Creep Effects Of Concrete Filled Steel Tubular Arch Bridges In Construction Stage

In concrete-filled steel tubular(CFST)structure,stress redistribution would occur between concrete and steel tube,and structural deformation would increase due to creep of the concrete core.Concrete-filled steel tubular arch bridges will be loaded at different stages of the construction process,and the concrete core would bear loads at early age.Due to incomplete hydration of the concrete core,the applied loads are relatively large,making the concrete creep difficult to be predicted.At present,few study focused on the creep of concrete in CFST arch bridge based on the monitoring of bridge in construction process.In this study,the deformations of arch ribs during construction are monitored,and the influence of concrete creep on structural deformation and stress in CFST is analyzed.The main works are as follows:(1)Two CFST arch bridges(with span lengths of 64m and 80m,respectively)in construction period were monitored and the vertical deformation of the arch ribs was measured.(2)Elastic modulus and compressive strength of the core concrete in CFST ribs at 7-day and 28-day were tested.Creeps of the core concrete loaded at 7-day and 28-day ages were also measured.Stress level of creep specimen was determined by the obtained compressive strength of concrete.(3)The test data of concrete creep were compared with the predicted value according to fib MC2010,B3 model,GL2000 model,and ACI209 model.Discrepancies between the predicted value and the tested data are obvious.Since the above-mentioned models are not suitable to predict creep of CFST ribs,a regressed model for creep of the concrete core was suggested in the analysis of CFST ribs.(4)Finite element models of the two CFST arch bridges were established with consideration of each building step.Development of the elastic modulus and the regressed creep model were introduced into the models to calculate the deformation of the CFST arch ribs.The analysis results agreed well with the monitored vertical deformations of the ribs,which verified the established model.Futher development rule of the steel tube and the concrete core on cross-sections at midspan and 1/4-span of the CFST ribs were calculated.Compared with the analyzing results without consideration of concrete creep,it is found that the creep plays an important role on the deformation of the arch ribs during construction.It can be concluded creep strain would be triple of the elastic strain.Influence of concrete creep on the stress redistribution between steel tube and core concrete in CFST can not be neglected.Stress of steel tube would increase 10%-30%,while that of core concrete would decrease 10%-50%,because of concrete creep.Stress distribution will be aggravated for the structure with larger spans.