Research On Effects Of Shrinkage And Creep In Concrete Filled Steel Tubular Arch Bridges

The effects of shrinkage and creep on Concrete-Filled Steel Tubular(CFST)arch bridges are significant,which include the influences on the deformation,internal forces and stresses and ultimate bearing capacity.However,study on these effects is less of profound and systematic.This paper carries out a comprehensive and systemic research on these effects by model tests,theoretical study and finite element analysis,the main works and achievements include follows:(1)Tests especially for shrinkage of CFST members are carried out.Test results show that the shrinkage deformation of CFST member decreases with the increase of the fly ash content and the steel ratio,increases with the increase of the concrete strength,while it is not influenced significantly by the diameter of the steel tube.Analysis results on predictions of the shrinkage deformation of the specimens by the common shrinkage models indicate that the ACI 209R-92 model could give good prediction results but the deviation will increase when the content of fly ash in the concrete is large.Based on the test results,a modified ACI 209R-92 is presented for the shrinkage of CFST member for concrete with a.fly ash content no less than 20%.(2)Case studies of 12 CFST arch bridges were carried out on shrinkage effects.The results indicated that both the direct calculation method and equivalent cooling temperature method can be used to calculate the shrinkage-induced secondary internal forces of a statically indeterminate CFST arch.However,it is difficult to give out an equivalent cooling temperature value,so the equivalent cooling temperature method is not suggested to be used in design calculation.The shrinkage-induced secondary stress is small and could be ignored in preliminary design,whereas the shrinkage-induced self-equilibrium stress is quite large and should be taken into account in design calculation.(3)Creep tests of CFST specimens were conducted.Prediction analysis of the specimens’creep deformation by five common creep models indicate that the prediction results by ACI 209R-92 model and CEB-FIP MC90 model are more precise than those by the other three.Creep effects of 12 CFST arch bridges are calculated by the ACI 209R-92 model and CEB-FIP MC90 model,the differences between the predicted results by these two models are smaller than 5%.The CEB-FIP MC90 model is recommended to be used in the GB 50923-2013 because it is more widely used and familiar by Chinese designers.(4)The case study of CFST arch bridges indicate that the increment of displacement at crown section caused by creep could be up to 103.6%,the most unfavorable stress increment of steel tube at springing section could be 100.6%,and the maximum stress decrement of core concrete at springing section is up to 59.8%.Therefore,the creep effects should be considered in the design of CFST arch bridges.However,creep will also decrease the shrinkage-induced secondary internal forces in a statically indeterminate CFST arch,where a reduction factor of 0.66 may be considered for it.(5)Ultimate bearing capacity of the long-term loaded CFST specimens is tested.Test results indicate that the shrinkage and creep will increase the stress of steel tube and make it earlier to reach the plastic-stage,therefore the ultimate stability bearing capacity is lower than that of specimens for short-term loading.The larger the slenderness ratio and the axial load ratio,the larger decreasing of the ultimate bearing capacity.The influence of slenderness ratio is larger than that of axial load ratio.(6)Degree of shrinkage-and-creep-induced stress of steel tube is proposed as a new concept in this paper.Formulae to calculate the factor kc of CFST strut column considered the influence of shrinkage and creep are deduced by the limit equilibrium theory.The mechanism of the ultimate bearing capacity for axially-loaded CFST strut columns influenced by shrinkage and creep is uncovered.A practical table is presented for practice calculation of the factor kc.(7)Finite element models are established for ultimate stability bearing capacity of CFST slender columns considering the effects of shrinkage and creep,which is validated against the test results by case study.FE parametric study is conducted,a simply method is proposed for calculation of the factor kc caused by the shrinkage and creep effects on the ultimate stability bearing capacity of CFST column,which can be used in the practical engineering and can be references for editing or revision of criterion of CFST structures.For the ultimate load-carrying capacity of CFST arch calculated by the equivalent beam-column method,case study considered the influence of shrinkage and creep is carried out.It is found from the study that the influence of shrinkage and creep is the quite large and should be taken into account in design calculation.