Studies On Short-and Long-term Mechanical Behavior Of Prestressed Concrete Box-Girder

In the wake of development in transportation engineering,prestressed concrete(PC)box-girder bridges have become the preferred bridge type in small and medium span bridges,due to their satisfactory mechanical performances and superiorities in construction.Combining inadequate consideration in design with overloading and poor maintenance in service,defects such as excessive deflection along with cracking appear in prestressed concrete bridges with service times for more than 20 years.For decades,considerable researches have been investigated in three separate ways of theoretical analysis,finite element analysis(FEA),and experimental studies.However,an advanced model reflecting degradations in mechanical performances of PC bridges is still lacking.In this context,the author intended to study short-and long-term performances of PC box-girders based on the long-term load test and flexible failure test on two PC box-girders.The research described in this paper was financially supported by the Scientific Research Project of Hunan province High School(key project,Grant No.12A027).The main contents of this thesis are listed as follows:(1)Long-term load test on two simply-supported PC box girders for 470 days were carried out.The results show that strain and deflection are greatly impacted by shrinkage and creep of concrete.Affected by cracking,stiffness of cracked girder is smaller than that of un-cracked girder.Hence,concrete stress is releasing,creep effect is weakening,and the growth of deflection in cracked girder is faster than that of un-cracked counterpart.(2)Flexural failure test was carried out on two simply-supported PC box-girders which have been used in the prior test.The measured data show well coincidence in cracking load and ultimate load between cracked and un-cracked girders.The curves of load versus deflection of two girders showed differences in the period between cracking of girders and yield of plain reinforcements.It indicates that cracked girder enters elastic-plastic status earlier than un-cracked girder and that the existence of cracking can weaken operational performances of PC structures without reducing their ultimate bearing capacity.Furthermore,FEA model used to simulate flexural failure test of un-cracked girder was established by a common software,ABAQUS.In view of the well agreement between calculated and measured data,the accuracy of the model have been checked.(3)Long-term load test on two continuous PC box-girders for 470 days were carried out.It turns out that the additional deflection,strain,and support reaction develop fast at the early stage,slow down then,and keep stable in the end.At a certain moment,the additional deflection is in linearly relation with load,and the increasing patterns of deflection in separate cross section are similar.The increment of concrete strain in compression is evident large than that in tensile,which conforms the statement that compressed creep is the main component contributing to additional deformation of concrete structures under sustained load.Affected by shrinkage and creep of concrete,prestressing loss after anchoring,and additional deformation of supports,redistribution of support reaction appears over time when the variation range of mid-support reaction decrease and end-support reaction increase is around 7%.To describe the variation of support reactions over time,a fitting formula in the form of exponential function was proposed finally.(4)Theoretical analysis of additional deflection of PC continuous box-girders was made.Frist of all,discussions on analysis models of concrete creep and prestressing loss after anchoring and the two dominant factors leading to the appearance of additional deformation were conducted.Then,an advanced and practical analysis model considering coupling effect of two factors was presented based on the measured data of deflection.Afterwards,the additional deflections of the PC box-girders on the theoretical level were worked out combined with the method of structural mechanics and effective elastic modulus.The results were in well agreement with that of measure.