concrete is developed. Time-varying model of reinforcement area at crack sites is built after the influence of reinforcement corrosion and crack change the rate of concrete carbonization is analyzed. 3. The ultimate carrying capacity of a bridge is estimated with spatial finite element software-Ansys. Then the computing results are compared with the results computed by simplified plane beam elements, and the modified coefficient of resistance model of the bridge under the action of motor is gained. 4. On the basis of fuzzy failure criterion, model of reliability of concrete box-girder bridge is set up, and the reliability of existing bridge is calculated.
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