Short And Long-Term Behavior Of Concrete Box Girders

Thin-walled concrete box girder prestressed with externally CFRP tendons would become a good structural style for the newly built bridges and the structural strengthen of existing bridges, it has no report on the research so far at home and abroad. At present the research on the performance of continuous box girder is only stagnate at short-term and elastic stages, it is almost no research on the structural internal force redistribution and long-term performance of continuous concrete box girder with wide flange after concrete cracking. For too many thin-walled concrete box girders universally cracked, this paper had an indepth experimental study on short-term and long-term, in order to obtain short-term and long-term performance of thin-walled concrete box girder, this paper has finished the following works:(1)Short-term performances of thin-walled concrete box girder prestressed with CFRP tendons were tested. Based on transformed-section method, adopting variational principle, it deduced shear lag coefficient distribution law of simply supported concrete box girder under uniform load and central load. Analyses indicate that effective distribution width coefficient of flange at mid-span section would change within 5% due to concrete cracking.(2) Short-term performance of concrete continuous box girder were tested, it shows that concrete cracking and uneven settlement of latex support would induce structural internal force redistribution of continuous box girder. Applying variational principle, shear lag effect of continuous box girder under uniform load were analysed adopting dismemberment mothed and transformed-section method. The analysis indicate that, after the simply supported beam which begins from the side support to inflection point, and the simply supported beam between the points of inflection, were cracking, it would appear negative shear lag effect nearby cracked point, the influence of concrete cracking on shear lag coefficient would be increased than elastic state at mid-supported section. The value calculated by variational principle would be increased by 9.1%¹4.8%, the value calculated by test results would be increased by 7.7%¹3.7%. The influence on effective distribution width coefficient of flange at mid-span section would change within 4% due to concrete cracking.(3) Long-term behaviors for 673 days were tested on 10 rectangle beams, including 6 ordinary reinforced concrete beams and 4 beams strengthened with CFRP laminates. Long-term deflection of concrete beams were predicted by means of creep transformed-section method, It is found that the existing of CFRP laminates is insignificant to the long-term deflection of concrete beams. Parameters were analysed on beams strengthened with CFRP laminates. The long-term sectional stress were analysed on R.C beams, the theoretical values agree well with the test date. It is preferable to predict the long-term deflection of rectangle concrete beams according to upstate design code.(4) Long-term behaviors were tested for 1001 days on thin-walled concrete box girder prestressed with CFRP tendons under uniform load. The strain of tensile steel bars would change relative small under long-term sustained load, the strain of compressional steel bars would change relative big, strain distribution of top flange would assume uniform trend along with time growth. Calculating the characteristic crack width according to upstate code JTG D62-2004, the property coefficient which related to the members C₃=1.25 is suggested. Calculating the maximum crack width according to upstate code GB 50010-2002, the property coefficient which related to the membersα_cr =2.7 is suggested, so the theoretical crack width would be agree well with the test date. The stress and strain distribution rule of prestressed concrete box girder under sustained uniform load was analyzed by creep transformed-section method, the theoretical predicted values agree well with the test date. By means of bilinear method and the sectional curvature method, the long-term deflection of prestressed concrete box girder was predicted. The actual increscent coefficient of mid-span deflection for 1001 days would bigger than that prescribed by upstate code of 18.5%. The increscent coefficient of long-term deflectionη_θ=2.45 is suggested, so the predicted long-term deflection would be agree well with the test date.(5)Long-term behaviors were tested for 1006 days on reinforced concrete continuous box girder under uniform load. The strain of tensile steel bars would change relative small, the compressional strain of steel bars and concrete would change relative big under sustained load, and the strain increase at mid-span section would bigger than that at mid-supported section. The strain distribution of top flange would assume uniform trend along with time growth. The internal force redistribution laws of the experimental model were tested for 1006 days, the theoretical values agree well with the test date. By modifying the relative parameters to upstate design code, the theoretical crack width at dominant sections would agree well with actual values. The actual long-term deflection at mid-span would be bigger than theoretical values prescribed by upstate design code of 11.4%.