| The early crack and long term deflection in large span PC box girder bridges were very widespread. Combined with the construction of Tan Bridge of Jingyue Yangtse River Highway Bridge (7spans PC continuous bridge, the sapan was100m+5×154m+100m) and the Main Bridge (asymmetric combine girder cable stayed bridge, the main span was816m, and the38.5m wide concrete side-box girder was adopted in the south side span.), these two problems were studied. The main purpose was to build these two bridges with early crack and deflection controlled. The study would be of great practical and theoretic significance.Based on two Ministry of Communications’ subjects, which were "the Crack and Deflection Prevention of Long Unit and Large Span PC Continuous Box Girder Bridge", and a subitem "the Mechanical Behavior and Reasonable Design of Concrete Side-box Girder" of "The Critical Technique of Jingyue Yangtse River Highway Main Bridge", some detailed test and analyses were carried out. The main contents included some typical problems, the hydration heat of concrete box girder, the mechanical behavior experiment of a full scale modal of a30m span PC box girder, the rational shrinkage and creep modal for the large span PC box girder bridges, the cable tension measurement and the shear lag of concrete side-box girder.1. The hydration heat and thermal effect on concrete box girder(1) The strength of large span PC box girder bridge concrete at early age was tested. The standard and site curing specimens, which was sampled from the construction site, was made from pumping concrete. The results demonstrate that the curing condition had great influence on the concrete strength at early age. The strength of standard curing specimens was higher than the site curing one. As the age became longer, this difference in strength was smaller.(2) The hydration heat of two long-span PC box girder bridges was tested and the development of hydration heat was obtained. The results showed that the peak temperature would appear about1day after the concrete was poured, and the maximum temperature difference between inside and outside would appear within the1st to4th day, which could induce the early crack.(3) Based on the mechanical properties of specimens cured at construction sites, the temperature field and stress field of PC box girder during hydration were simulated in time varying model with finite element method. The results demonstrated that the hydration heat was one of the main reasons which caused the early cracks in the web of box girder. Effective measures should be taken to control the temperature difference between the inside and the surface less than30℃, and the stripping time should be more the4days under the Beaufort-scale3.2. The experiment and analysis on full-scale model of30m span PC simply supported box girder(1) The experiment was carried out on full-scale model of30m span PC simply supported box girder. The test results demonstrated that the deformation behavior was very excellent. The maximum deformation was267.85mm, which was1/108of the span. The prestress bar would be tensile failure, and then the beam would be broken. The ultimate bearing capacity of the beam was sufficient.(2) Based on the test results, the ultimate flexural strength, deformation and crack width of the box girder were calculated with the relative equations in five different domestic and international codes. The results showed all of the codes could be applied to the ultimate flexural resistance calculation. ACI318M-05and CEB-FIP MC90were suituable for the deformation determination, while GB50010-2002and TB10002.3-05were applicable for the crack calculation of the box girder.(3) A full range nonlinear program was developed and parametric study was conducted. The calculated value agreed well with experiment. Further analyses was conducted by this program, the results showed that as the reinforcement ratio or the tension coefficient of the prestress increased, the residual deformation at the mid-span and the ductility performance were decreased. While the prestress ratio of the tendons increased, the residual deformation at the mid-span and the ductility were decreased, the maximum deformation increased.(4) The results of experiment and analyse demonstrated that when the load was1872kN the prestress bar would be tensile failure. But the strain of the concrete of the top flat at mid-span was only1456με. Therefore, the ratio of reinforcement could be increased. Taking account of the ductility, prestress bar would be6bunches (The original design was6bunches,44(?)s S15.24strands). When the beam was broken, the strain of the concrete of the top flat at mid-span was only2925με. The ultimate bearing capacity was2790kN, which was1.47times of the original design. 3. The shrinkage and creep of large span PC box girder bridge in nature environment(1) The shrinkage and creep experiment of100mm×100mm×400mm prisms was carried out in the laboratory. Analyse was conducted with AASHTO, ACI209R, JTG D62and GL2000, the results demonstrated JTG D62and GL2000were more suitable for the shrinkage and creep calculation. The fly ash had a significant influence on shrinkage and creep. It could restrain the shrinkage. For creep, it could increase the creep coefficient if the load age was very short, decrease the creep coefficient if the load age was long.(2) In order to study the shrinkage and creep of concrete at nature environment, the shrinkage and creep experiment of200mm X200mm X515mm prisms was carried out in the construction site. The results showed that JTG D62and GL2000could reflect the shrinkage and the creep of concrete at nature environment.(3) The shrinkage and creep experiment of two30m span PC box girders were carried out in nature environment. Analyse was conducted with superposition principle and all of the parameter corrected. The results demonstrated that GL2000was more accurate to reflect the shrinkage and creep than JTG D62. Therefore, GL2000could be applied to calculated shrinkage and creep in the nature environment.(4) The shrinkage and creep of Tan Bridge was analysed based on GL2000model.20mm (50mm) precamber due to creep was applied in the side (main) span. The stress and deflection at the critical section of Tan Bridge were observed during the construction and service stage. The observed deflection due to creep agreed well with calculated one, and the observed deflection was less than the calculated value, which indicated that the deflection had been controlled.4. The cable tension measurement and the shear lag of concrete side-box girder(1) Based on the transverse vibration equation of a cable, a practical formula expressed in a simply explicit form to estimate cable tension was proposed. The formula was the unified expression covering both cases from string theory and beam theory. The capability of this formulation was verified by the real cable in Jingyue Yangtse River Main Bridge, which indicated that the formula was sufficiently accurate and could be conveniently applied to field measurement.(2) The stress of concrete side-box girder was tested during the typical construction stages, and analyse was conducted. The results demonstrated that the calculated result agreed well with the measured one. Positive shear lag occurred in the concrete side-box girder at construction stage. Shear lag became larger when the section closer to the main tower when bridge was completed. For the mid-span section (without transverse diaphragm), the shear lag coefficient was1.04～1.13and1.03～1.10in top flat and bottom plate, separately. The results showed that the transverse diaphragm had a significant influence on the shear lag of side-box girder.When the bridge was successfully erected and evaluated by the third party, the result showed that there was not visible crack in the side-box girder of the Main bridge and the box girder of Tan bridge of Jingyue Yangtse River Main Bridge. The results of observed deflection after opening to traffic showed that the deflection agreed well with calculated one. And the observed deflection was less than the calculated value, which indicated that the deflection was within expectation. |
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