Research On Residual Resistance Assessment Method Of The Existing Prestressed Concrete Girder Bridges

Compared with the reinforced concrete bridges, the research on the methods of remaining capacity assessment of existing prestressed concrete bridges is not mature,it is a problems that has to be settled urgently in bridge engineering.For prestressed concrete bridge, concrete structural damage and prestress loss were the main reasons that led carrying capacity decay. In order to provide a theoretical basis for residual capacity assessment of prestressed bridge, this paper will introduce prestress loss to the evolution of bridge bearing capacity theory, and analysis the failure mechanism and damage to bearing capacity of concrete carbonation, chloride penetration, steel corrosion, alkali-aggregate reaction, freeze-thaw damage, concrete cracking, prestress loss and other factors in detail. Reference to" General specification for highway bridges and culverts"(JTG D60-2004 ), this paper will research the bearing capacity assessment methods of the existing prestressed concrete beam bridge under ultimate limit state and serviceability limit state respectively,main content as follows:(1)Concrete structure damage and prestress loss were the main reasons leading to bearing capacity decay of concrete girder bridge.The mechanism of concrete carbonation, chloride penetration, steel corrosion, alkali-aggregate reaction, freeze-thaw damage, concrete cracking and the influence on bearing capacity were systematically discussed in this paper. The detection of the prestress loss, relationship between effective prestress and the remaining capacity and bearing capacity assessme-nt based on vibration testing method were also introduced in detail.(2)The resistance of prestressed concrete bridge structure was seem as a random process, and its section random variable was assumed to be lognormal distribution, and its mean and standard deviation was obetained from the Taylor series and the error transfer formula, thereby, the stochastic process computational model of the existing prestress concrete beam bridges resistance was established.(3)The geometric parameters was seem as random variables, and the material properties such as concrete strength, general strength of reinforced and prestressed concrete, coordination coefficient between steel and concrete was seem as random process.Referring to "reinforced concrete and prestressed concrete highway bridge design code ", the time-dependent bending capacity model and time-dependent shear capacity model of prestressed concrete T-beam bridge under ultimate limit state were derived.(4)The main loads of the existing bridges were dead load, auto load and crowd load because of construction error and the impact of wear and tear during use, dead load was seem as normal random variables; becase of great variation with time and space, auto load was generally seem as random process.The auto load and maximum probability distribution were analysed and Time-dependent model of vehicle load effect was established in this paper. The crowed load was analysed by stationary binomial random process, the probability model of the bridge crowed load effect was established in the assessment base period.(5)Because of the resistance decay and the varying of the using period,the varing of target reliability index with time must be taken into consideration.The methods of time-dependent reliability index calculation when the resistance and load was both normal distribution was derived in this paper. On the basis of structural resistance,dead load and live load time-dependent probabilistic model, the reliability evaluation model of existing prestressed concrete bridge under ultimate limit stat was Established.The assessment mode was used to assess a thirty years old prestressed concrete beam bridge and predict its remaining life, good results were achieved.(6)The key of bearing capacity assessment of prestress concrete bridges under the serviceability limit state was to get the effective prestress.The correlation ship between the natural frequency and pre-stress has been proved by the theory of structural dynamics and model test.According to the theory of dynamic testing, the natural frequency of the prestress concrete bridges was analysed by Nonlinear finite element method, the Mapping relationship between effective prestress and natural frequency were simulated by Support Vector Machine, and then,the effective prestress expressions of Support Vector Machines was established.The frequency samples got from prestress dynamic tests was inputed to the trained support vector machine,and effective prestress of the actual prestressed conctete was identified. Substituted into the bearing capacity calculating formula, it can be used to assess the cracking capacity of prestressed concrete bridges under the serviceability limit state.