Research On Contact Stress, Pre-stressed Frictional Loss And Elongation In PC Bending Duct

Pre-stress technology has been widely used in bridge engineering field for its superior performance. There are two aspects worthy of further exploration about the contact problem between pre-stressing steel strand and concrete in bending duct. On the one hand, since the radial component of the tensile force of steel beam to concrete pressure, we need to correctly analyze the pressure distribution of contact area to check the local stress of concrete. On the other hand, since contact pressure lead to relatively large channel friction, we need to accurately calculate the value of pre-stressing loss and elongation. Currently, there are lack of special theoretical and experimental study about the contact problem between pre-stressing steel strand and concrete in bending duct. Its law has not been fully understood and there is even some misconception.Based on the elastic contact theory analysis, this thesis design and implement the CT model experiment, using the ANSYS finite element software obtains the contact stress distribution rule between curved pre-stressing strand and concrete. The law of pre-stressed friction loss in bending channel is deeply discussed, and the elongation problems about long curved pre-stressing strand and U-shaped pre-stressing strand are explained. The main content and achievements of research work is as follows:1. After summarizing the characteristic of contact problem between pre-stressing steel strand and concrete in bending duct, the processing method that reduce it to two plane contact problems is introduced: for the contact problem within the cross-section of duct, a Hertz contact stress calculation method based on the cushion layer hypothesis is put forward; for the contact problem within the centerline plane, using elastic plane half space line load theory the uniform pressure assumption is falsified. Then three possible non-uniform distribution assumptions are put forward and their stress distribution functions are respectively deduced.2. The simulation experiments of bending contact is designed and successful implemented, using CT technology the contact area stress value is calculated by ANSYS software. Required angle, the relationships of curvature radius, force, friction factor and the normal stress distribution are discussed. Through the stress data fitting, it is concluded that contact pressure close to cosine distribution as duct bending angle is small. Based on the above results, the Deviator local stress of externally pre-stressed concrete structure is analyzed.3. Analyzing the defect of current pre-stressed friction loss formula in Rules, the thesis derives the friction calculation formula of three kinds of non-uniform distribution and uniform distribution. Comparison analysis show that contact friction in bending duct is overestimated by Rules and the pre-stressed friction loss formula is approximately applicable when bending angle is small. Based on the basic theory of modern solid friction, it is pointed out that Coulomb Friction Law has its range. Through the test analysis in bending duct of real bridge, curve segment pre-stressed loss and the change rule of friction coefficient is analyzed. And put forward that the friction resistance caused by low duct quality is the third factor, which should be separately considered by Rules.4. For the problem that actual elongation of small radius U-shaped pre-stressing steel strand is out of limits, three factors are put forwarded: uneven steel strand stress; hole-wall radial displacement; the actual friction less than theoretical value. The extra elongation resulted from three factors are calculated combining a case, and the influence degree of the three factors are analyzed. Finally some relevant control measures are proposed.