Theoretical Analysis And Experimental Study On Effective Prestress Of The Long Span PC Bridge In Curved Duct

Prestressed concrete bridge has a large number in all kinds of built bridges in China. Although the structure form and the span of prestressed concrete bridge are in the constant innovation, the number is also constantly increasing, but the phenomenon of downwarping and cracking across the middle of box girder of many long span prestressed concrete bridge in operation is very common. These damages make some bridges failed or destroyed earlier, cause large threat to the bridge structure. The elongation of concrete beam of prestressed concrete continuous beam bridge in the operation indicates that downwarping, cracking and other diseases of many existing bridge structures have the matter with the insufficient estimation of effective prestress in the provision of Chinese "Highway Code". And during the construction of prestressed concrete continuous beam bridge the elongation of long cable under the designed tensile force is not enough. Both of these problems reflect the current formula does not give a good description of the tension variation of the pretress strands.Based on the Hertz elastic contact theory, this paper studies the formula of friction loss in the Chinese "Highway Code" and does some theoretical analysis and experimental studies on effective prestress of prestressed concrete continuous beam bridge in the curved duct. The main content includes:1. Study on the prestress loss of prestressed structure in curved duct. This paper pointed out problems of the prestressed friction loss formula of Chinese "Highway Code" in the deriving process at first. Based on force equilibrium conditions, this paper derived the friction loss formulas in any stress distribution mode. From the theoretical analysis and calculations of prestress loss of continuous and non-continuous circular of curved channel, this paper proved that friction losses of prestress in several curved ducts can not add up, but should be calculated in relatively independent arc as a unit. This paper then used the friction test of curved channel and the test data of a real bridge to prove the current general formula of prestresse friction loss unreasonable. Finally, prestressed loss formula between prestressed strands and pipe wall was shown.2. Finite element analysis of contact analysis between prestressed strands and concrete. With common finite element analysis software ANSYS, this paper analyzed the mechanical properties of prestressed strands and concrete in curved channel, and got the law of prestress friction loss and the interface contact stress distribution patterns between prestressed strands and concrete. Through the comparisons of the results by the code formula, the simplified formula and the finite element calculation, it is found when the central angle of the bend channel is less than 120°, we can use the simplified formula for friction loss of curved channel; when the central angle of the bend channel is greater than 120°, the results must be considered the value, of item B which can impact on the final friction losses. Assume that the distribution of contact stress along the arc is in three sections of lines, the value of item B can be expressed by a formula. This paper provided a reference of friction loss calculation in theory. It also proved derived formula will give an accurate value of friction loss as long as the assumption of contact stress distribution model is correct, and then validated the applicability of the derived friction loss formula.3. Parameter identification for formula of friction loss of prestress. For friction coefficient?and deviation coefficient k of the pipe is indeterminate in the construction process of a real bridge structure, this paper used the derived friction loss formula and field data to identify parameters?and k by BP neural network. The results shown that BP neural network algorithm for the identification of friction parameters is feasible; the values of friction loss by identification parameter are in good agreement with the experimental values. It proved the derived formula is effective for calculating prestress friction loss. The calculation of friction loss for flat curve and space curve prestressed strands were feasible and the accuracy is higher than the code formula. According to the strain test results of circular prestressed strands, it gave the calculation method of the specified point within an arc of curved duct.4. Design and comparison of prestress loss of a real bridge. Combined with a continuous rigid frame bridge, this paper analyzed the values of prestress loss. It calculated the prestress loss of friction, elastic compression, and looseness of strands, shrinkage and creep of concrete, and the ratios of the above four to the total prestress loss. This paper investigated the sensitivity of deflection and stress of the bridge structure with deviation of?and k. It also compared the influence of derived friction loss formulas and the formula of current code.