Theory And Methodology Of Predication On Effective Prestress For Beam Structures With Complicated Prestressing Systems

Since the 1930s, prestressing technology has been widely used in bridge engineering and more than 70 percent of bridges are of the prestressed concrete structures at present. However, in the procession of its practical application, serious defect have been found out in a considerable number of PC (Prestressed Concrete) bridges, especially to those existing bridges via many years. Altogether the material disease is mainly shown in two aspects:one is concrete deterioration and the other is steel tendon defect. Current studies about inspection and evaluation on attenuation degree of effective prestress are confined to the experimental datum on simply-supported beams. Therefore it still has a long way to go.The research is sponsored by the science & technology program for west communication construction of MOC which is the project of NDE (Nondestructive Examination) on prestress tendon for mid-span or long-span PC bridges (Grant No.2005 318 812 15). This paper relies on the fifth sub-project of the programm focusing on the inspection study of effective prestress for prestressed reinforcement. Therefore, the research objectives of this paper are getting standard testing technology on effective prestress and setting up unified theoretical system of prediction on its attenuation, which may cover both long-span bridge with complicated prestressing systems like continuous rigid frame bridge and mid-span bridge with simple prestressing systems for practical application.Main research contents of this paper are as follows:(1) Based on comparatively sophisticated prestress loss method in current code, combined with study on the inverse problem to the principle of longitudinal distribution for effective prestress, model for predicting on attenuation of effective prestress was built up which included both analysis module and simulation module. Altogether there existed four key aspects as follows:a) According to the route of summing up characteristic types and geometry of prestressed reinforcement in a large number of highway bridges, for the sake of matching with the DD (Directly Detection) testing technique, standardization and classification on effective prestress inspection had accompolished. The prestressed reinforcement was divided into two categories, namely, the directly-measured prestressed reinforcement and indirectly-predicted ones. As far as the directly-measured prestressed reinforcement was concerned, its LSFR (Limited Stress Fluctuating Rate)βalong longitudinal direction used as index of testing classification was defined, which considered the influence of spatial geometry factors such as radius, angle and length of projection and so on to the prestress losses. This type of tendon was categorized as VT (Violent Tendon) and MT (Moderate Tendon). This academic route has greatly strengthened normalization of the inspection technology.b) Numerical method for effective prestress prediction between facial tendon and inner tendon in the same section was built up, through which effective prestress for each tendon could be deduced. This method was suitable not only for tendons with symmetric geometry types but also for asymmetric ones.c) Through decomposition of geometric types and simplified analysis on prestress losses for VT, the algorithm for virtual frictional loss functionσlana(x) was put forward in a universal way. On this basis, software named Prestressing Parameter Solution to virtual frictional loss function (PresPS V1.0) was programmed and copyright of the software had been registered. Thus the analogy method for VT was got eventually. Numerical analysis showed that the method was more accurate and easier in use than the simplified method in current code.d) Aiming at the analogy method for MT, different cross section types, construction techniques and prestressing forms were concluded. Furthermore, three subordinated modes including GM (Geometric Mode), AM (Arithmetic Mode) and FM (Functional Mode) were put forward. After recognition on these modes, distribution regularity and effective prestress value for moderate tendon could be simulated.(2) Based on section analysis, studies of the integral methods for evaluation on attenuation of effective prestress were conducted. Altogether there existed three key aspects as follows:a) Researched on the influence of time-dependent effects such as shrinkage, creep and prestressing relaxation upon composite PC cross-section, a simplified analytical method of stress transformation analysis was presented according to the features of cross-section stress redistribution during the period between the end of construction and the start point of inspection.b) Two practical analysis models for resultant force were available through DD (Direct Detection) and RD (Relieve Detection) testing techniques separately. It made up for the deficiency in nonlinear analysis when applying finite element method for beam structures and possessed advantages of practicability.c) Quantitative analysis for the deviation of Prestress Reservation Degreeλin critical sections between ideal state and service state was executed based on cross-section analytical method. By means of grading index system for deficit of effective prestress, evaluation of PC bridge performance had accomplished and was verified by a synthesis example.(3) A special instrument named Stretching Force Tester for prestressing strand (model: LCZL-50) was invented, through which real-time acquisition of effective prestress force could be realized and the patent had been applied. It provided powerful support for the basic experiments on "static parameter" testing under the condition that bellows were locally split.(4) Special experiment for measuring the tension force of prestressed steel strand was implemented on static load anchoring platform. Both of the damage range and boundary condition were considered as the key index through which the directly-detected measurement on static and dynamic parameters was applied. Simultaneously, test errors were minimized by the aid of mathematic and regression method. Two of the practical techniques for prestressing force inspection which called SPLM (Static Parameter Loading Method) and MPIM (Modal Parameter Incentive Method) separately were got ultimately. Noticeably, SPLM was suitable for existed PC beam and MPIM was privileged in pre-tensioned voided slab respectively.(5) Consulting the common geometry type and disposal way of prestressing tendon in highway bridges, large-scale beam model was made and experimental study had been launched. By simulation of taking place, developing and changing state of prestress losses, the precision for empirical formula was tested and their adaptability study were accomplished. Meanwhile through experiment on actual distribution principle of inherent prestressed tendon along longitudinal direction, a significant deduction correlated with instantaneous prestress losses was demonstrated.(6) Taken five engineerings which composed of common types of PC beam bridges as the background project, the research results were put into practical use. On this basis, numerical simulation and comparative analysis on the key techniques for system of inspection and evaluation on effective prestress were applied to engineering practice successfully so as to fulfill the expected research target eventually.Generally speaking, four innovative points of this paper are summed up:(1) For the first time, model for predicting on attenuation of effective prestress was built up which included both analysis module and simulation module. It solved the problem of simulation on practical value and deviation regularities for effective prestress.(2) For the first time, the integral methods for evaluation on attenuation of effective prestress were put forward based on critical section analysis. In this method, double resultant force models were built up according to two of the parallel ways which are DD (Direct Detection) and RD (Relieve Detection) testing techniques. Furthermore, grading index system for deficit of effective prestress was set up and performance evaluation of beam structures with complicated prestressing systems had been accomplished.(3) For the first time, two of the practical techniques called SPLM and MPIM were got ultimately, which could execute prestressing force inspection on the condition of splitting the concrete and bellows locally.(4) A special instrument named Stretching Force Tester for prestressing strand (model: LCZL-50) was invented. For the first time, it integrated griping, holding, drawing and detecting on prestressing strand to realize real-time acquisition of effective prestress force.